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Ⅰ IntroductionThere are 400 types of scent receptors in a typical human nose that allow us to detect about 1 trillion different odors. But many of us also can't define the form or concentration of gas in our atmosphere. There are several kinds of sensors to calculate various parameters and a gas sensor is one that is useful in applications where we have to detect changes in the concentration of harmful gases to keep the device safe and prevent any unwanted threats. To detect gases such as oxygen, carbon dioxide, nitrogen, methane, etc., there are different gas sensors. They can also be widely used in devices that are used in factories and offices to detect the leakage of toxic gases, track air quality, etc. We will learn more about gas sensors, their construction, types, function, and how they can be used to calculate the type and concentration of gas required in our atmosphere in this article. There are several kinds of gas sensors, but gas sensors of the MQ type are frequently used and widely popular, so this article will concentrate more on these types of sensors.CatalogⅠ IntroductionⅡ Introduction of Gas SensorⅢ Different Types of Gas sensorsⅣ Construction of Gas SensorⅤ Working of Gas SensorⅥ Working Principle of Gas SensorⅦ How can a gas sensor be used?Ⅷ Gas Sensors List and What Gases They SenseⅨ Gas Sensors ApplicationsⅩ FAQⅡ Introduction of Gas SensorA gas sensor is a device that senses the atmosphere's presence or concentration of gases. The sensor creates a corresponding potential difference depending on the concentration of the gas by adjusting the resistance of the material within the sensor, which can be determined as the output voltage. The type and concentration of the gas can be calculated based on this voltage value.The type of gas that can be detected by the sensor depends on the sensing material within the sensor. As shown above, these sensors are usually available as modules with comparators. A specific threshold value of the concentration of gas may be set for these comparators. The automated pin goes up when the concentration of the gas reaches this threshold. It is possible to use the analog pin to measure the gas concentration.Ⅲ Different Types of Gas sensorsBased on the type of sensing element it is designed with, gas sensors are usually categorized into different categories. Below is the classification based on the sensing aspect of the different types of gas sensors that are commonly used in different applications:• Metal Oxide Based Gas Sensor.• Optical Gas Sensor.• Electrochemical Gas Sensor.• Capacitance-based Gas Sensor.• Calorimetric Gas Sensor.• Acoustic Based Gas Sensor.Ⅳ Construction of Gas SensorThe Metal Oxide Semiconductor Based Gas Sensor is the most widely used gas sensor of all the above types. A sensing component containing the following elements will consist of all gas sensors.• Gas Sensing Layer• Heater Coil• Electrode Line• Tubular Ceramic• ElectrodeThe picture below shows the components of a metal oxide gas sensor.The goal of each of these elements is as shown below.:• Gas sensing layer: It is the main component of the sensor that can be used to detect changes in gas concentration and produce electrical resistance changes. The gas sensing layer is essentially a chemical resistor that adjusts its resistance value depending on the environment's real gas concentration. The sensing factor consists of a Tin Dioxide (SnO2) here, which generally has excess electrons (donor element). Therefore, the resistance of the element changes and the current flown through it varies as toxic gases are detected, which reflects the shift in gas concentration. • Heater coil: The purpose of the heater coil is to burn the sensing component to increase the sensitivity and efficiency of the sensing component. It is made of nickel-chromium with a high melting point that allows it to remain heated without melting. • Electrode line: Since a very small current is generated by the sensing element when the gas is detected, preserving the efficiency of carrying those small currents is more critical. So Platinum wires come into play where they help to efficiently transfer the electrons. • Electrode: It is a junction where the sensing layer output is attached to the line of the electrode. So that the output current may flow to the terminal that is needed. Gold (Au-Aurum), which is a very good conductor, is an electrode here. • Tubular ceramic: There is a tubular ceramic made from aluminum oxide between the heater coil and the gas sensing layer (Al2O3). As it has a high melting point, it helps to preserve the sensing layer's burn-in (preheating), which provides the sensing layer with high sensitivity to obtain an effective output current. • Mesh over the sensing element: A metal mesh is used over it to cover the sensing elements and the setup, which is also used to prevent/hold dust particles from entering the mesh and to prevent damage from corrosive particles to the gas sensing layer.Ⅴ Working of Gas SensorA gas sensor's ability to detect gases relies on the chemiresistor to conduct current. Tin Dioxide (SnO2), which is an n-type semiconductor with free electrons, is the most widely used chemical resistor (also called a donor). The atmosphere usually contains more oxygen than combustible gases. The particles of oxygen attract the free electrons present in SnO2, bringing them to the surface of SnO2.  As there are no free electrons available, there will be a zero output current. The gif below shows the oxygen molecules (blue color) within the SnO2 attracting the free electrons (black color) and preventing them from having free electrons to perform the current.Ⅵ Working Principle of Gas SensorThis decreasing gas (orange color) interacts with the adsorbed oxygen particles when the sensor is put in the atmosphere of toxic or combustible gases and breaks the chemical bond between oxygen and free electrons, thereby releasing free electrons. As the free electrons return to their original location they will now conduct current, this conduction would be proportional to the number of free electrons available in SnO2 if more free electrons are available for the gas to be highly toxic.Ⅶ How can a gas sensor be used?There are 6 terminals in a simple gas sensor in which 4 terminals (A, A, B, B) serve as input or output and the remaining 2 terminals (H, H) are used to heat the coil. Of these 4 terminals, 2 terminals from each side can be used as either input or output (as seen in the circuit diagram, these terminals are reversible) and vice versa.MQ2 Gas sensor PinoutThese sensors are usually available as modules (shown on the right), consisting of a gas sensor and an IC comparator. Now let's see the gas sensor module pin definition that we normally use with an Arduino. The module for the gas sensor consists of 4 terminals.• Vcc – Power supply• GND – Power supply• Digital output – This pin produces an output that is either logically high or logically low (0 or 1), indicating that it shows any harmful or combustible gases near the sensor.• Analog output – This pin provides a continuous voltage output that varies depending on the gas concentration added to the gas sensor.The output of a gas sensor alone would be very small (in mV) as discussed earlier, so an external circuit has to be used to get a digital high low output from the sensor. A comparator (LM393), adjustable potentiometer, some resistors and capacitors are used for this purpose. The goal of LM393 is to get the sensor output, compare it to a reference voltage, and show whether or not the output is logically high. Whereas the potentiometer is intended to set the gas threshold value needed above which the digital output pin should go high. A simple circuit diagram of a gas sensor in a gas sensor module is shown in the diagram below.The input and output terminals here are A and B (these are reversible - meaning either of the paired terminals can be used as input or output) and H is the terminal for the heater coil. The purpose of the variable resistor is to change the voltage of the output and maintain high sensitivity. If the heater coil has no input voltage, so the output current would be much smaller (which is negligible or approximately 0). The sensing layer wakes up when an appropriate voltage is applied to the input terminal and heater coil and is ready to detect any combustible gases near it. Let's first presume that there is no poisonous gas near the sensor, so the layer's resistance does not shift and the output current and voltage are also unchanged and insignificant (approximately 0). Now, let's say that poisonous gas is nearby. Since the heater coil is pre-heated, any combustible gases can now be easily detected. The resistance of the material varies as the sensing layer interacts with the gases, and the current flowing through the circuit often varies. This variation shift can then be observed in the load resistance (RL). The load resistance (RL) value can be anywhere from 10K to 47K. It is possible to pick the exact value of the load resistance by calibrating it with the known gas concentration. The circuit has lower sensitivity if the low load resistance is chosen, and if the high load resistance is selected, then the circuit has high sensitivity.Ⅷ Gas Sensors List and What Gases They SenseSensor NameGas to measureMQ-2Methane, Butane, LPG, SmokeMQ-3Alcohol, Ethanol, SmokeMQ-4Methane, CNG GasMQ-5Natural gas, LPGMQ-6LPG, butaneMQ-7Carbon MonoxideMQ-8Hydrogen GasMQ-9Carbon Monoxide, flammable gassesMQ131 Ozone............Ⅸ Gas Sensors Applications• It is used to track the concentration of toxic gases in industries.• Used in homes to recognize activities in an emergency.• The concentration of the gases that are emitted is tracked at oil rig locations.• Used at hotels to discourage smoking by clients.• Used in workplace air quality inspections.• It is used to track CO2 levels in air conditioners.• Used in fire detection.• Used for gas concentration regulation in mines.• The analyzer of breath.