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Imagine a system crashing unexpectedly during a critical operation. Such failures can lead to downtime, data loss, or even safety risks. Watchdog timers act as your system's safety net. They monitor operations and ensure processes run smoothly. If a problem occurs, they reset the system to restore functionality. This mechanism significantly enhances reliability by preventing prolonged failures. Whether you’re managing industrial machines or consumer devices, these timers help maintain consistent performance and protect against costly disruptions.Understanding Watchdog TimersWhat Is a Watchdog Timer?A watchdog timer is a specialized hardware or software component designed to monitor the operation of a system. Its primary role is to detect and respond to faults that could disrupt the system's stability. Think of it as a vigilant guardian that ensures your system stays on track. If the system stops functioning as expected, the watchdog timer steps in to reset it, restoring normal operations.To understand its importance, consider how systems can freeze or crash due to software bugs, hardware malfunctions, or unexpected conditions. Without a watchdog timer, these issues could lead to prolonged downtime or even permanent damage. By resetting the system when it detects a problem, the watchdog timer prevents these scenarios and maintains stability.Key technical specifications of watchdog timers enhance their reliability. Here's a quick overview:SpecificationDescriptionTimeout PeriodThe duration before the timer triggers a reset, balancing between avoiding unnecessary resets and preventing system hangs.Reset MechanismOptions for soft or hard resets, depending on application needs, affecting recovery strategies.Error-Handling MechanismsImplementation of robust error logging and corrective actions to enhance system reliability.Power ManagementAbility to control system states for low-power operation, crucial for battery-operated devices.Application CriticalityImportance in mission-critical systems like medical devices and automotive systems for safety.These features make watchdog timers indispensable for systems where stability and reliability are non-negotiable.How Do Watchdog Timers Work?Watchdog timers operate by continuously monitoring the system's performance. They rely on a simple yet effective mechanism: the system must regularly "kick" or reset the timer to indicate it is functioning correctly. If the timer isn't reset within a specified period, it assumes the system has encountered a fault and takes corrective action.Here’s how watchdog timers detect and rectify faults:They activate fail-safe circuitry to force control outputs to safe states, preventing injuries or equipment damage during faults.In a two-stage watchdog, the first timer activates fail-safe outputs and starts a second timer that resets the system if the fault persists.They can trigger the recording of system state or debug information for fault recovery, allowing time for data recording before a reset.The system conducts fault detection tests to determine if it is functional before kicking the watchdog timer.Upon detecting a fault, the system may attempt corrective actions, including a software-initiated reboot, with a hardware watchdog as a backup.If the watchdog timer elapses due to a fault, it generates a timeout signal to initiate corrective actions, such as rebooting the computer.This process ensures that even if a fault occurs, the system can recover quickly and maintain stability. By implementing these mechanisms, watchdog timers provide a robust solution for fault detection and recovery, making them essential for reliable system performance.Watchdog Timers and System ReliabilityWhy Are Watchdog Timers Crucial for System Reliability?You rely on systems to perform consistently, whether in industrial automation, medical devices, or everyday electronics. However, software errors and unexpected conditions can cause systems to freeze or malfunction. This is where watchdog timers become indispensable. They act as a safeguard, ensuring your system remains operational even when faults occur.Watchdog timers play a vital role in maintaining system reliability. They detect and correct software errors that could lead to system lockups or malfunctions. This ensures the system continues to function as intended. Their importance becomes even more evident in safety-critical applications, such as industrial automation or medical devices, where a failure could have severe consequences.Watchdog timers are essential for ensuring system reliability and robustness in embedded systems.They maintain system integrity by detecting and correcting software errors.Their role is critical in applications where reliability and safety are paramount."Pathfinder, a mission whose software also famously hung, was saved by a reliable watchdog. The software team found and fixed the bug, uploading new code to a target system 40 million miles away, enabling an amazing roving scientific mission on Mars."By incorporating watchdog timers, you enhance fault tolerance. They provide a mechanism for automatic recovery from software failures, reducing the risk of prolonged downtime. This makes them a cornerstone of reliable system design.Preventing Failures and DowntimeFailures and downtime can disrupt operations, leading to costly consequences. Watchdog timers help you prevent these issues by continuously monitoring your system's performance. If the system encounters a fault, the watchdog timer steps in to reset it, restoring normal functionality.Here’s how watchdog timers reduce unexpected failures and downtime:Enhanced System Reliability: They prevent system lockups and malfunctions caused by software errors.Improved Fault Tolerance: They enable automatic recovery from software failures, ensuring minimal disruption.Increased Safety: They mitigate risks associated with system failures in safety-critical applications.Imagine a manufacturing line suddenly halting due to a software glitch. Without a watchdog timer, the downtime could last until someone manually intervenes. With a watchdog timer, the system resets itself, minimizing delays and keeping operations running smoothly. This proactive approach not only saves time but also reduces the risk of data loss or equipment damage.In safety-critical environments, such as medical devices or aerospace systems, watchdog timers provide an additional layer of protection. They ensure that even if a fault occurs, the system can recover quickly and continue to operate safely. This level of fault tolerance is crucial for maintaining trust in these high-stakes applications.By implementing watchdog timers, you can significantly reduce the impact of failures and downtime. They ensure your systems remain reliable, even in the face of unexpected challenges.Benefits of Watchdog TimersEnhanced Safety in Critical ApplicationsIn safety-critical systems, even a minor failure can lead to severe consequences. Watchdog timers act as a safeguard, ensuring your system operates reliably. They monitor the system's performance and reset it when faults occur. This proactive approach prevents malfunctions from escalating into dangerous situations. For example, in medical devices, a watchdog timer ensures continuous operation, protecting patients from harm. Similarly, in industrial automation, it prevents equipment failures that could endanger workers.You can rely on watchdog timers to enhance safety in environments where reliability is non-negotiable. Their ability to detect and correct faults makes them indispensable for maintaining system reliability in critical applications.Boosting System Uptime and PerformanceWatchdog timers play a vital role in keeping your systems running smoothly. By resetting the system during faults, they minimize downtime and ensure consistent performance. Imagine a server handling thousands of requests per second. A software glitch could bring it to a halt. With a watchdog timer in place, the system resets itself, restoring functionality without manual intervention.This automatic recovery process not only saves time but also boosts overall system uptime. You can trust watchdog timers to maintain high performance, even under challenging conditions. Their ability to prevent prolonged failures ensures your operations remain uninterrupted.Cost-Effectiveness in Long-Term UseInvesting in watchdog timers can save you money in the long run. By preventing system failures, they reduce the need for costly repairs and minimize downtime. This is especially important in industries where every minute of downtime translates to significant financial losses. Watchdog timers also extend the lifespan of your equipment by ensuring it operates within safe parameters.You benefit from their cost-effectiveness by avoiding frequent maintenance and replacements. Over time, the reliability they provide outweighs the initial investment, making them a smart choice for any system.Applications of Watchdog TimersImage Source: pexelsEmbedded Systems and Consumer ElectronicsWatchdog timers play a vital role in embedded systems and consumer electronics. These systems often combine complex hardware and software, making them prone to unexpected failures. Watchdog timers detect abnormalities and restore normal operations promptly. For example:They reset systems when software or hardware failures occur, preventing unresponsive states.They ensure devices restart automatically during