Fully electronic switching power supply design using capacitive sensor

This design replaces an electromechanical switch with a capacitive sensor button (circular or square pad on a PCB). The PIC12CXXX MCU is well suited for this application, and a cheap, fully electronic switch can be designed with a small number of components. This approach uses a simple RC delay circuit (Figure 1) whose time constant changes when the sensor is pressed.

To read the status of the sensor, the microcontroller must perform the following two steps (Figures 2 and 3):

1: Change the output state from "0" to "1" (write operation - Twr)

2: Read input status (read operation-Trd)

If the result of the read operation is "0", it means that the sensor is pressed. The finger capacitance is connected in series with capacitor C, making the circuit time constant larger.

Because the finger capacitance is small and the interval between Twr and Trd is less than 1 to 2μs, it is recommended that the PIC12CXXX internal clock use 4MHz.

The hardware circuit diagram in Figure 4 contains two sensor buttons. The trick is to read sensor SB0: GP0 is set as input and GP1 is set as output; and to read SB1: GP0 is set as output and GP1 is set as input. GP4, 5 are connected to LEDs to indicate the status of the sensor buttons. The other pins of PIC12C508 are not discussed here, they can be used as SYNCRO input and TRIAC DRIVER output.

The value of resistor R1 must be adjusted, it determines the sensitivity of the button. The value of capacitors C1 and C2 is not too important. It is recommended to use the same type as the microcontroller input, with equal input impedance.

The software flow is shown in Figure 5.