Wiring Schematic Diagram

This circuit uses three bipolar transistors to accomplish the Same result (previous post) with the touch contact referenced to the negative or ground end of the supply. The 12volt 20W lamp will be turn on when the contacts are touched with skin resistance about 2M or less. This circuits are available also for other applications, for additional current the lamp could be replace with a 12V relay and diode across the coil.

Since the base of a bipolar transistor draws current and the current gain is usually Less than 200, three transistors are needed to raise the microamp current levels through the touch contacts to a couple amps needed by the light.