malfunctions, maintaining efficient operation.They enhance embedded system reliability by minimizing downtime through automatic recovery.In consumer electronics, watchdog timers protect devices like smartphones and smart appliances from freezing or crashing. Their ability to monitor and reset systems ensures smooth performance without requiring manual intervention. This makes them indispensable for maintaining reliability in everyday devices.Industrial Automation and Control SystemsIndustrial environments demand systems that operate reliably under challenging conditions. Watchdog timers ensure uninterrupted performance by monitoring and resetting systems during faults. In automation, they prevent production delays caused by software glitches or hardware malfunctions.You can rely on watchdog timers to safeguard critical processes. They detect errors early and initiate corrective actions, reducing downtime. For instance, in a factory setting, a watchdog timer can reset a malfunctioning robotic arm, allowing operations to continue seamlessly.Their role extends to control systems, where they maintain stability in processes like temperature regulation or assembly line coordination. By implementing watchdog timers, you ensure consistent performance and protect against costly disruptions.Automotive and Aerospace SystemsIn automotive and aerospace systems, watchdog timers are essential for maintaining reliability and stability. These applications require tools that can handle high-security demands and space constraints.FeatureDescriptionFunctionalityStandalone devices for high-security applicationsPackage TypesCompact SOT23 and SC70 for space-conscious designsApplicationUsed in automotive systems to enhance reliabilityThe LabVIEW Real-Time Module includes a watchdog timer designed for real-time systems. It ensures consistent performance in critical applications like flight control systems or vehicle safety mechanisms.To maximize reliability, you should regularly test the watchdog timer and choose a timeout period that suits the application. This approach ensures systems remain operational even in multitasking environments. Watchdog timers provide the fault tolerance needed to keep automotive and aerospace systems running smoothly.Medical Devices and Healthcare EquipmentMedical devices and healthcare equipment demand the highest levels of reliability. A single malfunction could jeopardize patient safety or disrupt critical treatments. Watchdog timers serve as a vital component in these systems, ensuring continuous operation and preventing failures.You can find watchdog timers in devices like ventilators, infusion pumps, and patient monitoring systems. These devices often operate in life-critical environments where downtime is unacceptable. A watchdog timer monitors the system's performance and resets it if it detects a fault. This ensures the device remains functional, even under unexpected conditions.Here’s why watchdog timers are indispensable in healthcare equipment:Patient Safety: They prevent malfunctions that could harm patients by resetting the system before a failure escalates.Continuous Operation: They ensure devices like ventilators and monitors run without interruptions, even during software glitches.Data Integrity: They safeguard critical patient data by enabling systems to recover quickly from faults.Tip: When selecting a watchdog timer for medical devices, prioritize features like low power consumption and high reliability. These characteristics are crucial for battery-operated equipment and mission-critical applications.For example, consider a heart monitor in an intensive care unit. If the software freezes, the watchdog timer detects the issue and resets the system. This quick recovery ensures the device continues to provide accurate readings, protecting the patient from potential risks.By incorporating watchdog timers, you enhance the reliability of medical devices. They act as a safety net, ensuring these systems perform as intended, even in challenging scenarios. This makes them an essential component in modern healthcare technology.Implementing Watchdog Timers EffectivelySelecting the Right Watchdog TimerChoosing the right watchdog timer is essential for ensuring system reliability and performance. You need to evaluate your system's requirements and match them with the features of available watchdog timers. For instance, some timers, like the MAX6369-74 series, offer selectable timeout periods ranging from 1.7 milliseconds to 104 seconds. These timers also provide low power consumption, making them ideal for battery-operated devices.FeatureDescriptionTimeout PeriodsAdjustable from 1.7 milliseconds to 104 seconds in seven steps.Output Pulse WidthsOptions for 1.7 or 170 milliseconds.Current ConsumptionTypical consumption of 8 μA, with a maximum of 20 μA.Package TypeCompact SOT23-8 packages for space-constrained designs.When selecting a watchdog timer, consider its ability to handle safety-critical design requirements. For example, in medical or aerospace systems, the timer must provide enhanced safety and reliability. However, you should also account for potential drawbacks, such as increased system complexity or false triggers.To make an informed decision, analyze your system's performance under different conditions. Identify the maximum acceptable time for detecting and recovering from failures. This ensures the timer aligns with your system's fault tolerance needs.Configuring and Testing for ReliabilityProper configuration and testing are crucial for ensuring your watchdog timer operates effectively. Start by setting the timeout interval carefully. A short interval may cause unnecessary resets, while a long one could delay fault detection. Place the reset command strategically in your program, such as at the end of a main loop or after critical tasks.Testing the watchdog timer under various conditions is equally important. Follow these steps to validate its configuration:Define the maximum time your system can tolerate for fault detection and recovery.Select a watchdog timer type that matches your system's requirements.Configure the timer interval to balance sensitivity and false positives.Test the timer in real-world scenarios to ensure it functions without conflicts.Document the configuration for future reference.You should also integrate fail-safe mechanisms to enhance reliability. For example, use a window watchdog timer or variable time intervals to improve fault detection. Rigorous testing ensures the reset circuit operates reliably under all conditions, making your system more robust.Tip: Each periodic task in your system should contribute to resetting the watchdog timer. This ensures every task is functioning as expected, preventing unnecessary resets.Watchdog timers are essential for ensuring system reliability. They act as a safeguard, detecting faults and resetting systems to maintain smooth operations. By preventing failures and enhancing reliability, they keep your systems running efficiently. Whether you manage industrial machines or medical devices, adopting watchdog timers strengthens your system design. Their ability to minimize downtime and protect critical processes makes them a valuable addition to any application. Start integrating them into your systems today to achieve consistent performance and long-term dependability.FAQWhat happens if a watchdog timer is not reset in time?If you fail to reset the watchdog timer within the specified interval, it assumes the system has encountered a fault. It then triggers a corrective action, such as a system reset, to restore normal functionality. This prevents prolonged system failures.Can a watchdog timer cause unnecessary resets?Yes, improper configuration can lead to false triggers. Setting the timeout period too short may cause unnecessary resets. To avoid this, you should carefully configure the timer based on your system's operational requirements and test it under real-world conditions.Are watchdog timers suitable for all systems?Watchdog timers work well for most systems, especially those requiring high reliability. However, they may not be necessary for simple systems with minimal risk of failure. Evaluate your system's complexity and criticality before deciding to implement one.How do you choose the right watchdog timer?You should consider factors like timeout period, power consumption, and application requirements. For example, battery-operated devices benefit from low-power timers. Analyze your system's needs and select a timer that aligns with its fault tolerance and performance goals.Do watchdog timers require regular maintenance?No, watchdog timers typically operate autonomously once configured. However, you should periodically test them during system updates or maintenance to ensure they function correctly. This helps maintain reliability and prevents unexpected issues.

