Infrared detection switch circuit

The infrared detection switch circuit made by the author here is a low-cost, high-reliability, short-distance reflective induction switch with an effective distance of 1.5m to 2m. The shortest distance is 30cm. It can be widely used in home, hand dryer, automatic faucet, anti-theft alarm, or unit automatic switch, public toilet water tank automatic control and other occasions, and output with Z1 socket. Because of the simple circuit and few components, it is most suitable for beginners and technical school students. I believe you will benefit a lot from learning and doing in the process of making.

The circuit is shown in the attached figure. The circuit is mainly assembled by the infrared reflection detection module IC and a small number of peripheral components. After the 220V AC power is stepped down by capacitors C1 and C2, rectified by diodes VD1~VD4, filtered by capacitor C3 and stabilized by voltage regulator VD5, a smooth DC voltage of 12V is obtained for infrared detection.

There are infrared light-emitting tubes and infrared receiving tubes installed side by side inside the IC. After power is turned on, the infrared light-emitting tube will emit modulated infrared light with a frequency of 40kHz. When there is no object blocking the direction of the detection head, the infrared receiving tube cannot receive the infrared light emitted by the infrared light-emitting tube. There is no signal output at the output end OUT of the IC, and the transistors VT1 and VT2 are cut off. Relay J does not work, and the entire circuit is in a static state. When there is a person or object in a certain range in front, the infrared light is reflected back and received by the infrared receiving tube. After amplification, demodulation, shaping, and comparison processing by the internal circuit of the module, the output end OUT of the IC is high. It is sent to the base of VT1 through diode VD7 and resistor R1, and VT1 and VT2 are turned on. Relay J is energized and its normally open contact JO is closed, and the circuit power supply works at this time. At the same time, the power supply charges capacitor C4 through VD7.

When a person or object leaves the infrared reflection detection module IC, it returns to the initial static state. VT1 is saturated and turned on again, but VT2 cannot be turned off immediately. Under the discharge voltage of C4, VT2 remains turned on until the discharge current of C4 is less than the base working current of VT2. VT2 is turned off and the relay K is released, and the execution circuit stops working. VD6 is a freewheeling diode. The length of this delay time depends on the parameters of R1 and C4. The role of VD7 is to prevent C4 from discharging through the turned-on IC, so that VT2 can be reliably turned on during the delay time.