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LM567 Based Infrared Speed Sensor: Easy Way to Test Speed

  • Contents

I Introduction

When testing environmental protection, safety, and economic indicators such as noise, acceleration performance, maximum speed, and fuel consumption of motor vehicles, it is necessary to measure and control the vehicle speed. Most existing vehicle speed measurement methods use electronic timing devices or stopwatches to measure vehicles. The time it takes to travel a fixed distance and then find the average speed.

Generally, the processor and the display part of the electronic timing device are basically the same, but the speed sensor part is different, and the characteristics of the sensor directly affect the accuracy of the measurement result. At present, the commonly used speed sensors include pressure-sensitive sensors, COMS  cameras, and parallel light sources. The former has a simple structure, but it is cumbersome to lay, the sensor is easily damaged, and the sensitivity is reduced after long-term use, which affects the measurement results. The latter has high sensitivity and accurate measurement. However, the cost is too high, involves more equipment, and has higher requirements for the placement of the light source.

Considering the above problems, it is a feasible method to design a new infrared speed sensor using LM567. The sensor is small in size, low in cost, simple in operation, easy to use, has high sensitivity, accuracy, stability and anti-interference ability, and is suitable for measuring the average speed of a vehicle within a fixed driving distance.

Figure 1. LM567Figure 1. LM567

Catalog

I Introduction

II Working Principle of Traditional Speed Measuring Device

2.1 Using Pressure Belt Road Tester

2.2 Using Laser Road Tester

III Working Principle of Infrared Speed Sensor Based on LM567

3.1 Internal Structure and Function of LM567

3.2 Principle of Infrared Speed Measurement Based on LM567

IV Conclusion

Ordering & Quantity

II Working Principle of Traditional Speed Measuring Device

The following uses the measurement of motorcycle acceleration noise as an example, to introduce the principles, and advantages & disadvantages of the conventional speed measuring devices currently commonly used. Figure 1 is a simplified layout of the motorcycle acceleration noise test.

2.1 Using Pressure Belt Road Tester

For the speed measurement method using the pressure belt road tester, place the pressure belt at AA’, BB’, CC’, DD’ respectively and stick the pressure belt to the road surface well. The distance between AA’ and BB’, CC’ and DD’ (that is, the speed measurement zone) is 1 meter, and the pressure belt and the road tester are connected in sequence with a cable.

When the vehicle passes the pressure belt at AA’, the pressure-sensitive sensor in the pressure belt is triggered, and the trigger signal is sent to the road tester to start the timing of its internal timing device;

When the vehicle passes BB', a trigger signal is generated again to stop the timing device. Using the internal processor of the road tester, the time taken to pass the distance between AA' and BB' is converted into vehicle speed and displayed on the LCD screen.

Figure 2. Layout Diagram Using Pressure Belt Road Tester

Figure 2. Layout Diagram Using Pressure Belt Road Tester

Similarly, a vehicle speed value can be measured between CC’ and DD’ to meet the requirements of noise measurement. The working principle of this speed measurement method is simple, but the equipment is more troublesome to lay, and the sensor is easily damaged. After long-term use, the sensitivity will be reduced, which will affect the measurement result.

2.2 Using Laser Road Tester

For the speed measurement method using the laser road tester, four parallel laser light sources are placed at four positions of A, B, C, and D, and four are placed at four positions of A'B'C'D'. CMOS camera for receiving laser signals. The light source can be adjusted so that the laser light emitted is aligned with the center of the camera, and the camera is connected to the road tester in sequence.

When the vehicle passes AA’, the light is blocked, and the camera generates a trigger signal to make the internal timing device of the road tester work;

When the vehicle passes BB', a trigger signal is generated again to stop the timing device, and the internal processor of the road tester is used to convert the time spent through the distance between AA' and BB' to the vehicle speed and display it on the LCD screen. on.

Figure 3. Layout Diagram Using Laser Road Tester

Figure 3. Layout Diagram Using Laser Road Tester

Similarly, a speed value can be measured between CC’ and DD’. The sensitivity and measurement accuracy of this speed measurement method is very high, but the operation is extremely inconvenient. Not only does each laser light source require an independent power supply, but also the laser signal must be directed to the center receiving point of the camera, which places high requirements on the placement of the light source, otherwise the sensor will be difficult to work properly.

III Working Principle of Infrared Speed Sensor Based on LM567

This blog uses a phase-locked loop audio decoding chip LM567 to design a new infrared speed sensor. Its circuit diagram and working principle are as follows.

3.1 Internal Structure and Function of LM567

LM567 is specially used to demodulate a single tone frequency modulation signal, and its operating frequency can be as high as 500kHz. It is widely used in industrial automatic control, remote control telemetry, security alarm and other fields.