IntroductionFor people who have been in touch with digital circuits or analog circuits, the 555 IC is definitely classic work. With its low cost and reliable performance, it is widely used in various electrical appliances, including instruments and meters, household appliances, electric toys, and automatic control. The 555 timer only needs a few external resistors and capacitors to realize pulse generation and conversion circuits, such as multiple oscillators, monostable triggers and schmitt triggers. So how does it work in the circuit? What the role of its circuit? Here gives several typical 555 circuit examples for specific analysis.555 Timers Circuit LearningCatalogIntroductionⅠ Basic 555 Timer Circuit AnalysisⅡ 555 Multivibrator Circuit AnalysisⅢ 555 Timer Monostable Flip Flop Circuit AnalysisⅣ Classic 555 Timer Circuits DiagramsⅤ 555 Timer IC ModesⅠ Basic 555 Timer Circuit Analysis555 Means What?555 timer is a convenient and powerful IC, which is widely used in signal generation, conversion, control and detection. The origin of this name, because it is divided by three 5KΩ resistors. The 555 timer is a simple integrated circuit that can be used to make many different electronic circuits. With the following circuits analysis you will know how 555 IC works.Figure 1. Basic 555 Timer Circuit✔️ Circuit AnalysisR is not the reset terminal, when set to 0, Q is 0,  is 1, Uo outputs 0, and is 1 added to the base of the transistor T, the transistor is in the conducting state.① When R=0, Q=1, uo=0, T is saturated and turned on.② When R=1 (there is no reset function at this time):UTH>2VCC/3, UTR>VCC/3, C1=0, C2=1, Q=1 or =0, uo=0, T is saturated and turned on. (Analysis: C1's positive input terminal is 2VCC/3, C1's negative input UTH terminal is greater than the positive input terminal, working in saturation, and output 0. C2's negative input terminal is 1VCC/3, which is smaller than the positive input Terminal UTH, and outputs 1. There is a horizontal line above RD and SD, which means low level, meaning is Reset. C1 outputs 0, RD is valid, then Q is 0, not 1, Uo outputs 0, and is not acting on the base of the triode.)③ When R=1, UTH<2VCC/3, UTR>VCC/3, C1=1, C2=1, Q and remain unchanged, uo and T remain unchanged. (Analysis is the same as above)④ When R=1, UTH＜2VCC/3, UTR＜VCC/3, C1=1, C2=0, Q=0, =1, uo=1, T is cut off. (Analysis is the same as above) Learn how the inputs interact with the supply voltage to trigger and reset the output high and low. Find out which pins can be used to adjust the threshold at which that change happens.Ⅱ 555 Multivibrator Circuit AnalysisFigure 2. 555 Multivibrator Circuit Analysis Figure 3. 555 Multivibrator Circuit Example✔️ Circuit Analysis First, the power supply VCC charges the capacitor C through R1 and R2, and the voltage of the capacitor must be relatively small, less than 1VCC/3. Similarly, the positive terminal of C1 is 2VCC/3, the negative terminal of C2 is 1VCC/3, and the TH and TR terminals are connected At the same time, it is less than 1VCC/3 at the beginning. At this time, C1 outputs 1, C2 outputs 0, and the set terminal is valid (with detailed confirmation): Q is 1, is not 0, and uo is 1, the transistor is cut off, and outputs high level. At this time, the power supply is still charging the capacitor. When the TH and TR terminals are connected together, the voltage is less than 2VCC/3 and greater than 1VCC/3; C1 outputs 1, C2 outputs 1, the transistor is cut off, and uo is 1. When the capacitor is greater than 2VCC/3, C1 outputs 0 and C2 outputs 1. At this time, Q is 0, is not 1, uo is 0, the output is low, and the transistor is turned on. The capacitor will be discharged through pin 7. After this, the voltage at the point where TH and TR connected will gradually decrease, less than 2VCC/3 and greater than 1VCC/3, and then it will be less than 1VCC/3, to form a harmonic oscillator.The pulse width tp1 of the first transient state, that is, the time required for uc to rise from VCC/3 charging to 2VCC/3 (charged through two resistors):The second transient state pulse width tp2, that is, the time required for uc to discharge from 2VCC/3 to VCC/3:Duty cycle: the time that the high level occupies the entire cycle., it can be seen that its duty cycle is always greater than 50%.Examples 1Circuit with Adjustable Duty Cycle (add an adjustable resistor)Figure 4. Circuit with Adjustable Duty Cycle (add an adjustable resistor)It can be calculated:Where T1=0.7R1C (T1 is charging time), T2=0.7R2C (T2 is discharging time)Total time T=T1+T2=0.7(R1+R2)CSo R1, R2, and C are determined, and the period T is also determined.Duty Cycle Calculation Example 2Circuit with Adjustable Duty Cycle (1KHz)Figure 5. Circuit with Adjustable Duty Cycle (1KHz)✔️ Circuit AnalysisT = 0.7(R1+R2)C, f = 1/T, the duty cycle circuit only needs to adjust the resistance value. Ⅲ 555 Timer Monostable Flip Flop Circuit AnalysisWorking Characteristics① It has two different working states: steady state and transient state.② Under the action of an external trigger pulse, it can switch from the steady state to the transient state. After the transient state is maintained for a period of time, the circuit can automatically return to the steady state.③ The transient state cannot be maintained for a long time, and the duration of its sustaining time depends on the parameters of the circuit itself and has nothing to do with the trigger pulse. So what is the principle of a monostable circuit?Figure 6. 555 Timer Monostable Circuit Analysis Figure 7. 555 Timer Monostable Circuit Example✔️ Circuit AnalysisFirst, the TR terminal is at a high level ui, which must be greater than 1VCC/3. At this time, C2 outputs 1, and the power supply charges capacitor C through R. The charging voltage is less than 1VCC/3 (TH), CO voltage is equal to 2VCC/3, C1 outputs 1, and it is in the holding state at this time. Assuming that the non-reset terminal of R is reset before power on, the output of uo is 0, and then the previous state is still maintained and the output is 0 at this time. is 1, the transistor is turned on, the capacitor is discharged through pin 7, and uc is zero level. At a certain moment, ui is low, C1 still outputs 1, C2 outputs 0, Q is 1, is 0, uo outputs 1 (high level), and the transistor has been in the cut-off state. At this time, VCC can charge the capacitor (uc is getting larger). When uc is between 1VCC/3~2VCC/3, assuming that the TR terminal returns to the original state (high level), C1 outputs 1 , C2 outputs 1, at this time uo keeps in original state, it is still 1, and the transistor is in the cut-off state. When uc is greater than 2VCC/3, C2 is still 1, C1 output is 0, Q is 0, is 1, and uo is 0, the transistor is turned on and in a discharging state, at this time, uc is getting smaller and smaller.Summery:1. As long as a low-level trigger signal is given, the temporary stable stay time is the charging time of voltage 0V~ 2Ucc/3 (the time represented by tp).2. Charging time Tp=1.1RC3. It can be used as a timing circuit, and the time can be determined by RC.Example: Timing Circuit Design (1s delay time)Figure 8. 555 Timer Delay Circuit ExampleⅣ Classic 555 Timer Circuits DiagramsThere are A LOT of projects out there using the 555 in various ways and it’s easy to find schematics to make a project that has already been proven. Here lists some typical projects using 555 timer in circuits. Let’s have a look. 🔺 Car Tachometer🔺 SIREN🔺 Flashing Lights🔺 Knight Rider Circuit🔺 Laser Ray🔺 Latch🔺 LED Dimmer🔺 555 Amplifier🔺 Light Detector🔺 Machine Gun🔺 Metal Detector🔺 Motor PWM🔺 Music Box🔺 Zener Diode Tester Ⅴ 555 Timer IC Modes555 timer will use different models in different circuits to meet circuit requirements. Therefore, it has many derivative models produced by different companies with different pin functions, and uses CMOS design. What;s more, some chips include several integrated 555 timers. Some common models of the 555 chip family are as follows:ManufacturerModelRemarksCustom Silicon SolutionsCSS555/CSS555CCMOS chip, minimum working voltage 1.2V, IDD < 5µACEMIULY7855*ECG SemiconductorsECG955MTimer Single Rc-type OscillatorExarXR-555Highly stable controllerFairchildNE555/KA555Time-delay or mono-stableHarrisHA555*IK SemiconILC555CMOS chip, minimum working voltage 2VTexas InstrumentsSE555/NE555*RenesasICM7555CMOS RC timersLithic SystemsLC555Available in Industry's Smallest 8-Bump DSBGAMaximICM7555CMOS RC timers, minimum working voltage 2VMotorolaMC1455/MC1555Monolithic timerNational SemiconductorLM1455/LM555/LM555C*National SemiconductorLMC555CMOS chip, minimum working voltage 1.5VNTE SylvaniaNTE955MAccurate time delaysRaytheonRM555/RC555*RCACA555/CA555C*STMicroelectronicsNE555N/ K3T647*Texas InstrumentsSN52555/SN72555*Texas InstrumentsTLC555CMOS chip, minimum working voltage 2VZetexZSCT1555Precision single cell timerNXPICM7555CMOSHitachi SemiconductorHA17555Accurate time delays or oscillations Frequently Asked Questions about 555 Timer Circuit1. What does a 555 timer do in a circuit?The 555 timer IC is a very cheap, popular and useful precision timing device which can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing a string of stabilised waveforms of varying duty cycles from 50 to 100%. 2. How much voltage can a 555 timer take?The standard TTL 555 can operate from a supply voltage between 4.5 volts and 18 volts, with its output voltage approximately 2 volts lower than its supply voltage VCC. The 555 can source or sink a maximum output current of 200mA, (but it may get hot at this level), so the circuit variations are unlimited. 3. What are the modes of operation of a timer?The timer registers can be used in two modes. These modes areTimer mode and the Counter mode. The only difference between these two modes is the source for incrementing the timer registers. 4. What are the basic operation modes of the 555 timer?The operating modes of a 555 timer are astable, bistable and monostable. Each mode of operation signifies with a circuit diagram and its output. 5. What is the maximum frequency of a 555 timer?2MHzaccording to the website, the 555 timer has a maximum frequency of 2MHz.