Ⅹ FAQ1. What is a gas sensor?As the name suggests, it senses gas. It's a component used to detect fluctuations in the gaseous state. There are so many gas sensors based on the element they sense, some are given below:• Carbon Dioxide Sensor: Used for detection of pollution caused by vehicles emitting CO2.• Alcohol Sensor: I know alcohol is not a gas but it senses the smell of it. Traffic police use devices based on this sensor.• LPG Sensor: It is used for avoiding the destruction caused by leaked LPG cylinders. 2. What are gas sensor arrays?A gas sensor that is commonly available in the market is an MQ-x series sensor - MQ2, MQ3, MQ6, etc. The module for this series sensor gives a digital output but can even be modified to have an analog output. MQ2 sensor can detect gasses like propane, butane, LPG, smoke and alcohol. An array of gas sensors would mean that all the sensors of this category are interfaced to the same controller and are laid at different locations to monitor the aspect. 3. How does a gas sensor work?Gas detectors use a sensor to measure the concentration of particular gases in the atmosphere. The sensor serves as a reference point and scale, producing a measurable electric current when a chemical reaction caused by a specific gas occurs. 4. How do MQ5 gas sensors work?In any sensor, a physical change contributes to a chemical change that generates an electrical impulse which then drives a circuit.Similarly here, the MQ5 gas sensor which is made of SnO2 is less conductive normally. In LPG or any combustible gas environment like propane-butane etc., it becomes more conductive.A circuit similar to the Wheatstone bridge will be available inside the sensor with one of its resistance made of SnO2. Assume a bridge balanced condition. When the conductivity changes bridge becomes unbalanced. Hence current flows through the center galvanometer showing deflection.  5. What is the difference between a gas analyzer and a gas detector?A gas detector has simple structures, consisting only of the sensor and sensor conversion circuit. However, a gas analyzer not only has sensors inside but also has a complete set of pneumatic systems, which introduces the sample gas into the instrument firstly and then discharges or retrieves the gas. 6. What is the use of a gas sensor?Gas sensors (also known as gas detectors) are electronic devices that detect and identify different types of gasses. They are commonly used to detect toxic or explosive gasses and measure gas concentration. 7. How long do gas detectors last?The typical life span of an electrochemical sensor is usually between 2-3 years. Whereas a more exotic gas sensor may only last 12-18 months. I would advise anyone who uses a gas detector to get the instrument serviced every 6 months as this will ensure that your instrument will be working perfectly. 8. What is gas sensor sensitivity?Usually, sensitivity (S) can be defined as Ra/Rg for reducing gases or Rg/Ra for oxidizing gases, where Ra stands for the resistance of gas sensors in the reference gas (usually the air) and Rg stands for the resistance in the reference gas containing target gases. 9. What is the most important sensor in a gas monitor?The most important technical aspect of all gas detectors is the heart of the instrument - the sensor. All the bells and whistles that can be crammed into one instrument can't take away from the simple fact that sensors must technically process incoming gases and vapors and provide an accurate response. 10. What is a gas sensor made of?Highly sensitive and selective sensors are needed for the detection and prevention of hazardous gas leaks from industries. Generally used gas-sensing materials comprise vapor-sensitive polymers, semiconductor metal oxides, and other porous materials such as silicon.

IntroductionThe humidity sensor is a sensor that measures relative humidity, absolute humidity, or dew point. At present, such sensors are widely used, and are gradually developing in the direction of excellent environmental resistance, long life, and low price. This article will introduce the working principle, type, classification, application of humidity sensor, and an Arduino tutorial on real-time temperature and humidity monitor. In addition, some basic knowledge like the calculation of relative humidity and absolute humidity is also covered. Let’s waste no more time! The humidity sensor is a sensor that measures relative humidity, absolute humidity, or dew point. At present, such sensors are widely used, and are gradually developing in the direction of excellent environmental resistance, long life, and low price. This article will introduce the working principle, type, classification, application of humidity sensor, and an Arduino tutorial on real-time temperature and humidity monitor. In addition, some basic knowledge like the calculation of relative humidity and absolute humidity is also covered. If you are only interested about the Arduino guide, you can jump to that part from the category, and the video below is about the accuracy test of different temperature and humidity sensor for arduino, which can help you to choose the best one. Let’s waste no more time!Check this video to look for the best temperature and humidity sensor for ArduinoCatalogIntroductionCatalogI How do Humidity Sensors Work?1.1 Humidity1.2 Absolute Humidity and Relative Humidity1.3 How Humidity Sensors WorkII Classification and Common Types of Humidity Sensors2.1 Classification2.2 Comparison of Resistive and Capacitive Humidity Sensors2.3 Four Types of Humidity Sensors with More Applications2.4 Commonly Used Humidity Sensor ModelsIII Application of Humidity Sensors3.1 Typical Uses3.2 Application FieldsIV Arduino Entry Project: Real-time Temperature and Humidity Detector4.1 Hardware Preparation4.2 Software Preparation4.3 Circuit Connection4.4 Implementation CodeⅤ FAQI How do Humidity Sensors Work?1.1 HumidityBefore learning the humidity sensor, let's take a look at what humidity is. Humidity, a physical quantity indicating the degree of air dryness. At a certain temperature, the less water vapor contained in a certain volume of air, the drier the air; the more water vapor, the more humid the air. The degree of air humidity is called "humidity". In this sense, it is often expressed by physical quantities such as absolute humidity, relative humidity, comparative humidity, mixing ratio, saturation difference, and dew point; if it represents the percentage of the weight of water vapor in the wet steam to the total weight (volume) of the steam, it is called the humidity of the steam. The humidity that the human body feels comfortable with is: the relative humidity is lower than 70%. So the question comes again, what is the relative humidity?Figure1. What is Humidity?1.2 Absolute Humidity and Relative HumidityThe degree of dryness and humidity of the air, or the physical quantity that indicates how much water vapor it contains, is called humidity. The mass of water vapor contained in a unit volume of air is called absolute humidity. Because it is difficult to directly measure the density of water vapor, it is usually expressed by the pressure of water vapor. The absolute humidity of the air does not determine the speed of water vapor on the ground and the perception of humidity. People call the absolute humidity of air at a certain temperature and the percentage of saturated air pressure at the same temperature as relative humidity. To be more specific, absolute humidity refers to the mass of water vapor contained in a certain volume of air, and its unit is generally g/m3. The maximum absolute humidity is the highest humidity undersaturation. Absolute humidity is only meaningful together with temperature, because the amount of humidity that can be contained in the air varies with temperature, and the absolute humidity is also different at different temperatures, because the volume of air also changes with temperature. But the closer the absolute humidity is to the highest humidity, the smaller its change with temperature. The following is the formula for calculating absolute humidity:The symbols are:e-vapor pressure, the unit is Pascal (Pa)-The gas constant of water = 461.52J/(kg K) T-Temperature, the unit is Kelvin (K)m-the mass of water dissolved in the air, in kilograms (kg)V-The volume of air, in cubic meters (m). Relative humidity (RH)A hygrometer is recording relative humidity. The relative humidity is the ratio between absolute humidity and maximum humidity. Its value shows how high the saturation of water vapor is. Air with a relative humidity of 100% is saturated air. Air with a relative humidity of 50% contains water vapor that reaches half the saturation point of air at the same temperature. Water vapor in the air with a relative humidity of more than 100% generally condenses.  As the temperature increases, the air can contain more water, that is to say, the relative humidity will decrease when the temperature increases with the same amount of water vapor. Therefore, while providing relative humidity, temperature data must also be provided. The dew point can also be calculated from the relative humidity and temperature. The following is the formula for calculating relative humidity:The symbols are:ρw-absolute humidity, in grams/cubic meterρw,max-the highest humidity, the unit is g/m3e-water vapor pressure, the unit is PascalE-saturated vapor pressure, the unit is Pascals-specific humidity, the unit is g/kgS-the highest specific humidity, the unit is g/kg 1.3 How Humidity Sensors WorkGenerally, humidity sensors use the following four methods to detect humidity or condensation:(1) Measure the change in electrical impedance or capacitance caused by moisture absorption, separation, or condensation of moisture-sensitive materials.(2) Measure the difference in gas thermal conductivity due to changes in humidity.(3) Measure the change in the resonance frequency of the crystal vibrator due to changes in humidity or condensation.