Infrared detection alarm

The circuit principle of the device is shown in Figure 1. It is composed of infrared sensor, signal amplifier circuit, voltage comparator, delay circuit and audio alarm circuit. When the infrared detection sensor IC1 detects the infrared signal radiated by the human body in front, the ② pin of IC1 outputs a weak electrical signal, which is amplified by the first-stage amplification circuit composed of the transistor VT1, etc., and then input to the operational amplifier IC2 through C2 for high gain. , low noise amplification, at this time the signal output by IC2 pin ① is strong enough. IC3 is used as a voltage comparator. Its ⑤ pin is provided with the reference voltage by R10 and VD1. When the signal voltage output by IC2 ① pin reaches the ⑥ pin of IC3, the voltages of the two input terminals are compared. At this time, the ⑦ pin of IC3 is changed from the original High level becomes low level. IC4 is an alarm delay circuit, R14 and C6 form a delay circuit, and its time is about 1 minute. When IC3's ⑦ pin becomes low level, C6 is discharged through VD2. At this time, IC4's ② pin becomes low level. It is compared with the IC4's ③ pin reference voltage. When it is lower than its reference voltage, IC4's ①The pin becomes high level, VT2 is turned on, and the buzzer BL is energized to sound an alarm. After the infrared signal of the human body disappears, the ⑦ pin of IC3 resumes high-level output, and VD2 is cut off at this time. Since the voltage at both ends of C6 cannot suddenly change, C6 is slowly charged through R14. When the voltage at both ends of C6 is higher than its reference voltage, the ① pin of IC4 changes to low level for about 1 minute, that is, the alarm continues for 1 minute. . The power-on delay circuit is composed of VT3, R20 and C8, and the time is about 1 minute. Its setting is mainly to prevent the user from alarming immediately after powering on, so that the user has enough time to leave the monitoring site, and at the same time, it can prevent the user from restarting after a power outage. A false alarm is generated during an incoming call. The device is powered by a 9-12V DC power supply, which is stepped down by T, rectified by full bridge U, and filtered by C10. The detection circuit is powered by IC5 78L06. This device can be used for both AC and DC, with automatic and uninterrupted conversion.