IntroductionAlarm Circuits and Control Circuits are pretty common in daily life. A control circuit is a special type of circuit used to control the operation of a completely separate power circuit. Alarm circuit is a security circuit to reduce life loss and poverty under the excepted prevailing conditions. Both are quite famous and you probably could have seen plenty of different versions of them. Here, we will introduce several available home/office hobby circuits for smoking alarm, temperature controlling and timing, with their design principles, and component selection. They will provide more convenience for your daily life.A Simple Guide to Electronic Components in CircuitsCatalogIntroductionⅠ Indoor Monitoring Circuit DesignⅡ Smoke Alarm Circuit DesignⅢ Temperature Controlled Circuit Using NE555Ⅳ Temperature Sensor Circuit for Temp MeasuringⅤ Water Tank Temperature Controlled ProjectⅥ Cyclic Timing Circuit DiagramⅦ Indoor Overvoltage Protection Circuit DesignⅧ Temperature Fan Controller DiagramⅨ Water Boiling Alarm Circuit DesignⅩ FAQEvery alarm circuit and control circuit is composed of a number of basic components connected together to achieve the desired performance. The following lists some common and simple alarm and control circuits diagrams to share different ideas for protecting your home/office and making your life more easier.Ⅰ Indoor Monitoring Circuit DesignThe monitor can detect infrared rays emitted by the human body, and when a person enters the monitoring area, it can sound for alarm. It is suitable for homes, offices, warehouses, laboratories and other important occasions.👍 Circuit Working ModelFigure 1. Infrared Detection Alarm CircuitThe device consists of an infrared sensor, a signal amplifier circuit, a voltage comparator, a delay circuit and an audio alarm circuit. When the sensor IC1 detects the infrared signal radiated by the human body in front, it outputs a weak electrical signal from the pin②. It is amplified by the first-stage amplifying circuit formed by the transistor VT1, and then input to the operational amplifier IC2 through C2 with high gain and low-noise amplification. IC3 acts as a voltage comparator. Its pin⑤ reference voltage is provided by R10 and VD1. When the signal voltage output by IC2 pin① pass to the IC3 pin⑥, the voltages of the two input terminals are compared. At this time, IC3 pin⑦ changes from the high level to the low level. IC4 is an alarm delay circuit, formed by R14 and C6. Its continuous time is about 1 minute.When IC3 pin⑦ becomes low level, C6 discharges through VD2, IC4 pin② becomes low level,  which is compared with IC4 pin③ reference voltage. When it is lower than its reference voltage, IC4 pin① changes to high level, VT2 is turned on, and the buzzer BL is powered on and emits an alarm sound. After the infrared signal of the humans disappears, IC3 pin⑦ outputs to high level, and VD2 is cut off at this time. Since the voltage at both ends of C6 cannot change suddenly, charge C6 slowly through R14. When the voltage at both ends of C6 is higher than its reference voltages, IC4 pin① becomes low level for about 1 minute. That is, the alarm time lasts for 1 minute.The power-on delay circuit is composed of VT3, R20, and C8. It is mainly to prevent alarming immediately when powering on, so that the user has enough time to leave the monitoring site, and at the same time can prevent a false alarm occurred during a power cut. The device uses 9-12V DC power supply, with T step-down, full-bridge rectification, and C10 filtering. The detection circuit uses IC5 for power supply, and automatic non-stop conversion with AC and DC. 👉 Components SelectionIC1 adopts imported device Q74, the wavelength is 9~10um. IC2 uses op-amp LM358, which has high gain and low power consumption. IC3 and IC4 are dual voltage comparators LM393 with low power consumption and low offset voltage. Among them, C2 and C5 must use tantalum capacitors with small drain electrodes, otherwise the debugging will be affected. R12 is the key element to adjust sensitivity, and linear high-precision sealed type should be selected. Other components can be selected as shown in the circuit diagram. 👉 DIY and AdjustmentWhen making, a Fresnel lens is installed in front of the IC1 sensor. Since the frequency range of the human body is 0.1~10Hz, it is necessary to use the Fresnel lens to multiply the frequency of the human body. After installation finished, connect the power supply for debugging. Let a person walk about 7-10m in front of the detector, adjust R12 in the circuit, and make the buzzer alarm. As long as the other parts are of good quality and welded correctly, they can work normally without debugging. The static working current of this machine is about 10mA. It will enter the waiting state about 1 minute after the power is turned on. As long as someone enters the monitoring area, it will alarm, and stops in 1 minute. In addition, if the buzzer is changed to a relay to drive other devices, it will be used for other controls. Ⅱ Smoke Alarm Circuit DesignThis smoke alarmer can be used in family rooms or various places where smoking is forbidden (such as hospitals, conference rooms, etc.). When someone smokes, the no-smoking warning device will emit a warning sound of "No Smoking!" to remind the smoker to stop smoking consciously.👍 Circuit Working ModelFigure 2. Smoke Alarm CircuitThe no-smoking warning circuit is composed of a smoke detector, a monostable trigger, a voice generator and a power amplifier circuit. The smoke detector consists of potentiometer RP1, resistor R1 and gas sensor. The monostable trigger has time-base integrated circuit IC1, resistor R2, capacitor C1, and potentiometer RP2. The voice generator circuit is composed of voice integrated circuit IC2, resistors R3-R5, capacitor C2 and Zener diode VS. The audio power amplifier circuit includes transistor V, boost power amplifier module IC3, resistors R6 and R7, capacitors C3 and C4, and speaker BL.When the gas sensor doesn’t detect smoke, the resistance value between A and B is relatively large. IC1 pin2 is high level (higher than 2VCC/3), pin3 outputs low level, while voice generator circuit and the audio power amplifier circuit does not work. When someone smokes and the gas sensor detects the smoke, the resistance value between the A and B becomes smaller, causing the voltage of IC1 pin2 to drop. When the voltage of this pin drops to VCC/3, IC1 pin3 changes from low level to high level. Pass through current limiter R3, filter C2 and Zener diode VS, the high level will generate 4.2V DC voltage, which is supplied to voice IC2 and crystal arm. After IC2 energizes and works, it outputs a voice electrical signal. After the signal is amplified by V and IC3, it makes BL to emit a voice warning sound of "No Smoking!" 👉 Components SelectionRl~R7 selects 1/4W carbon film resistor or metal film resistor for use. RP1 and RP2 can choose small linear potentiometer or variable resistor. C1, C2 and C4 all use aluminum electrolytic capacitors with a withstand voltage of 16V; C3 uses monolithic capacitors. VS selects the silicon Zener diode of 1/2W, 4.2V for use. V uses S9013 or C8050 silicon NPN transistors. IC1 uses the NE555 timer IC; IC2 uses the voice integrated circuit; lC3 uses the WVH68 boost power amplifier thick-mode IC. BL selects 8Ω, 1~3W electrodynamic speakers. The gas sensor is MQK-2 type sensor. 👉 DIY and AdjustmentThis no-smoking warning device can be used as a smoke alarm to detect fires or harmful gases, and combustible gases. Adjusting the RP1 resistance can change the heating current of the gas sensor (usually about 130mA). And adjusting the RP2 resistance can change the sensitivity of the monostable trigger circuit. Ⅲ Temperature Controlled Circuit Using NE555This circuit is an automatic temperature controller composed of a 555 timer IC and a few peripheral components. Because the voltage at each point in the circuit comes from the same DC power supply, it does not need a high-performance regulated one. Using the capacitor step-down method can work reliably. The circuit components are low in price, small in size, and easy to self-made under amateur conditions. The automatic temperature controller made by this circuit can be used for electric heating control in industrial production and household use, with good effect.👍 Circuit Working ModelFigure 3. 555 Timer Based Circuit for Temperature ControlWhen the temperature is low, the resistance of the thermistor Rt with a negative temperature coefficient is large, the potential of pin2 of the 555 timer IC is lower than 1/3 of the voltage of Ec (about 4V), and its pin3 output high level. At this time, V conducts, the heater RL is heating, and the timing cycle starts. When the temperature of the thermistor Rt is higher than the set value and the timing cycle has not been completed, the heater RL will cut off after the timing cycle stops. When the Rt temperature drops below the set value, V will conduct again and turn on the heater RL for heating. In this way, automatic temperature control can be achieved. 👉 Components SelectionIn this circuit, the thermistor Rt can be a negative temperature coefficient type MF12 or MF53, or other types of negative temperature coefficient thermistors with different resistance values, as long as Rt＋VR1= 2R4 is satisfied under the temperature condition to be controlled. A larger potentiometer VR1 can have a larger adjustment range, but its sensitivity will decrease. The bidirectional thyristor V can also be selected according to the size of the load current. There are no special requirements for other components. Choose according to the parameters given in the circuit diagram. 👉 DIY and AdjustmentThe whole circuit can be installed on PCB. Generally, debugging is not required. The time interval is 1... .1R2×C3, which should be smaller than the thermal time constant of the heating system, but not too small, otherwise it will cause excessive radio frequency interference due to the thyristor V turns on or off rapidly. After installation and debugging, it can be put into a small plastic box, and the thermistor Rt can be led to the required place. Ⅳ Temperature Sensor Circuit for Temp MeasuringThis circuit is a thermometer made by AD590 special integrated temperature sensor, which has the characteristics of simple structure, reliable use and high precision.👍 Circuit Working ModelFigure 4. Digital Thermometer CircuitAfter the 100V AC voltage passes through the transformer T1, the rectifier bridge stack UR and the capacitor C1, the DC voltage is obtained, and then the adjustable voltage regulator circuit μA723C provides a stable working voltage for the temperature sensor AD590. AD590 is a new type of current output temperature sensor, composed of multiple transistors and resistors with the same parameters. When a specific DC working voltage is applied to both ends of the sensor, if the sensor temperature is 1 degree Celsius, the output current of the sensor changes by 1 μA. The changing current of the sensor is converted into a voltage signal through the resistor R5 and the variable resistor RP2, and then output to the digital meter, which displays the temperature change. 👉 Components SelectionThe IC selects AD590-series temperature sensor. There are no special requirements for other components of this circuit, and can be selected according to the parameters given in the circuit diagram. 👉 DIY and AdjustmentBy adjusting the value of resistor R5 and variable resistor RP2, the sensitivity of the circuit output can be improved. Ⅴ Water Tank Temperature Controlled ProjectAn automatic fish tank water temperature controller uses a negative temperature coefficient thermistor as a temperature sensor to automatically heat the fish tank through heating gas. The transient time of this circuit is small, which is beneficial to the accuracy of temperature control. And it is suitable for various sizes of fish tanks.