Touch switch made of music doorbell chip KD-9300

The figure below is a touch switch made of the music doorbell chip KD-9300. The circuit is novel and interesting. It cleverly uses the function of the internal circuit of the doorbell chip to control the bistable trigger inside the chip with the help of the leakage current of the human body. We know that the music integrated circuit has a very high input impedance. When a person touches the "open" electrode M1, the leakage current of the human body is injected into the trigger terminal TRI of the music doorbell chip through R1, causing the bistable trigger inside the chip to flip, so that the entire doorbell chip presents a low resistance, the transistor VT1 is turned on, the relay K is energized, and the normally open contact can connect the controlled electrical appliance. When you need to turn off the electrical appliance, just touch the "off" electrode M2, and the leakage current of the human body is injected into the Vss terminal of the chip through R2, forcing the bistable trigger inside the integrated circuit to flip again, making the entire chip present a high resistance, VT1 enters the cut-off state, K loses power and releases, and the controlled electrical appliance is powered off.

To ensure safety, the resistance connecting the touch electrode sheet must not be lower than 4.7MΩ high-quality high-resistance metal film resistor.