A touch switch sensor can be built using this simple electronic from the figure below. The touch switch sensor is built using a CMOS circuit, a resistor and a capacitor.
When the voltage separation stage (CMOS) is connected to the ground by touching the bottom contact, the output goes to logic state 0. This state is maintained until the sensor is touched up. Capacitor C1 make out to be continuously in 1 state after connecting the logic supply voltage. This circuit can be supplied with a DC voltage between 3 and 15 volts.