👍 Circuit Working ModelFigure 5. Automatic Control of Fishbowl Water TemperatureAfter being rectified by diodes VD2~VD5 and filtered by capacitor C2, a voltage of about 12V is provided to the control part of the circuit. 555 timer is connected as a monostable flip-flop, the transient state is 11s. Set the control temperature to 25ºC, adjust the potentiometer RP, to get RP + Rt = 2R1 ( Rt is the thermistor with negative temperature coefficient). When the temperature is lower than 25ºC, the Rt resistance value increases, and the pin2 of the 555 timer is low level, then the pin3 output changes from low level to high level. The relay K is turned on, and its contact is closed. The heating tube starts to heat until the temperature returns to 25ºC, the Rt resistance value becomes smaller, the pin2 of the 555 timer is at high level, and the pin3 outputs low level. The relay K loses power, its contact is open, and the heating stops. 👉 Components SelectionIC uses NE555, NA555, SL555 and other 555 timer ICs; VD1 uses IN4148 silicon switching diodes; LED uses common light-emitting diodes; VD2~VD5 uses IN4001 silicon rectifier diodes; Rt uses 470Ω MF51-type negative temperature coefficient thermistors at room temperature; RP uses WSW organic solid trimming potentiometer; R1and R2 uses RXT-1/8W carbon film resistors; C1 and C3 uses CD11-16V electrolytic capacitors; C2 uses CT1 ceramic dielectric capacitors; K uses 12V JZC-22F electromagnetic relay. 👉 DIY and AdjustmentThe temperature sensor probe connects the thermistor Rt with wires, and then seals the solder joint with epoxy glue, to avoid water erosion. As long as the circuit is correct in the DIY process, this circuit is easy to operate. If the component performance is good, it can be used without debugging after installation. Ⅵ Cyclic Timing Circuit DiagramThe circuit can set the cycle time of the equipment and each time it works, allowing the equipment to work continuously according to the set time. This circuit can be applied to control occasions such as timing pumping, timing ventilation, and timing cut off.👍 Circuit Working ModelFgiure 6. Cycle Timing CircuitAfter the circuit is stepped down through the capacitor C2 and the bleeder resistor R3, and then rectified by the bridge stack IC2, and stabilized by VD2, a DC voltage of about 12V is obtained to supply power to IC1 and other circuits. IC1 is a 14-bit binary counter/frequency divider integrated circuit. A clock oscillator with a certain frequency is formed by the internal circuits of R1, R2, C1 and IC1 to provide clock pulses for timing IC1. When the circuit is powered on, it first enters the working gap waiting time of the device. IC1 internally realizes the delay by counting and dividing the clock pulse.  When the timing is up (according to the parameters in the figure, about 3 hours), the Q14 terminal of IC1 outputs high level, making the transistor V conducts. The relay KA gets to work, and drives the controlled equipment to start working. At this time, IC1 starts to count the working time of the device again. When the timing expires (according to the parameters in the figure, about 20 minutes), the Q14 terminal outputs low level. So that V is cut off and the device stops working. And meanwhile, IC1 automatically resets and starts the next timing. So that the device can perform timing cycle according to requirements. In the figure, VL is a working indicator. 👉 Components SelectionIntegrated circuit IC1 chooses 14-bit binary counter/frequency divider CD4066, or CC4066 or other digital circuit integrated blocks with the same function. IC2 selects a 1A, 50V bridge stack, or can be connected with four 1N4007 diodes. Transistor V uses NPN-type transistor 8050, and other transistors such as 9013 or 3DG12 can also be used. VD1 selects rectifier diode 1N4007; VD1 selects 1W, 12V silicon regulator tube, such as 1N4742; VD3 ~VD5 use switching diodes 1N4148; VL selects ordinary light-emitting diodes. Resistors R1, R2, R4, R6 and R7 use 1/4W metal film resistors; R3 and R5 use 1/2W carbon film resistors. C1 selects polyester or monolithic capacitors; C2 selects polypropylene capacitors with a withstand voltage of 450V and above; C3 selects aluminum electrolytic capacitors with a withstand voltage of 16V. Relay KA chooses a miniature relay with a coil voltage of 12V, and the contacts capacity is determined according to the power of the controlled device. 👉 DIY and AdjustmentAfter the circuit is installed, it can work normally without debugging. When you need to adjust the control time, you can adjust the parameters of R1, and C1. Also you can change the position of the IC1 output control terminal (Q4 ~Q14). Ⅶ Indoor Overvoltage Protection Circuit DesignBecause the instability of the mains, the household appliances often affected, their service life may reduce. In serious cases, it is easy to burn out due to voltage surge. The circuit described in this example can solve this problem well.👍 Circuit Working ModelFigure 7. Overvoltage Protection Circuit for AppliancesThe mains supply provides a stable 12V working voltage for the switch integrated circuit via C1, VD1, and DW1. VD3, R2 and RP1 form a voltage divider sampling circuit. When the mains voltage is normal, DW2 cannot be turned on, the working voltage of TWH8778 pin⑤ is lower than 1.6V. The relay J does not pull in, and the mains supplies the CZ socket through the J-1 normally closed contact. When the mains voltage is high than the normal setting, DW2 breaks down, the potential of TWH8778 pin⑤ rises to 1.6V, causing the IC to flip, pin ③ outputs high level. At this time, the relay is pulled in, and the electrical power supply is immediately cut off, avoiding the overvoltage affects electrical appliances. 👉 Components SelectionC1 uses 0.47µ/400V electrolytic capacitor, relay J uses 6V DC contactor; RP uses ordinary trimming potentiometer, chip IC can be TWH8778-type electronic switch or TWH8752-type electronic switch. 👉 DIY and AdjustmentAfter the device is welded correctly, connect the mains power to the input end of the voltage regulator, cooperate with the voltage regulator and carefully adjust RP1, so that the relay J is closed when the voltage is 250V, and then the circuit is connected to the mains power grid. Ⅷ Temperature Fan Controller DiagramThis is an automatic fan temperature controlled governor, which can automatically adjust the speed according to the temperature change. The circuit can be adjusted, so it can be used for the control of other electrical equipment.👍 Circuit Working ModelFigure 8. Automatic Temperature Control and Speed Regulation in Fan CircuitThe IC in the picture is a 555 timer IC, which forms a multivibrator with components of R2, R3 and C2. It can send out a rectangular wave signal with an adjustable duty cycle. When the temperature changes, the resistance value of the thermistor changes, so the duty cycle of the square wave output by the multivibrator changes. Adjusting the conduction angle of the bidirectional thyristor VT changes the voltage across the fan electrodes, which automatically adjusts the speed of the electric fan. 👉 Components SelectionThe integrated circuit IC selects NE555 timer, and models such as LM555 and TLC555 can also be used. VT is a bidirectional thyristor, its withstand voltage should be above 400V, and the rated current should be reasonably selected according to the capacity of the electric fan to be controlled. Resistor R1～R5 can choose ordinary 1/8 or 1/4W carbon film resistors; Rt is a negative temperature coefficient thermistor, and can choose a thermistor with a resistance of about 10KΩ at room temperature. Capacitor C1 uses ordinary aluminum electrolytic capacitors; Capacitors C2 and C3 are polyester capacitors. VD is a Zener diode with a steady voltage of 9.1V. 👉 DIY and AdjustmentYou can make your own PCB, or use a universal one. After the circuit is installed, the temperature of the thermistor Rt can be artificially changed to observe the speed of the fan motor. If the temperature control effect is not ideal, the resistance value or temperature change range of the thermistor can be adjusted appropriately. Ⅸ Water Boiling Alarm Circuit DesignOnce the water boils in the kitchen, if it is not turned gas off in time, the boiling water will overflow and extinguish the flame. The gas may spill, which is very unsafe. This problem can be solved by using the water alarm.👍 Circuit Working ModelFigure 9. Boiling Water Alarm CircuitThis circuit uses thermistor as the temperature sensing element. When the water temperature rises, the resistance of the thermistor decreases and the potential at point A increases. When the potential at point A is higher than the conversion voltage of the IC-1 inverter, the IC -1 will output low level, IC-2 will output high level. Making the audio oscillator composed of IC-3 and IC-4 work, and the piezoelectric ceramic sheet makes sound. When IC-2 outputs low level, the audio oscillator doesn’t work, and the piezoelectric ceramic chip is silent. 👉 Components SelectionIC uses C066 two input terminal four NAND gate, working voltage 3V~18V, power supply is 3V~6V; RT thermistor selection resistance value is about 1kΩ; piezoelectric ceramic chip diameter is 27mm; resistor selection is ordinary 1/8 or 1/4W metal film resistors. 👉 DIY and AdjustmentFind two starter shells of waste fluorescent lamps, use iron sheet as a clip, close the tops of the two starters, and fasten them with screws. One of the starters can be set on the mouth of the kettle to obtain the temperature of the water. The two pins of the thermistor are welded on the cover of the other starter and put into the shell. Note that the thermistor must be close to the inner shell wall to facilitate heat transfer. Solder the outer lead of the thermistor and the temperature sensor. After all the components are welded and checked, you can turn on the power for debugging. Put the temperature sensor on the mouth of the kettle, and adjust the RP when the water boils to make the piezoelectric ceramic sheet sound. Repeat it several times before this circuit can be used normally. If you want to change the sound frequency, you can change the C2 capacity. If you feel that the sound is light, you can connect an external transistor to the IC-4 output terminal to amplify the sound.  Ⅹ FAQ1. How do you make a security alarm circuit?The cathode of the photodiode is connected to the supply while the anode is connected to a 10KΩ resistor. Another end of the resistor is connected to the ground. The anode terminal of the photodiode is also connected to pin 5 of the LM358 op-amp, which is the non-inverting terminal. 2. What is the technique of alarm circuits?In a closed-circuit system, the electric circuit is closed when the door is shut. This means that as long as the door is closed, electricity can flow from one end of the circuit to the other. But if somebody opens the door, the circuit is opened, and electricity can't flow. This triggers an alarm. 3. What is a security alarm circuit?This circuit will help you to guard your precious documents as well as jewelry from intruders or theft. All you need is just to place this circuit in front of the locker or below the mat so when any unknown person comes and walks over the switch, the circuit will trigger and the sound of an alarm comes. 4. What type of circuits are security alarms made of?The simplest type of contact-operated security circuit consists of an alarm bell (or a buzzer or electronic 'siren sound' generator, etc.), wired in series with a normally-open (n.o.) close-to-operate switch; the combination being wired across a suitable battery supply, as shown in the basic 'door-bell' alarm circuit ... 5. What are the three basic parts of an alarm system?The main components of an alarm system would be a sensor, a camera, a motion detector, a buzzer a flash light and batteries. It is a component that is usually used to detect noise or movement. Sensors are usually connected to the circuit. 