LM567 is mainly composed of quadrature phase detector, phase-locked loop and amplifier. Its internal structure is shown in Figure 2. Pins 5 and 6 of LM567 are externally connected with timing resistors and capacitors R, C. R and C determine the center frequency f0 of the phase-locked loop internal voltage controlled oscillator, that is, f0.

Resistor R is connected between pins 5 and 6, of which pin 6 is grounded through capacitor C (Uss). If R is 2~20kΩ, the LM567 can extract the tone signal in the range of 0.01~500kHz. Pins 1 and 2 of LM567 are respectively connected to the ground with a capacitor to form an output filter network and a phase-locked loop low-pass filter network. The capacity of the external capacitor C2 on pin 2 determines the capture bandwidth of the phase-locked loop, and its size is Bw ≈1070.

Uin is the effective value of the sine wave signal voltage input from pin 3, and requires Uin ≥25mV, generally between 100~200mV. The external capacitor C1 of pin 1 is the output filter capacitor of the quadrature phase detector, and its capacity is more than twice the capacity of the capacitor C2 connected to pin 2, which should satisfy C1≥2C2.

Figure 4. Top View of LM567

Figure 4. Top View of LM567

(1) Using LM567 as Frequency Modulator

Pin 2 is connected to the input of the low-pass filter of the phase-locked loop. The modulated signal added from pin 2 is filtered by a low-pass filter to remove out-of-band noise and noise, and then added to the center frequency f0 of the voltage-controlled oscillator for frequency modulation, and then the pin 5 outputs the FM signal. The center frequency f0 of the FM signal is determined by the parameters of the RC resistor-capacitor network connected to pins 5 and 6. When LM567 is used as the frequency modulation circuit, only its internal phase-locked loop low-pass filter and voltage-controlled oscillator are used. Changing the parameter value of the RC network can realize modulation to different frequencies.

(2) Using LM567 as Frequency Demodulator

The modulated signal is input from pin 3. When the center frequency of the input signal is equal to the center frequency f0 of the voltage-controlled oscillator in the LM567, the low-pass filter (pin 2) of the loop outputs the demodulated signal.

3.2 Principle of Infrared Speed Measurement Based on LM567

The circuit diagram of the infrared speed sensor based on LM567 is shown in Figure 3. The internal oscillator of the LM567 provides a square wave signal to drive four LEDs to emit infrared light, and its frequency is determined by R2 and C4.

Figure 5. Circuit Diagram of Infrared Speed Sensor

Figure 5. Circuit Diagram of Infrared Speed Sensor

Place the four sensors in the four positions A, B, C, and D in Figure 1. When the vehicle passes the sensor, the infrared rays emitted by the LED are reflected by the vehicle body. The photosensitive tube Q1 receives the reflected light, is amplified by the transistor and converted into a voltage signal, and is sent to the internal phase detector of the LM567 for synchronous demodulation, and then converted into a digital signal by the comparator inside the LM567 and output from pin 8. The output signal is transmitted to the road tester, which triggers the timing device in the road tester to start timing. Similarly, when the vehicle passes the sensor at point B, a trigger signal is generated to stop the timing device and pass the road tester. The internal processor operates to obtain the speed of the vehicle as it passes AA' and BB'.

LM567 is a phase-locked loop audio decoding circuit. In the circuit, it is used for frequency selection, that is, the circuit outputs low level only when the frequency of the 3-pin input signal is consistent with the frequency of the LM567 internal oscillator, otherwise the output is high. Level. In other words, only when the reflected infrared light received by Q1 comes from the LED in its own circuit, the LM567 will output a trigger signal from high to low to the road tester.

The biggest feature of this circuit is to realize the automatic synchronization of the infrared emission frequency and the working frequency of the receiving circuit; That is, there is no special pulse generating circuit in the infrared transmitting part, and the pulse is directly introduced from the detection circuit of the receiving part (LM567 phase-locked center frequency signal).

In this way, the wiring and debugging work is simplified, avoiding inconsistent transmission and reception frequencies caused by changes in the surrounding environment and component parameters, eliminating mutual interference between adjacent sensors, and greatly enhancing circuit stability and anti-interference capabilities.

IV Conclusion

The infrared speed sensor designed based on the LM567 modulation and demodulation function realizes the automatic synchronization of the infrared transmission frequency and the working frequency of the receiving circuit. In addition, it has the characteristics of strong anti-interference ability and stability, low cost and simple structure. Therefore, it can be widely used to measure the average speed of vehicles such as automobiles and motorcycles.

Figure 6. LM567

Figure 6. LM567

V FAQ

What is tone decoder?

A tone decoder is a device that can interpret the sound frequencies and their wavelengths on telephones and other digital tone devices.

 

What is LM567?

The LM567 and LM567C are general purpose tone decoders designed to provide a saturated transistor switch to ground when an input signal is present within the passband.

 

After reading the blog, have you better understand LM567? 

Finally, if you have any questions about LM567, please do not hesitate to leave a message in the comment section below!

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