Infrared blocking alarm device

It is very suitable to use the invisible light emitted by infrared light-emitting tubes to make alarm devices. When an object or a person passes through the warning area, the infrared light is blocked and the alarm device will immediately send out an alarm signal. It has the advantages of high reliability, long life, and strong anti-interference ability.

The alarm device circuit is shown in the figure. After the power is turned on, the GaAs infrared light emitting diode emits infrared light. These infrared lights are received by the phototransistor 3DU, and the internal resistance of 3DU decreases, making BG1, BG2, and BG3 turned on, BG4 turned off, and relay J does not work. When the infrared light is blocked by an object or a human body, the internal resistance of 3DU increases, making BG1, BG2, and BG3 turned off, BG4 turned on, and relay J works, turning on the alarm device and sending a signal.

When selecting components, the light-emitting diode should be selected according to the size of the control distance. When the control distance is small, a low-power light-emitting diode, such as the HG410 series, can be selected. When using it, be careful not to exceed 30 mA in working current; when the control distance is large, the HG500 series or GH520 series can be selected.

When debugging, you should first connect the infrared light-emitting diode and make it have a certain working current, and then connect 3DU, BG1, and BG2. At this time, use a multimeter to measure the voltage across R5. When there is infrared light, the voltage across R5 should increase; if the voltage across R5 does not change, you can adjust R2 and check whether the circuit is wrong. If the circuit is correct, you can appropriately reduce R1 to increase the working current of the light-emitting tube, or you can use the method of shortening the distance between the light-emitting diode and the photoelectric triode to solve it. After the voltage across R5 is normal, connect BG3 and BG4, and adjust the resistor R7 to make the relay work normally.