IntroductionTime relay refers to a kind of relay whose output circuit needs to make an obvious change (or contact action) after adding (or removing) the input action signal in a specified and accurate time. It is an electrical component used in a circuit with a lower voltage or a smaller current to switch on or off a circuit with a higher voltage and larger current.  With the development of electronic technology, electronic time relays have become mainstream products in time relays. Electronic intelligent digital display time relays using large-scale integrated circuit technology have many working modes, which can not only achieve long delay time but also have high time-delay accuracy, small size, convenient adjustment and long service life, making the control system simpler and more reliable. The time relay also has the function of automatic monitoring. Time relay and other equipment together can form a program space route to realize the automatic operation of the equipment.Time Relay Basics ExplainedCatalogIntroductionⅠ Time Relay Basics  1.1 What is a Time Delay Relay?  1.2 Time-delay Relay Working Principle  1.3 Timer Relay Structure  1.4 Timer Relay Parameters  1.5 Four-type Time Relay Contacts Ⅱ Understanding Time Delay in Relay CircuitⅢ Time Relay Classifications  3.1 According to Working Principle  3.2 According to the Delay ModesⅣ How to Wire Time Relay?Ⅴ Time Relay ApplicationsⅥ Time Relay SelectionⅦ Timer Relay Using Instructions  7.1 General Ideas  7.2 Two Points for Attention in Using Time RelaysⅧ Case Study: Time Relay Switch in Light CircuitⅨ Frequently Asked Questions about Time Delay Relay BasicsⅠ Time Relay Basics1.1 What is a Time Delay Relay?The time relay is a very important component in the electrical control system. In many control systems, use the time relay to achieve delay control. Time relay is an automatic control electrical appliance that uses the principle of electromagnetic or mechanical action to delay the closing or opening of contacts. Its characteristic is that there is a delay from the time the attracting coil gets the signal to the action of the contact. The time relay is generally used to control the motor starting process with time function. As above mentioned, the main function of the time delay is as an executive device in simple program control. When it receives the start signal, it starts timing. After the timing ends, its working contact opens or closes to promote the subsequent circuit work. Generally speaking, the delay performance of the time relay can be adjusted within the range of design, so as to facilitate the adjustment of its delay time. In addition, a time relay alone may not be able to do close. After closing for a period of time, it will open again. It is a cycle of time-delay closing and opening. However, configuring a certain number of time relays and intermediate relays can do it. 1.2 Time-delay Relay Working PrincipleTime relay is widely used in remote control, telecommunication, automatic control and other electronic equipment, and is one of the most important control components. When the coil is energized, the armature and the pallet are attracted by the core and move down instantaneously, making the action contact close or open. However, the piston rod and the lever cannot fall with the armature at the same time, because the upper end of the piston rod is connected to the rubber film in the air chamber.  When the piston rod starts to move downward under the action of the released spring, the rubber film is concave downward. The air in the air chamber becomes thinner, causing the piston rod to be damped and slowly descend. After a certain period of time, the piston rod descends to a certain position, and then the delay contact action is pushed through the lever to make the moving contacts open and close. The time from when the coil is energized to when the delay contact completes the action is the delay time of the relay. The length of the delay time can be changed by adjusting the size of the air inlet hole of the air chamber with a screw. After the suction coil is de-energized, the relay relies on the spring to recover. And the air is quickly expelled through the air outlet. 1.3 Time Relay StructureFigure 1. Air-damping Time Relay1 Coil5 Push plate9 Weak Spring13 Adjusting Screw2 Iron Core6 Piston rod10 Rubber Film14 Air Inlet3 Armature7 Lever11 Air Chamber Wall15 Micro Switch4 Reaction Spring8 Spring12 Piston16 Micro Switch 1.4 Time Relay ParametersTechnical parameters include rated voltage, contact working current, contact type and quantity, delay time, accuracy, ambient temperature, mechanical life and electrical life, etc. Now take the SJ23 series air-type time relay as an example, its technical parameters are as follows:1) Rated control capacity: AC300VA, DC60W (30W delay contact assembly).2) Rated voltage level: AC380V, 220V; DC220V, 110V.3) Rated voltage of the coil: AC110V, 220V and 380V.4) Maximum operating current of the contact: 0.79A at AC380V, 0.27A (momentary) and 0.14A (delay) at DC220V.5) Delay repeat error: ≤9%.6) Hot-state pull-in voltage: no more than 85% of the rated voltage of the relay. When the voltage drops from the rated value to 10% of the rated value in cold-state, it can be reliably released. And it can reliably release after reaching 110% of the rated voltage.7) The mechanical life is not less than 1 million times, and the electrical life is 1 million times (the DC life of the delay contacts assembly is 500,000 times). 1.5 Four-type Time Relay ContactsFigure 2. Time Relay SymbolsNOTC (normally-open, timed-closed): When the coil is not energized, the NOTC contact is normally open. It is closed by energizing the relay coil, but only within a specified time after the coil is continuously energized. The moving direction of the contact (close or open) is the same as that of a standard normally open contact. Since the delay occurs in the direction in which the coil is energized, this type of contact is normally open and on-delay. NOTO (normally-open, timed-open): Unlike the NOTC contact, the timed action occurs when the coil is de-energized. Since the delay occurs when the coil is de-energized, this type of contact is normally open and off-delay. NCTO (normally-closed, timed-open): When the coil is not powered on, the NCTO contact is normally closed. By energizing the relay coil, the contact is opened, but only within a specified time after the coil is continuously energized. The movement direction of the contact (closed or opened) is the same as the standard normally closed contact, but there is a delay in the opening direction. NCTC (normally-closed, timed-closed): The NCTC contact is similar to NCTO contact, because when the coil is normally closed when in de-energized and opened by energizing the coil. Ⅱ Understanding Time Delay in Relay CircuitSet delay time of a relay. Generally speaking, the delay performance of the time relay can be adjusted within the range of design, so as to facilitate the adjustment of its delay time in circuit. Time Delay Relay Circuit (Power-Off)If you are using an on delay relay, the delay will start immediately after the input signal is obtained. After the delay is completed, the executive part will output the signal to the control circuit. When the input signal disappears, the relay will immediately return to the pre-action status. It is opposite to an off delay relay. When the input signal is obtained, the execution part immediately has an output signal. After the input signal disappears, the relay needs a certain time to restore to the state before the action.Figure 3. Timer Relay StructureⅢ Time Relay Classifications3.1 According to Working PrincipleAccording to different working principles, time relays can be divided into air damping time relays, electric time relays, electromagnetic time relays, electronic time relays, etc. (1) Air damping time relayThe type is obtained by using the principle of damping when air passes through the small hole. Its structure is composed of three parts: electromagnetic system, delay mechanism and contact. The electromagnetic mechanism is a double-port direct-acting type, the contact system is a micro switch, and the delay mechanism adopts an airbag damper. (2) Electronic time relayUtilize the principle that the capacitor voltage in the RC circuit can't jump, and can only change gradually according to the exponential law, that is, the delay is obtained by electrical damping characteristics.Features: Wide delay range, high precision (generally about 5%), small size, shock resistance and easy adjustment. (3) Electric time relayUse the miniature synchronous motor to drive the reduction gear train to obtain the time delay.Features: The delay range is wide, up to 72 hours, and the delay accuracy can reach 1%. At the same time, the delay value is not affected by voltage fluctuations and environmental temperature.Its delay range and accuracy are unmatched by other time relays. Its disadvantages are complex structure, large size, short life, high price, and accuracy is affected by the power frequency. (4) Electromagnetic time relayUse the principle of slow attenuation of the magnetic flux after the electromagnetic coil is cut off to delay the release of the armature of the magnetic system to obtain the delay action of contacts. It is characterized by a large contact capacity, so the control capacity is large. However, the delay time range is small, and the accuracy is slightly worse. So it is mainly used in the control of DC circuits. 3.2 According to the Delay ModesBased on it, time relays can be divided into two types: on-delay type and off-delay type.(1) The on delay type time relay starts to delay immediately after receiving the input signal. After the delay is completed, its execution part outputs the signal to manipulate the control circuit. When the input signal disappears, the relay immediately returns to the state before the action.