Design of a touch-type anti-theft alarm circuit

The circuit is shown in the figure. IC 1 is the time base integrated circuit NE555, which forms a monostable trigger with R1, C1, C2, and C3. When no one touches the metal sheets M1~Mn, the circuit is in a steady state, that is, the ③ pin of IC1 outputs a low level, and the alarm circuit does not work. Once someone touches any of the metal sheets M1~Mn, the human body induced potential inputs a negative pulse (actually a noise pulse) to the ② pin of IC1, and the monostable circuit is triggered to flip into a temporary steady state, and the ③ pin of IC1 jumps from the original low level to a high level. The high-level signal turns on the transistor V1 through the current limiting resistor R2, so V2 is also saturated and turned on, and the audio integrated circuit IC2 is connected to the power supply to work. The audio signal output by IC2 is amplified by the complementary amplifier composed of transistors V3 and V4, and then drives the electric speaker BL to emit a loud alarm sound. As the monostable circuit is triggered to flip, the power supply starts to charge C2 through RI. After about 1.1R1C2 time, the monostable circuit automatically returns to a stable state, the output of pin ③ becomes a low level, the alarm stops sounding, and is in a warning state.

M1~Mn can be made of steel or aluminum sheets, with a small hole drilled in the middle, and connected to any metal part that needs protection. For example, door lock handles, metal shells of instruments, etc. The time base integrated circuit ICI can be selected from NE555, 5G1555, uA555, LM555, FX555, etc. V1, V2, and V3 can all use 9013, 9014, 3DG6, 3DG8, or 3DG12, etc., B>=100. V4 uses PNP type transistors, such as 3AX81, 3AX31, 9012, etc., B>=80. IC2 uses KD9561, which is a four-tone analog integrated circuit. The peripheral components of IC2 only have one oscillating resistor R3, which can be in the range of 180~510KO. The smaller the resistance of R3, the faster the alarm rhythm, and vice versa. BL uses 0.8W, 80 electric speakers, which can also be replaced by piezoelectric ceramics. A sound-assisting cavity must be installed. However, the volume is still much smaller. The device can be powered by a 4.5~6V battery or by a rectifier after stepping down and rectifying.

If this circuit is changed to connect a resistor to the positive pole of the power supply on pin 2 of IC1 and a mercury switch to the negative pole of the power supply, and the mercury switch acts as a trigger device, this circuit can be used as any vibration alarm, tilt alarm, earthquake alarm, temperature rise alarm, etc.