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I think you try the human infrared sensing, it can detect people, detect a delay for a period of time and then flush, so that the hardware circuit and software are relatively simple.
If there is a professional design, it can be constructed according to the drawings.
Theoretically, it is possible, when someone approaches, a relay signal output is triggered, and an external siren horn can be connected. But I don't know if you've noticed, that thing's sensing distance is very limited, is there twenty centimeters? It is not practical, and the sensing distance of infrared probes on the market is at least about ten meters.
Induction is a sensor, control is a solenoid valve or something.
* Motherboard adopts digital circuit, digital transmission, 16-bit digital is more accurate. >>>More
Then find a professional master to repair or replace the old with a new one!!