(2) The off delay type time relay is just the opposite. When the input signal is obtained, the execution part immediately has an output signal. After the input signal disappears, the relay needs a certain delay to restore to the state before the action. Ⅳ How to Wire Time Relay?The time relay is a very important component in the electrical control system. There are power-on delay types and power-off delay types. Based on the action type, there are electronic type and electric type, etc. Between them, the electronic type uses the principle of capacitor charging and discharging combined with electronic components to achieve delay action. There are many electric styles by using air bags and springs.Figure 4. Time Relay Wiring Schematics Time Relay Wiring:1) Control wiring: Consider it as a DC relay.2) Work control: Although the control voltage is connected, whether it plays a control role is determined by the timer on the panel.3) Function understanding: It is a switch,single-pole double-throw, with an active point, just like the active arm of a common knife switch.4) Load wiring: Connect the neutral wire of the power supply or the negative terminal.5) Working principle: When the timer is invalid, it is equivalent to the normal light in the switch-off state. When timing, the relay will act and the electrical appliances will be energized to work, which is equivalent to the normal light in the switch-on state.Take the power-on delay time relay as an example:Figure 5. On Delay Relay Contacts Wiring Ⅴ Time Relay ApplicationsIn Flash ControlTwo-time relays cooperate with each other to provide constant frequency on/off pulses of the contacts, sending intermittent power to the light.  In Furnace Safety Purge ControlBefore the combustion furnace can be safely ignited, the fan must run for a certain period of time to clean out any flammable or explosive steam in the furnace chamber. The time relay provides the required time parts for the furnace control work.In Electric Soft-start Delay ControlIt is not necessary to start a large electric engine by switching full power from a completely stopped state, and can reduce voltage softly start with less inrush current.In Conveyor Belt Sequence DelayWhen multiple conveyor belts are arranged to transport materials, the conveyor belts must be started in the reverse order (the last one is first, the first one is last) to prevent materials from accumulating on the moving conveyor which may be stop or move slowly. Ⅵ Time Relay SelectionThe selection of time relay is mainly due to delay mode and parameter coordination. The following aspects should be considered when selecting.(1) Delay mode selectionIt should be selected according to the requirements of the control circuit. The reset time after the action is longer than the inherent action time, so as to avoid misoperation or even no delay. This is especially important in the occasions of repeating delay circuits and frequent operations. (2) Type selectionFor occasions where the delay accuracy is not high, cheaper electromagnetic or air damping time relays are always used. On the contrary, for occasions where the delay accuracy is high, electronic time relays can be used. (3) Coil voltage selectionAccording to the voltage of the control circuit, the voltage at which the relay attracts the coil is selected. (4) Selection of power supply parametersIn the occasions where the power supply voltage fluctuates greatly, it is better to use air damping or electric time relays than the transistor type. And in the occasions where the power frequency fluctuates, electric time relays should not be used. In addition, when the temperature changes greatly, air damping type should not be used. When selecting a time relay, pay attention to the current type and voltage level of its coil (or power supply), and other factors, such as delay mode, contact form, delay accuracy and installation method according to the control requirements.Ⅶ Timer Relay Using Instructions7.1 General Ideas1) Keep the time relay clean, otherwise, the error will increase.2) Before use, check whether the power supply voltage and frequency are consistent with the voltage and frequency of the time relay.3) Choose the control time of the time relay according to user requirements. Regardless of the type of time relay, as long as the timing time is equal to the set time, its output contacts will act to achieve the purpose of the timing control circuit.4) For DC products, pay attention to wiring according to the circuit diagram and pay attention to the polarity of the power supply.5) After the time relay is out of working state, it should be reset immediately for the next use. If the repeated use interval is less than the preset time, the control circuit will be abnormal. What’s more, the power-on delay type is automatically reset after power off; and the power-off delay type is automatically reset after power on.6) Try to avoid using it in places with obvious vibration, direct sunlight, humidity and soil contact. 7.2 Two Points for Attention in Using Time RelaysThree Key Points1) Starting point of timingOn one hand, when selecting the timing point of the power-on delay time relay, you should choose to supply power to the time relay when the timing signal is sent by the control circuit that needs to perform timing. On the other hand, when selecting the timing point of the power-off delay type time relay, you should choose to cut off the power supply of the time relay when the control circuit that needs to send out the timing signal, so that the timing can be performed.2) Ending point of timingThe timing endpoint has two meanings: one refers to the point at which the set time is equal to the timing time; the other refers to the point at which the contract operates.3) Reset point of timingThe reset of the time relay is to clear the last timing content for the next use. If it is not reset, an abnormality will occur the next time it is used. Special attention should be paid to: the interval between two uses should be greater than the reset time, which is particularly important in electric time relays. The relationship between the starting point, ending point and reset point of timing1) After the time relay is used, there is a reset problem. Therefore, most of the control circuits are in the next level circuit by the time relay output. After the timing completion signal is accurately obtained, it is used to cut off the power supply of the time relay (power-on delay type), or power the time relay (power-off delay type).2) In the upper and lower control circuits of the time relay, there are components that cannot work at the same time. If the time relay cannot accurately operate the upper and lower control circuits at these points, it will cause the device to operate abnormally. Ⅷ Case Study: Time Relay Switch in Light CircuitControl requirements: Light 1 and light 2 are on at the same time, and light 2 is off in 30 seconds after light 1 is off. When light 1 is on, light 2 can be off at any time.According to the control requirements, explain through the following circuit diagram.Figure 6. Time Relay Switch in Light Circuit1) Press SB2, the contactor KM is energized and self-locked, and at the same time KT is also energized, and KT closes.2) After KT is turned on, the intermediate relay KA is also energized to work.3) At the same time, contact KM and contact KA are also closed at the same time, light 1 and light 2 are on.4) When the stop button SB1 is pressed, the contactor KM powers off, the contact KM opens, and the light 1 is off at the same time. Because of the existence of the power-off delay relay, KT is still on as well as the light 2. It goes out after the timing set by the time relay.5) When light 1 is on, and contact KA1 is turned on at any time, the time relay resets. KT disconnects and the light off.This is the typical application of an off delay relay. However, in the actual circuit, the control logic may be more complicated than this, so we must deeply understand the working principle and application of the time relay. Ⅸ Frequently Asked Questions about Time Delay Relay Basics1. What is time-delay relay?Time-delay, or time-release relays, allow necessary actions to happen at specific times in an electrical apparatus because they, in essence, act as a timer. 2. How does a time delay relay work?Time delay relays control the flow of electrical power and can be used to control power to many different types of electrical loads. Combining electromechanical output relay capability with control circuitry, these relays are pre-engineered to perform up to eleven time delay functions. 3. What is time delay relay circuit?Time Delay Relays. Time-Delay Relay. Relays are switches that are controlled by a circuit. Relays, in essence, send messages that tell something to start. When a car is started, the ignition only indirectly interacts with the battery of the car because a relay is sending the signal that tells the car to start. 4. How does a time delay relay work?Upon application of input voltage, the time delay relay is ready to accept trigger signals. Upon application of the trigger signal, the relay is energized and the preset time begins. ... Continuous cycling of the trigger signal at a rate faster than the preset time will cause the relay to remain energized. 5. How do you make a time delay relay?These relays provide a “Time Delay” between the energizing or de-energizing of the coil and movement of the armature. Such relays are called Time Delay Relays. A Time Delay Relay consists of a normal electromechanical relay along with a control circuit to control the relay operation and timing. 6. What is off delay relay?Abbreviated “NOTO”, these relays close immediately upon coil energization and open after the coil has been de-energized for the time duration period. Also called normally-open, off delay relays. 3: Normally-closed, timed-open. 7. How does an off delay timer relay work?Operation of Off Delay FunctionUpon application of input voltage, the time delay relay is ready to accept a trigger. When the trigger is applied, the output is energized. Upon removal of the trigger, the time delay (t) begins. At the end of the time delay (t), the output is de-energized. 8. What is the difference between off delay and on delay timer?As for Timer ON Delay, Timer starts by turning ON the timer trigger bit, and the timer output bit turns ON when the setup time has passed. As for Timer OFF Delay, the timer output bit turns OFF when the setup time has passed after the timer input bit had turned OFF. 9. How do you test a timer relay?Burden TestAdjust the timer with high time delay for example: 2 minutes.Energize the relay with 125V and measure the dc current.Note down the current before timer operates.After 2 minutes relay will pick up. Note down the current after operation.Calculate the relay power (W) = 125v x measured current. 10. What is the function of an time delay relay?Typical time delay functions include on-delay, repeat cycle (starting off), interval, off-delay, retriggerable one shot, repeat cycle (starting on), pulse generator, one shot, on / off delay, and memory latch.

