As shown in the figure, the home anti-theft fire alarm circuit. The circuit is mainly composed of a QM-N5 gas sensor and a monostable trigger composed of 555, a monostable delay circuit and a multivibrator. QM-N5 and potentiometer W1 form a smoke detector. When the flammable gas or smoke exceeds a certain concentration, the resistance between AB becomes smaller. As a result, the potential at point B rises and is enough to make the tube BG3 conduct, and the point C is high. The electric multi-vibrator consisting of IC3, R8, R9, C6, etc., is triggered to emit an audible alarm signal to alert the user. The oscillation frequency of IC3 is about f=1.44/(R8+2R9)C7, and the frequency corresponding to the parameter shown in the figure is about 1000 Hz. At the same time, the adjustment potentiometer W1 can change the alarm threshold. IC1 (555) and R1, C1 and so on constitute a monostable trigger, the metal piece M is connected with the metal part of the door lock. When someone unlocks, the human body's sensing signal causes the IC1 to trigger the set, and the high level of the 3-terminal output makes BG1 is turned on and BG2 is turned off. IC2 (555) and C4, R4 form a one-shot delay circuit, the delay time is td=1.1R4C4, and the delay time corresponding to the parameter shown in the figure is about 60 seconds, so that the owner turns off the alarm power after opening the door; For thieves, it takes a certain amount of time to unlock the door, which usually takes more than 60 seconds. The monostable pulse output from IC2 triggers the IC3 multivibrator to sound an alarm.
The circuit can be used together with an electronic lock, an electronic mechanical lock, an ordinary door lock, etc., and is used as an anti-theft fire alarm device for a household or a warehouse. When someone breaks the door to unlock or a gas leak or fire occurs, an alarm sounds immediately.
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