The 555 time base integrated circuit works in a single stable state, and usually the 3 and 7 pins are low. When the hand touches the metal sensing piece M, the human body's sensing signal is added to the foot of the 555 timebase integrated circuit through the 0.1μF capacitor, so that the circuit is turned over into a transient state, then the foot output high level is directly added to the trigger end of the doorbell chip, the chip is triggered and pushes the speaker through the transistor to sound. At the same time 7 pin also becomes high, the power supply through 100KΩ resistance to 4.7μF capacitor charge, when the voltage on the capacitor is charged to 2\/3 of the power supply voltage, the circuit turns over, transient stability ends, ③ the foot becomes low. When M is touched again, the above working process starts again. So every time you touch M, the doorbell is triggered. ③ The 0.01μF capacitor on the foot is an anti-interference capacitor, which can prevent the doorbell from being triggered by mistake.
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