(4) Measure the attenuation and light absorption and reflection caused by alpha rays passing through water droplets due to changes in humidity.For example, the characteristic of a humidity-sensitive resistor is to cover a film made of moisture-sensitive material on the substrate. When water vapor in the air is adsorbed on the moisture-sensitive film, the resistivity and resistance value of the element will change. One characteristic can measure humidity.Figure2. How a polymeric membrane humidity sensor worksII Classification and Common Types of Humidity Sensors2.1 ClassificationAt present, there are many types of humidity sensors on the market, and their application ranges are also different. They are roughly divided into the temperature and humidity ranges used, which can be listed in the following table.IndustryScope of applicationOperating temperature and humidity rangeUsesTemperature(℃)Humidity(%PH)Home appliancesAir conditioning machine5~40 40~70 Air conditioning equipmentDryer800~40Clothes dryingElectronic range5~1002~100Food heating and conditioning controlVTR-5~6060~100Prevent condensationCarAutomatic anti-fog-20~8050~100Prevent condensationMedical treatmentTreatment device10~3080~100Respirator systemIncubator10~3050~80Air conditioning equipmentIndustryFiber10~3050~100SilkDryer50~1000~50Kiln industry, wood dryingPowder moisture5~1000~50Ceramic raw materialsDry food50~1000~50Food preservationElectronic Components Manufacturing5~400~50Magnetic head, LSI ICAgriculture, Forestry and LivestockHouse air conditioning5~400~100Air conditioning equipmentTea smoke anti-frost-10~6050~100Prevent condensationReptile feeding5~400~50Increase humidity, health managementTestConstant temperature and humidity tank-5~1000~100Precision measurementRF detector-50~400~100High-precision meteorological measurementHygrometer5~1000~100Control recording deviceOtherSoil moisture-20~500~100Plant cultivation, soil and sand collapseIn the above, we have introduced four methods for detecting humidity or condensation. These measurement methods must be selected according to the different test environments. The most commonly used measurement method on the market is the first one because its measurement and signal selection methods are quite simple and cheap. It can be divided into the following six categories if it is distinguished by the difference of its manufacturing materials:(1) Electrolytes such as LiCl.(2) Semiconductor materials such as Se and Ge.(3) MgCr2O4, ZnCr2O4, TiO2, SnO2 and other metal oxide fusion products.(4) Porous metal oxide film such as Al2O3.(5) A material made by dispersing conductive powder in a polymer material such as nylon.(6) Organic or inorganic polymer electrolyte membrane. Sensors made of moisture-sensitive materials can be roughly divided into 7 categories:(1) Electrolyte humidity sensor(2) Polymer humidity sensor(3) Ceramic humidity sensor(4) Crystal oscillator humidity sensor(5) Semiconductor humidity sensor(6) Thick-film humidity sensor(7) Condensation humidity sensorClassificationMoisture Sensitive MaterialDetection methodElectrolyteLiCl+Polyviny1 Alcho1 PolystyreneResistorSulfated filmResistorPotassium sulfate membraneResistorLiC1 saturated solutionResistorSemiconductorSe (Ge or Si) vapor deposition filmResistorSi+SiO2+PAPA (Polyamino Phenylacetylene)ResistorMetal oxides (ceramics)Fe3O4 Colloid coating filmResistorCr2O3 Ni2O3 Fe2O3ResistorGlass ceramic filmCapacitorFe2O3-K2O ceramicResistorZnO-Li2O-V2O5 ceramicResistorMg Cr2O4 type ceramicCapacitorPolymersAl2O3+ epoxy resinResistor or CapacitorMulti-emulsion resin filmCapacitor Organic materialCelluloid+CarbonResistorButyr CellaloseCapacitorResin carbonResistorPolyamid+ crystal oscillatorResonance frequency2.2 Comparison of Resistive and Capacitive Humidity Sensors(1) Resistive humidity sensorThe resistive humidity sensor is a sensor that uses the electrical characteristics of the humidity sensor (such as resistance value) to change with humidity to measure humidity. The humidity sensor is generally immersed in an insulating material with a hygroscopic substance, or through evaporation, It is made by coating and other processes to prepare a layer of metal, semiconductor, polymer film and powdered particles.  During the moisture absorption and dehumidification process of the moisture-sensitive element, the conduction state of the ion H+ decomposed by water molecules changes, so that the resistance value of the element changes with humidity.Figure3. Resistive humidity sensorAdvantage1) Compared with the capacitive type, the structure is simpler, and it is easier to achieve mass production and low price.2) There is no need to consider the capacity between the leads like capacitive sensors, so the sensor can be stretched at will, with greater design freedom.3) Since the characteristic is a logarithmic change (the degree of change is large), the humidity change is small for the resistance change. (According to this point, for example, the degree of influence of the deviation of the electrode on the characteristics is small, and the instability is also small. Even if there is a slight change, it is difficult to show when converted to humidity.) Disadvantages1) The temperature characteristic is larger than that of the capacitive type (0.5%rh/℃), and temperature compensation is usually required.2) Since the characteristic is a logarithmic change, if the logarithmic conversion is not processed, the linear characteristic will not be obtained.3) The low humidity range is difficult to detect due to high resistance. (About 20% rh is the limit) In addition, it is easily affected by interference. (2) Capacitive humidity sensorA capacitive humidity sensor is a commonly used instrument in humidity sensors. It uses polymer humidity and humidity-sensitive capacitors as the basic humidity-sensing component, and uses a single-chip microcomputer to analyze, process, display and remotely transmit the measurement results. The measurement accuracy is ±2.5 %. The capacitive humidity sensor is mainly composed of a glass substrate, a lower electrode, a humidity-sensitive material, and an upper electrode. The two lower electrodes are connected in series with the humidity-sensitive material and the two capacitors formed by the upper electrode. Humidity-sensitive material is a high molecular polymer whose dielectric constant changes with the relative humidity of the environment.  When the environmental humidity changes, the capacitance of the humidity sensor changes accordingly, that is, when the relative humidity increases, the humidity sensitive capacitance increases, and vice versa (the capacitance is usually between 48 and 56 pf). The sensor's conversion circuit converts the humidity-sensitive capacitance change into a voltage change, which corresponds to a change in relative humidity from 0 to 100% RH, and the output of the sensor changes linearly from 0 to 1v.Figure4. Capacitive soil moisture sensorAdvantage1) Generally speaking, low humidity starting from 0% rh can be detected.2) The capacitance value is relatively close to linear, and no logarithmic change is required.3) The temperature characteristic is smaller than that of the resistance type (about 0.05～0.1%rh/℃), and temperature compensation is not required for general use.4) In order to increase the capacitance value, the structure is made into a thin film, and there are more products with a faster response speed than the resistance type. Disadvantage1) If the sensor is extended with a lead wire, the capacitance value will change, so it is not suitable to extend the sensor alone. Also, if it is assembled into the device, it is difficult to change the position by the lead wire, so the design freedom is small.2) The amount of change is relatively small, but a small change in capacitance will cause a large error. Therefore, inexpensive sensors have a large deviation. (The sensors used for measurement also have very high accuracy, but these estimation formulas have been processed.)3) The fact that the amount of change is small can be said in terms of dependence. But a small change in capacitance will produce a large error. Therefore, a sensor with poor reliability will have a large humidity change.4) Although the amount of change is small, the deviation and temperature characteristics of other circuit parts will have a greater influence, so be careful when selecting circuit parts. 2.3 Four Types of Humidity Sensors with More Applications(1) Lithium chloride humidity sensor● Resistive lithium chloride hygrometerCertain metal salts (such as lithium chloride LiCI) have strong moisture absorption properties in the air, and their moisture absorption is a certain function of the relative humidity of the air, that is, the greater the relative humidity in the air, the more the moisture absorbed by the lithium chloride. At the same time, the electrical conductivity of lithium chloride, that is, the size of the resistivity changes with the amount of moisture absorption, the more water absorbed, the smaller the resistivity, and vice versa.  