Infrared detection alarm

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

How it works

The circuit principle of the device is shown in Figure 1 (click to download the schematic diagram). It consists of an infrared sensor, a signal amplification 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 amplification 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 monitoring site. At the same time, it can prevent false alarms when the power comes back after a power outage.

The device is powered by a 9-12V DC power supply, which is stepped down by T, rectified by a full bridge U, filtered by C10, and the detection circuit is powered by IC5 78L06. This device can be used for both AC and DC, and can be automatically converted without interruption.
Component selection and production

The components 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 fine-tuning resistor) 6 VT1 Crystal transistor 9014 1
R7、R10、R11、R17 Resistor 10K 4 VT2 Crystal transistor MPSA13 0.5A 30V 1
R8、R16 Resistor 300K 2 VT3 Crystal 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 regulator 78L06 1
C1, C2, C6, C8, C9 Electrolytic capacitor 47u/16V (C2, C5 use tantalum electrolytic) 5 BL Electromagnetic sounder U=12V 1
C3, C5 Electrolytic capacitor 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 the 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. C2 and C5 must use tantalum capacitors with very small leakage, otherwise the 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 face of the IC1 sensor. Because the frequency range of human activity is 0.1-10Hz, a Fresnel lens is needed to double the frequency of human activity.

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, the other parts can work normally without debugging.

The static working current of this machine is about 10mA. It will enter the standby 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 control purposes.