In general applications, the induction spring shown in Figure 4 is used to increase the area pressed by a finger. The
induction spring is equivalent to a metal plate to the ground. There is a capacitor CP to the ground. After pressing the finger,
a capacitor CF is connected in parallel to the ground, as shown in the figure below.
The following is the description of the circuit diagram. CP is the distributed capacitance of metal plate and ground, CF
is the finger capacitance, they are connected in parallel and connected with C1 to divide the input 300KHZ square wave.
After being rectified by D1 and filtered by R2 and C2, the wave is sent to ADC. After pressing the finger, the voltage sent to
the ADC decreases, and the program can detect the key action.
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