SummarySome electornic or electrical appliances needs time limited power supply,or usage of some devices are depends on limitted time.To automate electrical devices depends on time simple and robust solution given based on arduino.Today let's make a arduino variable time relay together.By using this arduino variable timer relay we can control high voltage electrical appliances or electronic devices.Now let me share the process. To indicate the time duration and status 16×2 LCD display is included in this design, once the program uploaded to the Arduino then it can work independent with some external battery power source. Connection DiagramConstruction and Working In this project arduino uno board is used to control SPDT (Single pole double throw) Relay and 16 x 2 character LCD indicates the time duration status. Digital pins D2 to D7 are connected to the LCD display. VR1 varible resistor helps to control the contrast of LCD display, Transistor Q1 BC547 reacts as a Switching device and controls the power supply to the Relay coil depends on arduino output. There are three push buttons are placed to set different time durations, S1 Switch makes the count start, S2 changes the Hours and S3 changes the Minutes of time duration. Output signal from the Arduino is taken from D8 pin and it drives the Relay through transistor. After making the connection, upload the following arduino sketch and pretest the operation with real timer clock.Note:- Candle with extreme care if you using High voltage supply at the Relay end. Arduino Code #include <LiquidCrystal.h>LiquidCrystal lcd(7,6,5,4,3,2);const int set = 9;int hours=10;int start=11; int relay=8;int b=0,h=0,t=0;int buttonState = 0; int lastButtonState = 0; void setup() {    pinMode(set,INPUT);  pinMode(hours,INPUT);  pinMode(relay,OUTPUT);  pinMode(start,INPUT);  lcd.begin(16,2);  lcd.setCursor(0,0);  lcd.print("Adjustable Timer");  }int timer( int b,int h){         if(b<=9)           {            lcd.setCursor(3,1);            lcd.print(0);            lcd.setCursor(4,1);            lcd.print(b);          }     else{lcd.setCursor(3,1);lcd.print(b);}         lcd.setCursor(2,1);         lcd.print(":");     if(h<=9)           {            lcd.setCursor(0,1);            lcd.print(0);            lcd.setCursor(1,1);            lcd.print(h);          }     else{lcd.setCursor(0,1);lcd.print(h);}    }void loop()       {           buttonState = digitalRead(set);                         if (buttonState != lastButtonState)       {                 if(buttonState == HIGH)         {                       lcd.clear();           lcd.print("Set time in min:");                      ++b;           timer(b,h);                                   }                   lastButtonState = buttonState;          }       if (digitalRead(hours)== HIGH)          {              lcd.clear();              lcd.print("Set time in hours");              ++h;              timer(b,h);              while(digitalRead(hours)==HIGH);                                    }             if(digitalRead(start)==HIGH)          {             lcd.clear();             t=((h*60)+(b))*1000;             lcd.print("Timer is set for");             timer(b,h);             digitalWrite(relay,HIGH);             delay(t);             digitalWrite(relay,LOW);             while(digitalRead(start) == HIGH );                                   }                        } Have you make it successfully?  

A simple and efficient PWM lamp dimmer using timer IC NE555 is discussed in this article. Yesterdays linear regulator based dimmers can only attain a maximum efficiency  of 50% and are far inferior when compared to the PWM based dimmers which can hit well over 90% efficiency. Since less amount of power is wasted as heat, the switching elements of PWM dimmers require a smaller heat sink and this saves a lot of size and weight. In simple words, the most outstanding features of the PWM based lamp dimmers are high efficiency and low physical size. The circuit diagram of a 12V PWM lamp dimmer is shown below. As you can see, NE555 timer IC which is wired as an astable multivibrator operating at 2.8KHz forms the heart of this circuit. Resistors R1,R2, POT R3 and capacitor C1 are the timing components. Duty cycle of the IC’s output can be adjusted using the POT R3. higher the duty cycle means higher the lamp brightness and lower the duty cycle means lower the lamp brightness. Diode D1 by-passes the lower half of the POT R3 during the charging cycle of the astable multivibrator. This is done in order to keep the output frequency constant irrespective of the duty cycle. Transistors Q1 and Q2 forms a darlington driver stage for the 12V lamp. Resistor R4 limits the base current of transistor Q1.Understanding the variable duty cycle astable multivibrator.As I have said earlier, the variable duty cycle astable multi vibrator based on NE555 forms the foundation of this circuit and a good knowledge on it is essential for designing projects like this. For the ease of explanation the timing side of the astable multivibrator is redrawn in the figure below.Upper and lower halves of the POT R3 are denoted as Rx and Ry respectively. Consider the output of the astable multivibrator to be high at the starting instant. Now the capacitor C1 charges through the path R1, Rx, and R2. The lower half of POT R3 ie; Ry is out of the scene because the diode D1 by-passes it. When the voltage across the capacitor reaches 2/3 Vcc, the internal upper comparator flips its output which makes the internal flip flop to toggle its output. As a result the output of the astable multivibrator goes low. In simple words, the output of the astable multivibrator remains high until the charge across C1 becomes equal to 2/3 Vcc and here it is according to the equation Ton =0.67(R1+Rx+R2)C1.Since the internal flip flop is set now, the capacitor starts discharging through the path R2,Ry into the discharge pin. When the voltage across the capacitor C1 becomes 1/3 Vcc, the lower comparator flips its output and this in turn makes the internal flip flop to toggle its output again. This makes the output of the astable multivibrator high. To be simple, the output of the astable multivibrator remains low until the voltage across the capacitor C1 becomes 1/3 Vcc and it is according to the equation Toff = 0.67(R2+Ry)C1. Have a look at the internal block diagram of NE555 timer shown below for better understanding.How does the frequency remain constant irrespective of the position of POT3 knob?.What ever may be the position of  POT3 knob, the total resistance across it remains the same (50K here). If anything decreases in the upper side (Rx) the same amount will be increased in the lower (Ry) and the same thing gets applied to the higher(Ton) and lower(Toff) time periods. The derivation shown below will help you to grasp the matter easily.With reference to Fig 2, we have:Ton = 0.67(R1+Rx+R2)C1Toff= 0.67(R2+Ry)C1Total time period of the output waveform “T” is according to the equation :T = Ton + ToffThere fore, T = 0.67(R1+Rx+R2+R2+Ry)C1                        T= 0.67(R1+2R2+Rx+Ry)C1We know that Rx+Ry = R3There fore T = 0.67(R1+2R2+R3)C1Therefore frequency F = 1/(0.67(R1+2R2+R3)C1) From the above equation its is clear that the frequency depends only on the value of the components C1, R1, R2  and the over all value of R3 and it has nothing to do with the position of R3 knob.  Ref:KY32-NE555KY32-NE555.NE555DR.NE555P