Therefore, the relative humidity of the air can be determined according to the change in resistivity of lithium chloride. Lithium chloride resistance hygrometer is a meter made of the characteristics of resistivity change after lithium chloride absorbs moisture. The first lithium chloride electric humidity sensor based on the principle of resistance-humidity characteristics was developed by F.W.Dunmore of the American Bureau of Standards. This kind of element has high precision, simple structure, low price, suitable for a series of advantages such as measurement and control of normal temperature and humidity. ● Dew point lithium chloride hygrometerThe dew-point lithium chloride hygrometer was first developed by Forboro Company in the United States. This type of hygrometer is similar to the above-mentioned resistive lithium chloride hygrometer, but its working principle is completely different. In short, it uses the saturated vapor pressure of a saturated aqueous solution of lithium chloride to work with temperature. (2) Carbon humidity sensorThe carbon humidity sensor was first proposed by EKCarver and CWBreasefield in the United States in 1942. Compared with commonly used sounding elements such as hair, casing and lithium chloride, the carbon humidity sensor has a fast response speed, good repeatability, The advantages such as no erosion effect and narrow hysteresis ring are eye-catching. The uncertainty of measurement using carbon humidity sensor does not exceed ±5%RH, the time constant is 2～3s at positive temperature, the hysteresis is generally about 7%, and the specific resistance stability is also better. (3) Alumina hygrometerThe outstanding advantage of alumina sensor is that the volume can be very small (for example, the humidity sensor used in the radiosonde is only 90μm thick and 12mg weight), high sensitivity (the lower limit of measurement reaches -110℃ dew point), and the response speed is fast (generally 0.3 s to 3s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing program, and so on. In addition, it is also suitable for measuring moisture in liquids. (4) Ceramic humidity sensorCeramic humidity sensor is also called metal oxide humidity sensor, because its humidity sensing material is made of metal oxide powder through pressure molding and sintering into ceramics. Due to the degree of sintering, many porous objects can be obtained, and water vapor will be adsorbed on the porous surface to form an adsorption layer, and the H+ ions in the adsorption layer will form current carriers due to the adhesion of water vapor. When the humidity is high, the current attached to the layer of water vapor in the adsorption easily flows. The ceramic humidity sensor utilizes this property to convert the humidity change into the output of the impedance value change.Figure5. Heating purification type ceramic humidity sensor2.4 Commonly Used Humidity Sensor ModelsAt present, the main manufacturers and typical products producing integrated humidity sensors are Honeywell (HIH-3602, HIH-3605, HIH-3610), Humirel (HM1500, HM1520, HF3223, HTF3223), Sensiron (SHT11, SHT15) type). These products can be divided into the following four types: Linear voltage output integrated humidity sensorTypical products are HIH3605/3610, HM1500/1520. Its main feature is the use of constant voltage power supply, built-in amplifier circuit, can output a volt-level voltage signal proportional to the relative humidity, fast response, good repeatability, and strong anti-pollution ability. Linear frequency output integrated humidity sensorThe typical product is HF3223 type. It adopts a modular structure and is a frequency output integrated humidity sensor. The output frequency is 8750Hz (type value) at 55%RH. When the relative humidity changes from 10% to 95%, the output frequency is reduced from 9560Hz to 8030Hz . This kind of sensor has the advantages of good linearity, strong anti-interference ability, easy to be equipped with digital circuits or single-chip computers, and low price. Frequency/temperature output integrated humidity sensorThe typical product is HTF3223. In addition to the functions of HF3223, it also adds a temperature signal output terminal and uses a negative temperature coefficient (NTC) thermistor as a temperature sensor. When the ambient temperature changes, the resistance value changes accordingly and is drawn from the NTC terminal, and the temperature value can be measured with a secondary meter. Single-chip intelligent humidity/temperature sensorIn 2002, Sensiron took the lead in the world to successfully develop the SHT11 and SHT15 intelligent humidity/temperature sensors. The overall dimensions are only 7.6 (mm) × 5 (mm) × 2.5 (mm), and the size is similar to that of a match head. Before leaving the factory, each sensor has been precision-standardized in the temperature room, and the standard coefficients are compiled into corresponding programs and stored in the calibration memory.  The relative humidity can be automatically calibrated during the measurement process. They can not only accurately measure relative temperature, but also temperature and dew point. The measurement range of relative temperature is 0-100%, the resolution is up to 0.03%RH, and the highest accuracy is ±2%RH. The measuring temperature range is -40℃～+123.8℃, and the resolution is 0.01℃. The accuracy of measuring dew point is <±1℃. When measuring humidity and temperature, the digits of the A/D converter can reach 12 and 14 bits respectively.  Using the method of reducing the resolution can increase the measurement rate and reduce the power consumption of the chip. The products of SHT11/15 have good interchangeability, fast response speed, strong anti-interference ability, do not need external components, adapt to various single-chip microcomputers, and can be widely used in medical equipment and temperature/humidity adjustment systems.Figure6. HTS221 Capacitive Digital Humidity SensorIII Application of Humidity Sensors3.1 Typical UsesWork in any industry is inseparable from the air, and the humidity of the air is directly related to work, life, and production, making the monitoring and control of humidity more and more important. The main applications of humidity sensors are as follows: (1) Climate monitoringWeather measurement and forecasting are of great significance to industrial and agricultural production, military and people’s lives, and scientific experiments. Therefore, humidity sensors are essential humidity measuring equipment. For example, resin swelling humidity sensors have been used in meteorological balloon humidity measuring instruments. on. (2) Greenhouse breedingModern agriculture, forestry, and animal husbandry industries have a considerable number of greenhouses. The humidity control of the greenhouse is as important as temperature control. Controlling the humidity in a suitable range for the growth of crops, trees, livestock and poultry is one of the conditions for reducing pests and diseases and increasing yield. (3) Industrial productionIn the textile, electronics, precision machinery, ceramic industry and other sectors, air humidity directly affects the quality and output of products, and must be effectively monitored and regulated. (4) Storage of goodsVarious items have certain adaptability to the environment. If the humidity is too high or too low, the product will lose its original performance. For example, in high-humidity areas, electronic products are seriously damaged in the warehouse, non-metal parts will become moldy, and metal parts will corrode and rust. (5) Use protection of precision instrumentsMany precision instruments and equipment have higher requirements for the working environment. The environmental humidity must be controlled within a certain range to ensure their normal operation and improve work efficiency and reliability. For example, the working humidity of the telephone program-controlled switchboard is better at 55% ±10%. Too high temperature will affect insulation performance, and too low temperature will easily generate static electricity and affect normal operation.3.2 Application Fields(1) Humidity measurement system● When the temperature is below 70°C (usually above -40°C), if the environment is clean, use a polymer sensor, and use a ceramic sensor (heating cleaning regeneration type) for serious pollution. Because of its heating and cleaning process, it cannot be measured continuously and consumes a lot of energy (1-10W). However, it has a long life and can choose a sensor with a longer heating and cleaning cycle during use, such as a chloroapatite ceramic sensor, which is washed once every 2 to 3 months. In addition, the internal heating type consumes less energy than the external heating type. ● Measure the humidity in the range of 70～100℃, use ceramic sensors with heating and cleaning, and perform linearity and temperature compensation to improve accuracy. In order to achieve higher accuracy, a microcomputer is required. Frequent heating and cleaning are required at high temperature and humidity. For example, when the RH is above 80%, it needs 30S cleaning once. It is best equipped with an automatic heating cleaning device. ● Measure the humidity in the range of 100～150℃. In the world, ceramic humidity sensors are mostly used to make high temperature humidity meters. (2)Automatic control of industrial processesIn order to improve product quality and energy-saving, ceramic humidity sensors are usually used for control in product drying systems, reactor humidity control, boiler water vapor leakage detection, integrated circuits, or air conditioning in magnetic head processing plants; the humidity control of various air conditioning systems, medical systems can be carried out with polymer or ceramic humidity sensors. (3) Steam leak detection systemIn thermal power stations, nuclear power plants, steam locomotives, boilers and other high-temperature and high-pressure equipment, in order to prevent gas leakage and prevent personal accidents, humidity sensors can be used for leak detection. (4) Other systemsIn-home appliances, the humidity sensor can be used for humidity measurement of humidifiers, dehumidifiers, air conditioners, wine cabinets, clothes dryers, etc.IV Arduino Entry Project: Real-time Temperature and Humidity Detector4.1 Hardware PreparationArduino UNO oneA temperature and humidity sensorOne PCF8574T adapter board1602LCD oneA piece of breadboardSeveral connecting lines4.2 Software PreparationArduino IDE4.3 Circuit ConnectionThis project directly uses the PCF8574T adapter board to drive the 1602 LCD display, which will save a lot of Arduino IO ports and save a lot of wiring troubles. PCF8574T adapter board contains four interfaces: VCC, GND, SDA and SCL. Make these connections respectively: VCC - 5V, GND - GND, SDA - A4, SCL - A5. The temperature and humidity sensor contains 3 pins, viewed from the side with the mesh, from left to right are DATA, VCC, and GND. Make connections like this: DATA - A0, VCC - 3.3V, GND - GND.Figure7. Circuit Connection4.4 Implementation CodeFigure8. Experiment Result/** Use temperature and humidity sensor to detect information and display it on the LCD*/#include "Wire.h" // Import libraries needed to drive LCD#include "LiquidCrystal_I2C.h"#include "dht.h" // Import dht library for temperature and humidity sensor#define dht_pin A0 // Connect the data port of the temperature and humidity sensor to A0dht DHT;// Set up LCDLiquidCrystal_I2C lcd(0x27,16,2); // 0x27 is the address of the I2C busvoid setup() {// Delay waiting for system initializationdelay(1000);// Initialize LCDlcd.init();// Turn on the screen backlightlcd.backlight();// LCD screen displays Humidity(%):lcd.print("Humi(%): ");// LCD screen displays Temp(C):lcd.setCursor(0, 1);lcd.print("Temp(C): ");}void loop() {// Read the data of the temperature and humidity sensorDHT.read11(dht_pin);// LCD displays the collected temperature and humidity datalcd.setCursor(8,0);lcd.print(DHT.humidity,1);lcd.setCursor(8,1);lcd.print(DHT.temperature,1);delay(1000);} After the code is compiled without any problem, click the button to upload it to the Arduino UNO board. After the programming is no problem, you can observe the result on the LCD. If there is no change in the humidity, you can try to breathe a sigh of relief at the sensor and you can observe the change in value.