Infrared detection alarm circuit

Infrared detection alarm This alarm can detect infrared rays emitted by the human body. When a person enters the monitoring area of ​​the alarm, it will sound an alarm. It is suitable for anti-theft alarms in homes, offices, warehouses, laboratories and other important places. Working Principle 　　 　　The circuit principle of this 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 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. The component selection and production component list is shown in the table below. No. Name Model Quantity No. Name Model Quantity R1 Resistor 47K 1 C10 Electrolytic capacitor 470u/25V 1 R2 Resistor 1M 1 C11 Polyester capacitor 0.1u 1 R3 Resistor 1K 1 VD1－VD5 Rectifier diode IN4001 5 R4 Resistor 4.7K 1 U Full bridge 2A/ 50V 1 R5, R6, R9, R12, R13, R15, resistor 100K (R12 is a linear trimming resistor) 6 VT1 transistor 9014 1 R7, R10, R11, R17 resistor 10K 4 VT2 transistor MPSA13 0.5A 30V 1 R8, R16 Resistor 300K 2 VT3 Transistor 8050 1 R14 Resistor 470K 1 IC1 Infrared sensor Q74 1 R18 Resistor 2.4K 1 IC2 Operational amplifier LM358 1 R19 Resistor 220Ω 1 IC3 Comparator LM393 1 R20 Resistor 560K 1 IC4 Three-terminal voltage regulator 78L06 1 C1. C2, C6, C8, C9 electrolytic capacitors 47u/16V (C2, C5 use tantalum electrolysis) 5 BL electromagnetic sounder U=12V 1 C3, C5 electrolytic capacitors 22u/16V 2 T power transformer 12V 5W 1 C4 polyester capacitor 0.01u 1 S toggle switch 1 C7 electrolytic capacitor 220u/16V 1 IC1 uses imported device Q74, with a wavelength of 9-10um. IC2 uses the operational amplifier LM358, which has high gain and low power consumption. IC3 and IC4 are dual voltage comparators LM393, with low power consumption and low offset voltage. Among them, C2 and C5 must use tantalum capacitors with small drain electrodes, otherwise debugging will be affected. R12 is a key component for adjusting sensitivity, and a linear high-precision sealed type should be selected. During production, a Fresnel lens is installed in front of the end of the IC1 sensor. Because the human body's activity frequency range is 0.1-10Hz, a Fresnel lens needs to be used to multiply the human body's activity frequency. If the installation is correct, connect the power supply for debugging, let a person walk 7-10m in front of the detector, adjust R12 in the circuit, and make the sounder alarm. As long as the components are of good quality and the welding is correct, other parts can work normally without debugging. The static working current of this machine is about 10mA. It will enter the waiting state about 1 minute after the power is turned on. As long as someone enters the monitoring area, it will alarm. It will stop alarming about 1 minute after the person leaves. If the buzzer is changed to a relay to drive other devices, it can be used for other controls.