Design and production of a human infrared sensor alarm

Infrared alarm circuit and principle

This alarm can detect infrared rays emitted by the human body. When a person enters the alarm area, an alarm will sound. It is suitable for anti-theft alarms in important places such as homes, offices, warehouses, and laboratories.

The circuit principle of the device is shown in Figure 1. It consists of an infrared sensor, a signal amplifier circuit, a voltage comparator, a delay circuit and an audio alarm circuit . When the infrared detection sensor IC1 detects the infrared signal radiated by the human body in front, a weak electrical signal is output from the ② pin of IC1, which is amplified by the first-stage amplifier circuit composed of transistor VT1, and then input into the operational amplifier IC2 through C2 for high-gain, low-noise amplification. At this time, the signal output from the ① pin of IC2 is strong enough. IC3 is a voltage comparator, and its ⑤ pin is provided with a reference voltage by R10 and VD1. When the signal voltage output from the ① pin of IC2 reaches the ⑥ pin of IC3, the voltages of the two input terminals are compared, and the ⑦ pin of IC3 changes from the original high level to the low level. IC4 is an alarm delay circuit, and R14 and C6 form a delay circuit, which lasts about 1 minute. When the 7th pin of IC3 becomes low level, C6 discharges through VD2, and the 2nd pin of IC4 becomes low level at this time, which is compared with the reference voltage of the 3rd pin of IC4. When it is lower than its reference voltage, the 1st pin of IC4 becomes high level, VT2 is turned on, and the sounder BL is powered on to sound an alarm. After the infrared signal of the human body disappears, the 7th pin of IC3 resumes high level output, and VD2 is cut off at this time. Since the voltage across C6 cannot change suddenly, C6 is slowly charged through R14. When the voltage across C6 is higher than its reference voltage, the 1st pin of IC4 becomes low level, and the time is about 1 minute, that is, the alarm lasts for 1 minute.

The power-on delay circuit is composed of VT3, R20, and C8, and the time is also about 1 minute. It is mainly set to prevent the user from alarming immediately after turning on the machine, so that the user has enough time to leave the * site, and at the same time prevent false alarms when the power is restored after a power outage.
The device is powered by a 9-12V DC power supply, which is stepped down by T, rectified by full bridge U, filtered by C10, and the detection circuit is powered by IC5 78L06. This device is AC/DC dual-purpose, and automatically switches without interruption.