Ⅴ FAQ1. What are humidity sensors?A humidity sensor (or hygrometer) senses, measures and reports both moisture and air temperature. The ratio of moisture in the air to the highest amount of moisture at a particular air temperature is called relative humidity. Relative humidity becomes an important factor when looking for comfort. 2. How do humidity sensors work?Humidity sensors work by detecting changes that alter electrical currents or temperature in the air. ... A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide's electrical capacity changes with the atmosphere's relative humidity. 3. How many types of humidity sensors are there?There are three primary types of humidity sensors employed which are defined around what approach is used to sense humidity and deliver an electrical signal that can be used to establish the value. These types of humidity sensors include Capacitive humidity sensors. Resistive humidity sensors. 4. Is a humidity sensor analog or digital?Humidity sensors measure and report moisture levels in two distinct ways - analog or digital (aka discrete). Digital sensors are able to monitor conditions for operation within a specified range. ... Analog sensors are more advanced and provide continuous visibility to current conditions through accurate measurements. 5. Why is a humidity sensor used?Humidity sensors are electronic devices that measure and report the moisture and air temperature of the surrounding environment where they are deployed e.g., in air, soil, or confined spaces. Humidity measurements indicate the concentration of water vapor present in the air. 6. What is an absolute humidity sensor?The ABS-300 is a thermal conductivity absolute humidity sensor. This sensor measures absolute humidity by quantifying the difference in thermal conductivity of dry air and air containing water vapor. ... If temperature and pressure are known the absolute humidity easily converts to relative humidity. 7. How accurate is a humidity sensor?Digital relative humidity sensors are typically accurate to plus/minus 3% relative humidity throughout the entire 0-100% RH range, but closer to plus/minus 2% at 50% RH. ... The simplest way to calibrate a relative humidity sensor is with table salt and water in an airtight container. 8. What is a humidity transducer?Humidity transducers are normally used in laboratories connected to a controller to keep a constant humidity there. ... Humidity transducers can transform a physical quantity of air humidity into a standard signal which is transferred to a controller. 9. How does the humidity sensor sense the moisture in the air?A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide's electrical capacity changes with the atmosphere's relative humidity. These types of sensors are used for weather, commercial and industrial applications. Resistive humidity sensors utilize ions in salts to measure the electrical impedance of atoms. As humidity changes, so do the resistance of the electrodes on either side of the salt medium. State-of-the-art resistive humidity sensors use ceramics to overcome areas where condensation occurs. Thermal conductivity sensors measure changes in heat to detect humidity. Two thermal sensors conduct electricity based upon the humidity of the surrounding air. One sensor is encased in dry nitrogen as a comparison to the other sensor which measures the ambient air. The difference between the two measures the humidity. 10. What is the difference between a Temperature sensor and a Humidity sensor?Temperature Sensor: Temperature is the most common environmental parameter. Temperature plays an important role in our homes and industries. Over the past few years, we are able to monitor and control environmental parameters with the help of temperature sensing devices. A temperature sensor is an electronic device that is used to detect and measure accurate temperature levels in different environmental conditions. There are many affordable temperature sensors are available in the market to measure the accurate temperature level. Humidity Sensor: Humidity is another most measurable environmental parameter. The high humidity levels in our homes and warehouses increase the chances of damaged products and things. In the past, we were not able to detect the accurate humidity level due to a lack of sensing devices. The humidity sensor is an electronic device uses to measure the humidity level and make changes in the humidity level through our mobile phone from anywhere. The humidity sensor detects the humidity level in the water, air and in soil. We can easily access humidity sensors in our homes and business. 

I IntroductionThe light sensor is developed based on the photoelectric effect principle of semiconductors. It can be used to detect the intensity of ambient light, and it can also be used to detect the difference in light between different colored surfaces. Users can make projects that interact with light with it, such as smart dimming lights, a laser communication system or something more awesome.Light Sensor Using Arduino and LDR | Arduino Light SensorCatalogI IntroductionII Definition  2.1 What is a Sensor?  2.2 Definition of the Light SensorIII Spectrum and Photometric Physical Quantity  3.1 Spectrum  3.2 Photometric Physical Quantities  3.3 MID Display's Perception of Backlight Brightness Under Different IlluminationIV How the Light Sensor WorksV Types and Characteristics of Light Sensors  5.1 Photodiode Type  5.2 Photoresistor TypeVI Applications of Light Sensors  6.1 Types of Light Sensors in Application  6.2 Typical Applications  6.3 Practical Application CasesVII The Circuit Diagram of a Light Sensor  7.1 Model Introduction  7.2 Appearance and Size  7.3 Application  7.4 Functional Framework Diagram  7.5 Application CircuitVIII Programming Guide  8.1 mBlock Programming  8.2 Arduino Programming  8.3 SchematicIX A Related Question about Light Sensor  9.1 Question  9.2 AnswerⅩ FAQII Definition2.1 What is a Sensor?In a broad sense, a sensor is a sensor that converts a measurement into a signal that can be perceived or quantified. In a narrow sense, a device that senses the measurement and converts it into an output signal of the same or another nature according to a certain law. The sensor is generally composed of a sensor element, a conversion element, a measurement circuit, and an auxiliary power source. The sensor element and the conversion element may be combined into one, and some sensors do not require an auxiliary power source.2.2 Definition of the Light SensorThe light sensor usually refers to a device that can sensitively sense the light energy of ultraviolet light to infrared light and convert the light energy into an electrical signal. The light sensor is a kind of sensing device, which is mainly composed of light-sensitive elements. It is mainly divided into four categories: ambient light sensor, infrared light sensor, sunlight sensor, and ultraviolet light sensor. It is mainly used in the field of changing body electronics applications and intelligent lighting systems. Modern electrical measurement technology is becoming more and more mature. Due to its advantages such as high accuracy and easy microcomputer connection for automatic real-time processing, it has been widely used in the measurement of electrical and non-electrical quantities.  However, the electrical measurement method is susceptible to interference. In the AC measurement, the frequency response is not wide enough and there are certain requirements on the withstand voltage and insulation. Today, the rapid development of laser technology has been able to solve the above problems.Figure1. Light SensorIII Spectrum and Photometric Physical Quantity3.1 SpectrumThe spectrum is a pattern in which monochromatic light, which is dispersed by the dispersive system (such as a prism and a grating), is sequentially arranged according to the size of the wavelength (or frequency). The largest part of the visible spectrum is the visible part of the electromagnetic spectrum of the human eye. Electromagnetic radiation in this wavelength range is called visible light. The spectrum does not include all the colors that the human brain can distinguish, such as brown and pink.Figure2. Spectrum3.2 Photometric Physical Quantities3.2.1 Light Intensity(I/Intensity)(1) Definition: the intensity of light emitted by a monochromatic light source (frequency 540 × 1012 Hz, wavelength 555nm) in a unit solid angle in a given direction (radiation intensity in this direction is 1/683 watts per spherical degree) .(2) Unit: cd (Candela)(3) Luminous intensity of common light sources:●  Sun, 2.8E27 cd●  Highlight flashlight, 10000 cd●  5mm super bright LED, 15 cd 3.2.2 Luminous Flux(F/Flux)(1) Definition: The energy emitted by a point light source or a non-point light source in a unit time. Among them, the visual person (radiation flux that humans can feel) is called luminous flux.(2) Unit: Lm (lumens)(3) Efficiency of common light sources (lumens / watt, Lm / W)● Incandescent, 15● White LED, 20● fluorescent lamp, 50● The sun, 94● Sodium lamp, 120 3.2.3 E/Illuminance(1) Definition: Luminous flux irradiated onto a unit area.(2) Unit: Lx / Lux (1), 1 (Lx) = 1 Lm / m2.(3) Common Illumination (Lx):● Direct sunlight (noon), 110,000● Overcast day, 1000● Inside the mall, 500● Cloudy room with window, 100● Under normal room lighting, 100● Full moon, 0.2 3.2.4 L / Luminance(1) Definition: The intensity of light emitted by the unit light source area in the normal direction and within the unit solid angle.(2) Unit: nt (nits), 1 (nt) = 1 cd / m2.(3) Brightness of common luminous body (nt):● Solar surface, 2,000,000,000● Incandescent filament, 10,000,000● White paper under the sun, 30,000● Brightness that human eyes can get used to, 3,000● The human eye can better distinguish the brightness of the color, 1● No moon night sky, 0.00013.3 MID Display's Perception of Backlight Brightness Under Different IlluminationFigure3. Ambient Illumination-LUXIV How the Light Sensor WorksThe light sensor actually works according to the principle of the photoelectric effect. The so-called photoelectric effect refers to the phenomenon that certain special substances can convert light energy into electrical energy after absorbing light. The photoelectric effect can be divided into two types: an external photoelectric effect and an internal photoelectric effect. The external photoelectric effect refers to the fact that under light irradiation, electrons can be emitted from the inside of the material to generate electricity. The photocell and photomultiplier are originals based on the external photoelectric effect.  Correspondingly, the internal photoelectric effect occurs inside the substance. When light is irradiated onto the substance, the resistivity inside the substance is changed, thereby generating electromotive force. Photoelectric elements such as photoresistors and photovoltaic cells are made based on the internal photoelectric effect. Take the light sensor on the mobile phone as an example:The light sensor in a mobile phone should actually be an ambient light sensor, which is mainly composed of two parts, a light projector, and a light receiver. The white dot next to the front camera acts as a lens that focuses the light in the environment and transmits it to the receiver via the projector. According to the photoelectric effect, the light receiver can convert various light signals into corresponding electrical signals, and then further process them into various switching and control actions to realize the sensitivity adjustment of the mobile phone. An infrared cut-off film is often attached to the chip of the ambient light sensor to eliminate the interference of infrared light so that our electronic devices such as mobile phones and laptops can accurately detect the visible light intensity in the environment. When the display consumes too much power, the light sensor can also automatically reduce the screen brightness to extend the operating time of the battery. Figure4. Light Sensor in PhoneV Types and Characteristics of Light Sensors5.1 Photodiode TypePhotodiodes and semiconductor diodes are similar in structure, and their die is a PN junction with photosensitive characteristics, which has unidirectional conductivity, so a reverse voltage needs to be added when working. When there is no light, there is a small saturation reverse leakage current, that is, a dark current, at which time the photodiode is turned off. When exposed to light, the saturation reverse leakage current greatly increases, forming a photocurrent, which changes with the intensity of the incident light. When light irradiates the PN junction, an electron-hole pair can be generated in the PN junction, which increases the density of minority carriers. These carriers drift under the reverse voltage, causing the reverse current to increase. So you can use the light intensity to change the current in the circuit. It is turned off when there is no light and turned on when there is light. Features:(1) High sensitivity can reduce the influence of stray light(2) Photodiode (photodiode) is a photoelectric conversion device, which can convert the received light into a current change(3) The working mode of the photodiode (photodiode) is to increase the reverse voltage or not increase the voltage. When a reverse bias is applied to it, the reverse current in the tube will change with the intensity of the light. The greater the light intensity, the greater the reverse current.Figure5. Photodiode5.2 Photoresistor Type(1) PrincipleIt works based on the semiconductor photoelectric effect. The photoresistor is non-polar and is purely a resistive element. It can be applied with DC voltage or AC voltage.(2) Working characteristics of the photoresistor: When the light is on, the resistance is small; when the light is off, the resistance is large. The stronger the light, the smaller the resistance; when the light stops, the resistance returns to its original value.(3) Spectral range: from ultraviolet to infrared.(4) Features:● The internal photoelectric effect has nothing to do with the electrode (only related to the photodiode), that is, a DC power supply can be used.● Sensitivity is related to the semiconductor material and the wavelength of the incident light● Epoxy resin package, high reliability, small size, high sensitivity, fast response speed, and good spectral characteristics.Figure6. PhotoresistorVI Applications of Light Sensors6.1 Types of Light Sensors in Application(1) Ambient light sensorThe ambient light sensor can sense the surrounding light conditions and tell the processing chip to automatically adjust the backlight brightness of the display to reduce the power consumption of the product. On the other hand, the ambient light sensor helps the display provide a soft picture. When the ambient brightness is high, the LCD monitor using the ambient light sensor will automatically adjust to high brightness. When the external environment is dark, the display will be adjusted to low brightness to achieve automatic brightness adjustment. (2) Infrared light sensorThe infrared light sensor uses a charged thermopile and a scandium bromide iodide (KRS-5) window to sense wavelengths from 580 to 40,000 nm. The sensor can be used to measure a range of phenomena, including infrared radiation from the palm of your hand. (3) Sunlight sensorSolar sensor. It can recognize horizontal and vertical 360 degrees. The location of the sun, identification, cloudy, cloudy, semi-cloudy, sunny and evening during the day. Tracking bearing identification. Identification circuit processing and server drive. A digital chip is used to complete the processing of the above information. It can serve a variety of ordinary motors, stepper motors. The power consumption of the whole machine is 3mA, and the chip working voltage is 5V.  International advanced solar tracking equipment uses computer data theory, which requires data and settings for the latitude and longitude of the earth. The circuit principle and equipment technology are complicated. Intelligent sun tracker uses recognition theory technology, simple circuit and few components, no theory of latitude, longitude and data information. There is no need to consider the route that the sun runs through the year. From which direction the sun rises and from which direction it falls, it can accurately identify the position where the sun rises and falls. If he is placed on a walking car or boat, the tracker can face the sun no matter where he goes. (4) UV light sensorThe UV light sensor uses a filter to measure the UV light band (315nm-400nm). Remove the filter, the sensor can sense visible light at the same time. The sensor includes a UV filter, a sight, and a sensor handle. Figure7. Types of Light Sensors6.2 Typical ApplicationsBacklight adjustment: TV, computer monitor, LCD backlight, mobile phone, digital camera, MP4, PDA, GPS;Energy-saving control: outdoor advertising machines, induction lighting appliances, toys; instruments and meters: instruments and industrial controls for measuring light intensity;Environmentally friendly replacement: Replace traditional photoresistors, photodiodes, phototransistors6.3 Practical Application Cases6.3.1 Changing Body Electronics Applications(1) Ambient light detectionIn body electronics applications, ambient light sensors are used to adjust the backlight intensity of the dashboard, as well as the LCD backlight intensity in navigation systems (GPS), temperature control, and DVD screens. This is especially important for displays like BMW's iDrive and Prius' Multi-Info. For example, when daylight becomes dim and dark, the dashboard backlight will be adjusted to varying degrees to achieve the best visibility and reduce the glare that may be caused to the driver. Using these sensors eliminates the problem of turning on the headlights during the day, and the display automatically adjusts brightness. The key function of the ambient light sensor is to use the sensitivity visible wavelength of 380nm ~ 780nm to replicate the sensitivity of the human eye. (2) Tunnel detectionTunnel detection requires the input of two sensors. The first sensor has a wider field of view "looking up" and a relatively long average moving period, which prevents the lights from turning on and off. The second sensor has a narrower field of view "looking forward" and a relatively short average moving time. This allows the tunnel sensor to respond quickly to sudden changes in daylight, turn on the car's headlights, and adjust the display's backlight brightness when entering the tunnel. Forward-facing sensors eliminate the need to turn lights on and off when entering under a bridge or a tree covering the sun. In these cases, the sensor will still "see" the light ahead. When entering the tunnel, the signal from the tunnel sensor will drop, while the signal from the wide-field sensor will remain high; the headlights of the car will be turned on. When exiting the tunnel, the signal from the tunnel sensor will increase and the signal from the wide field of view sensor will decrease; the headlights of the vehicle will be turned off. With different average moving periods, the controller makes a clear distinction. 6.3.2 Intelligent Lighting SystemTo improve the comfort of the working environment, the lighting control system adopts a light sensor to automatically control the lighting equipment according to the illuminance of the current environment, so that the illuminance is controlled within a comfortable range. In traditional lighting control systems, ordinary light sensors are often combined with A / D converters (ADCs). Because the light signal detected by the light sensor contains both visible light components and infrared light components, the infrared light is filtered to detect the light sensor detection results.VII The Circuit Diagram of a Light Sensor7.1 Model IntroductionThe light sensor shown below is a low-cost I2C digital light sensor (ALS), which can convert light intensity into a digital output signal that can directly interface with I2C, providing a wide dynamic range from 0.01lux to 64K lux The linear response is very suitable for applications under high ambient brightness.Figure8. Model7.2 Appearance and SizeFigure9. Appearance and Size of the model7.3 Application(1) Back-lighting Control in mobile / portable devices(2) Touch Panel Control in mobile / portable devices7.4 Functional Framework DiagramFigure10. Functional Framework Diagram7.5 Application CircuitFigure11. Application CircuitVIII Programming GuideThe programming described below is based on the Me light sensor developed based on the photoelectric effect principle in semiconductors.8.1 mBlock ProgrammingThe light sensor module supports the mBlock programming environment. The following is a brief description of the module instructions:Figure12. Programming GuideHere is an example of how to use mBlock to control a light sensor moduleWhen the LED receives the light, M-Panda will move left and right and say I love sunshine; Cover the LED light, M-Panda will stop moving and say I love night. The results are as follows:Figure13. Result8.2 Arduino ProgrammingIf you write a program using Arduino, you should call the library Makeblock-Library-master to control the Me Light Sensor. This program instructs Me Light Sensor to read the current light intensity through Arduino programming.Figure14. Arduino ProgrammingFunction list of light sensor:Figure15. Function List of Me Light Sensor8.3 SchematicFigure16. SchematicIX A Related Question about Light Sensor9.1 QuestionHow to combine these 2 circuits together so that during complete darkness on the LDR, the LED would turn on instantly and when light falls on the LDR there would be around a 1 or 2-second delay before completely shutting off?The circuit would be running on a 5V DC power supply and powering an LED array.How to combine them together? Figure17.Circuit1Figure18. Circuit29.2 AnswerIn the 555 circuit the capacitor controls the wait time, if the capacitor is short-circuited the circuit will wat forever.In the LDR circuit the transistor acts like a switch but unfortunately it's switching to ground but the capacitor in the 555 circuit is connected to +9VTo resolve this I swapped the parts in the 555 circuit upside down to have the capacitor to ground. Then it was simple to I merge the two circuits.Figure19. AnswerIn the dark R1 turns Q1 on the keesp C1 duscharged so 555 output will be high.when there is light the LDR turns Q1 off and C1 charges , once it gets enough charge the 555 output goes low.We could have instead built the upside-down version of the LDR circuit using a BC557 transitor (or other similar PNP type) instead of the BC547 NPN transistor and merged that with the original 555 circuit.Ⅹ FAQ1. How is a relay added to a light sensor circuit?Presumably, your light sensor will be generating a variable voltage signal in response to how much light is hitting it, and you want to trip a relay when this light is above (or possibly below) a threshold. One way to do this is with a comparator circuit, which will compare two voltages and output a high or low depending on which one is higher. You then compare the signal from the light sensor to a reference voltage that you can set with a potentiometer and generate a high or low output signal from that. You can also use a microcontroller and read the signal from the light sensor with an analog input pin. This is more complex but useful if you want to implement features like hysteresis in the comparison. Now, the logic level signal can’t drive a relay coil directly, so you will need to use a transistor to switch the relay coil current. Which transistor to use will depend on the voltages involved and the amount of current you need to switch, but it’ll be a small signal transistor of some kind. You also need a current limiting resistor on the gate, possibly a pull-down on the gate as well, and a flyback diode across the relay coil. 2. What is a light sensor?Light sensors respond to changes in infrared light to detect motion or proximity to another object. Proximity sensors help robotic machines navigate obstacles and avoid bumping into objects. They are also used for devices in vehicles that sound an alarm when the vehicle is close to bumping into an object. 3. What are the disadvantages of a light sensor?Following are the disadvantages of Light sensor :• LDRs are highly inaccurate with high response time (about 10s or 100s of milliseconds).• Resistance varies continuously (analog) in photoresistors and is rugged in nature.• Photodiodes are temperature sensitive and are uni-directional, unlike photoresistors. 4. What does a light sensor do?Light sensors are electronic devices that indicate the intensity of daylight or artificial light. They convert light energy to electrical signal output. Light sensors have several uses in industrial and everyday consumer applications. 5. Where are light sensors used?Light sensors have a lot of uses. The most common use in our daily lives is in cell phones and tablets. Most portable personal electronics now have ambient light sensors used to adjust brightness. 6. How many types of light sensors are there?By using LDR as a circuit, we can calibrate the changes in its resistance to measure the intensity of Light. There are two other Light Sensors (or Photo Sensors) that are often used in complex electronic system design. They are Photo Diode and Photo Transistor. All these are Analog Sensors. 7. How long does a light sensor last?Long Duration Settings – In most cases, your motion detector light should only stay on for 20 to 30 seconds after it's triggered. However, you can manipulate the settings so it will stay on longer. For example, many lights come with settings ranging from a few seconds to an hour or more. 8. Is a light sensor analog or digital?Analog sensors that are used for detecting the amount of light striking the sensors are called light sensors. These analog light sensors are again classified into various types such as photo-resistor, Cadmium Sulfide (CdS), and, photocell. 9. What is a light sensor in a phone?Ambient-light sensors (ALS) are widely used in smartphones to provide information about ambient-light levels, in support of the backlight LED power circuit. 10. How do you wire a light sensor to an outside light?Connect one black wire on the photocell to the black wire that comes from the building. Be sure to twist the exposed copper wire so that it forms a tight connection. Connect the second black wire on the photocell to the black wire on your light fixture, making sure that the copper wire is twisted together completely.  

SummaryRihito Kuroda, a researcher at Tohoku University in Japan who develops UV imagers in his lab said“There's important information hidden in the UV,”. However, this information has been difficult to capture; silicon doesn't absorb ultraviolet wavelengths very well, and other semiconductors that play well with ultraviolet light make slow imagers with low frame rates. But that’s about to change. This week at the International Electron Devices Meeting in San Francisco, two research groups presented ultrathin, flexible UV sensor designs they hope will help make these devices more widespread.  UV images reveal spots that presage rot on mushrooms, dark lines along flower petals that guide insects to nectar, and clouds of acetone in water. And with their relatively short wavelengths, UV sensors could be well suited to more precise navigation for flying swarms of tiny drones. The research aimed at making UV sensors from specially formulated paperA group from King Abdullah University of Science and Technology in Saudi Arabia presented their research aimed at making UV sensors from specially formulated paper. Electrical engineering student Chun-Ho Lin explained that it’s difficult to make flexible UV sensors because they heat up under the high-energy rays. Typical flexible substrates, like plastic and paper, can’t wick away that heat quickly enough. He and other electrical engineers in Jr-Hau He’s lab made thermally conductive, UV-sensitive paper by combining boron nitride nanosheets with cellulose fibers. Flexible sensors made from this formulation can take the heat, withstanding temperatures up to 200 degrees Celsius. What’s more, they are blind to wavelengths above the deep UV band.Other researchers are sticking with silicon, but using graphene to help it along. Yang Xu, an electrical engineer at Zhejiang University in China, says there are good reasons to work with silicon, even though in its native state it is a strong reflector of UV rays. Silicon photodetectors can work quickly, enabling higher frame rates, and they can draw on a vast manufacturing infrastructure. Xu says his philosophy is, “Why not help silicon do better?” With that in mind, his team is pairing the semiconductor with graphene, which absorbs UV light like a champ. To make flexible silicon-graphene UV photodetectors, the Zhejiang University group uses etching and rubber stamps to transfer ultrathin silicon microstructures to a flexible plastic substrate, then coats the silicon with graphene and adds electrodes. This photodetector is blind to visible light, because the silicon layer is just 20 nanometers thick and cannot absorb it. This ultrathin device is flexible and performs as well as state-of-the-art UV photo detectors, says Xu. His lab is currently working on shrinking the size of the photodetectors to improve their resolution. 

Summary Energy-efficient sensor nodes are crucial to the development of the industrial internet of things (IIoT).Engineering team are trying to optimise energy efficient IIoT sensor nodes.In many cases, these devices will have to perform for years on a single battery charge. That calls for an implementation that is as energy efficient as possible. Achieving this demands a holistic approach to energy optimisation, one that reaches from the system level down to process and circuit-design choices.     Problem met Engineering team are trying to optimise the energy comsumption of an IIOT sensor node is that many of the design decisions interact with each other. And there are often hidden complexities of designs that lead to energy consumption being much higher than expected. For example, conventional wisdom points to the power consumption of an RF transmitter being a major influence on total energy. But, even though the receiver element may consume far less instantaneous power, system-level decisions that call for the device to listen for intermittent updates from a server can lead to it being left active for long periods of time – tens of seconds per hour versus tens of milliseconds for the transmitter. Because of the long operational life of a typical IoT sensor node, the energy used even when subsystems are sleeping can be responsible for a heavy drain on the battery.   Integration Despite the complex interaction between application design and implementation, there are some high-level choices that are likely to lead toward an optimal solution. One of these is the use of integration. Although it is entirely possible to use 2D-IC and 3D-IC multi chip packaging to assemble a compact IIoT sensor node from off-the-shelf components, integration into a single custom integrated circuit (IC) provides not just significant benefits in terms of cost and size but reductions in power consumption. In order to communicate with off-chip memories and analogue and RF on traditional PCB-based implementations, Input/Output (I/O) drivers with significant current draw are often required. A single system-on-chip (SoC) makes it possible to remove such power-hungry circuits.     The duty cycle and lifetime energy consumption The other fundamental consideration for designing energy-efficient IIoT sensor nodes is an understanding of the duty cycle and its impact on lifetime energy consumption. Simply minimising the power consumption of individual elements is not enough to guarantee that a remote or  inaccessible sensor can operate on a single battery charge for a decade or more. In such a situation, every microjoule the node requires from its battery is important. But that does not mean the system powered by a typical battery can consume no more than a few microwatts at any point in its life. Such a system would not be able to take measurements and communicate them wirelessly in any practical way. The use of duty-cycle planning makes it possible for the system to perform tasks that take significant amounts of power for short periods, trading those bursts against savings that can be made while much of the system is quiescent. For example, the RF subsystem of a wireless sensor node need only be powered when it is active. This is likely to be one of the most power-hungry parts of the overall design because of the need to supply enough transmitter power to ensure packets of data can be delivered reliably. However, the power consumed by the transmitter portion of the RF subsystem is relatively easy to control. Once a packet has been delivered the transmitter can be shut down. But there can still be significant power drawn by subsystems such as the RF receiver that continue to remain active once the transmitter has finished sending.   The RF receiver often needs to remain active because of timing uncertainty and this type of uncertainty has a major influence on overall energy consumption. Whereas the transmitter has predictable requirements – it need only be activated when data is ready to send – the receiver needs to be active for much longer. It needs to wait for acknowledgments from nodes to which it is sending data, and also needs to activate periodically to be able to listen for unsolicited messages. As a result, the overall energy consumption of the RF receiver will often exceed that of the transmitter over the lifetime of the sensor, even though its instantaneous power demand is lower. An efficient design will exploit power-saving techniques such as putting much of the circuitry into a low-activity state until an RF signal is detected. Another optimisation is to reduce the amount of time per minute the receiver is active at the cost of the sensor node’s responsiveness to external commands.     Although they might appear to be essential to all operations, the microprocessor core and its memory subsystem need careful duty-cycle management because they can demand very high levels of power. The problem for many designs is that software running on the processor is often responsible for core tasks such as fetching data from sensors and passing messages to the RF subsystem. This appears to mandate that the processor be fully active whenever sensor inputs need processing.   However, in many cases, the work performed by the software is very simple. It is quickly checking data values to see if they have passed a limit that might signal a problem, or for increased activity that needs closer inspection. Activating the processor to handle all the data is wasteful and can easily be offloaded to custom hardware or a programmable state machine. These circuits consume far less power and can run independently of the processor, so that and the memory array can be powered down.   Current leakage Even when most of the device is powered down, the power drawn during lengthy periods of sleep can be surprising. Energy lost through current leakage in subsystems that need to remain powered can incur a heavy overhead when analysed over the lifetime of the system because the time the system spends sleeping can be orders of magnitude longer than that during which the system is active. The problem of leakage calls for design techniques that limit leakage in subsystems such as real-time clocks and interrupt controllers to the nanoamp level. It might seem reasonable to disable interrupts for external events and only keep the real-time clock running in some applications. However, in that design the system needs to wake at regular intervals to check inputs that may incur unwanted energy consumption if there is no overall change to record. If the long-term energy usage of an interrupt controller is low enough, keeping that active to respond to events as they happen may make more sense.   When the processor and memory subsystem are powered down, a key decision is how to manage temporary data. One option is to use specialised retention register and memory cells, at the cost of some leakage power. Another is to put important data, such as calibration values, into non-volatile memory (NVM). This allows values to be restored quickly on restart but allows the leakage-prone SRAM arrays and registers to be powered down fully until then. But NVM choices are not always straightforward.   Processes that are optimised for low leakage and that support high-density NVM options may not have the performance required to support efficient RF modules on-chip. The energy needed for I/O drivers that transfer data to an off-chip RF transceiver may outweigh the power savings and security advantages obtained from implementing NVM on-chip. Careful analysis of the application’s requirements will indicate which choice is better for the custom SoC solution.   For the portions of the design that will be active for much of the device’s lifetime, careful attention to detail is required. Seemingly small details such as choosing to multiplex inputs into an analogue-to-digital converter (ADC) will help determine the architecture of choice for those circuits. A sigma-delta ADC may initially appear to offer a good trade-off between accuracy, energy efficiency and silicon area. But it is not suited to multiplexing. Often a successive approximation (SAR) architecture offers superior performance for industrial sensor signals. Advances in SAR design have pushed the energy per bit per conversion down into the range of tens of femtojoules.   Front-end analogue circuits are just as important as the ADC. Amplifiers and buffers that isolate and condition signals before conversion can consume high levels of power and they will be active for long periods of time. Analysis of the specific requirements for bandwidth and accuracy often allow for optimizations that reduce the energy of front-end circuitry and ADCs.   To tie all the subsystems together into a working custom SoC demands the use of power-aware design methodologies to ensure subsystems and circuits are activated properly when required, and can be powered down without disrupting the operation of other parts of the custom IC that need to stay running. Standards such as the Unified Power Format (UPF) have been designed to support such power-aware methodologies, but their application requires experience and attention to detail at different levels of abstraction.   Take an example For example, there may be a logical connection between two subsystems that demands they be active at the same time. But physical restrictions may call for them to form part of a larger power island – an area of the mixed-signal ASIC with a common set of power and ground rails – that includes other subsystems that are not required during that time. Design verification needs to ensure that the entire island is powered up correctly. If not, the final SoC will fail. Such physical design considerations may call for changes to the power-control architecture if the consumption of the whole island is higher than the budget allows. It may call for subsystems to be assigned to different power islands, for example.   Verification also needs to pay attention to on-chip noise, which may point to further optimization of the power-island strategy. For example, a low-noise LDO may be used to power sensitive mixed-signal sections that operate autonomously. Once measurements have been taken or RF communications have been completed, a higher-efficiency DC/DC converter may then be reactivated to analyse incoming data and make decisions.   Although the core requirements of energy efficiency in IIoT sensor nodes are readily understood, as can be seen, the implementation choices are complex and often subtle. Many factors affect the optimum solution for a given IIoT sensor node application, although a custom SoC will frequently be the best target in terms of energy and overall cost. Therefore, the ability to call on the expertise of design teams with extensive experience in custom mixed-signal IC implementation is key to success.  

SummaryResearchers at Caltech have developed a prototype miniature medical device that could ultimately be used in “smart pills” to diagnose and treat diseases. This is critical for the function of biosensors and smart pills. A key to the new technology—and what makes it unique among other microscale medical devices—is that its location can be precisely identified within the body, something that proved challenging before. The picture is about an ATOMS microchip localized within the gastrointestinal tract. bodyCalled ATOMS, which is short for addressable transmitters operated as magnetic spins, the new silicon-chip devices borrow from the principles of magnetic resonance imaging (MRI), in which the location of atoms in a patient's body is determined using magnetic fields. The microdevices would also be located in the body using magnetic fields—but rather than relying on the body's atoms, the chips contain a set of integrated sensors, resonators, and wireless transmission technology that would allow them to mimic the magnetic resonance properties of atoms. The ATOMS device seen next to a penny. The device has a surface area of 1.4 square millimeters, 250 times smaller than a penny. A key principle of MRI is that a magnetic field gradient causes atoms at two different locations to resonate at two different frequencies, making it easy to tell where they are. The researchers wanted to embody this elegant principle in a compact integrated circuit. ATOMS devices also resonate at different frequencies depending on where they are in a magnetic field. The scientists wanted to make this chip very small with low power consumption, and that comes with a lot of engineering challenges. They had to carefully balance the size of the device with how much power it consumes and how well its location can be pinpointed.The devices are still preliminary but could one day serve as miniature robotic wardens of our bodies, monitoring a patient's gastrointestinal tract, blood, or brain. They could measure factors that indicate the health of a patient—such as pH, temperature, pressure, sugar concentrations—and relay that information to doctors. Or, the devices could even be instructed to release drugs. Microscale and Biosensors You could have dozens of microscale devices and biosensors -traveling around the body taking measurements or intervening in disease. These devices can all be identical, but the ATOMS devices would allow you to know where they all are and talk to all of them at once. The researchers compare it to the 1966 sci-fi movie Fantastic Voyage, in which a submarine and its crew are shrunk to microscopic size and injected into the bloodstream of a patient to heal him from the inside—but, instead of sending a single submarine, you could send a flotilla. The researchers say the devices are still preliminary but could one day serve as miniature robotic wardens of our bodies, monitoring a patient's gastrointestinal tract, blood, or brain. The devices could measure factors that indicate the health of a patient—such as pH, temperature, pressure, sugar concentrations—and relay that information to doctors. Or, the devices could even be instructed to release drugs. This chip is totally unique: there are no other chips that operate on these principles. Integrating all of the components together in a very small device while keeping the power low was a big task. The final prototype chip, which was tested and proven to work in mice, has a surface area of 1.4 square millimeters, 250 times smaller than a penny. It contains a magnetic field sensor, integrated antennas, a wireless powering device, and a circuit that adjusts its radio frequency signal based on the magnetic field strength to wirelessly relay the chip’s location. In conventional MRI, all of these features are intrinsically found in atoms. Ther researchers still had to create an architecture that functionally mimics them for our chip. Article from CaltechArticle edit by kynix