Touch Activated Light

The circuits below light a 20 watt lamp when the contacts aretouched and the skin resistance is about 2 Megs or less.The circuit on the left uses a power MOSFET which turnson when the voltage between the source and gate is around6 volts. The gate of the MOSFET draws no current so thevoltage on the gate will be half the supply voltage or6 volts when the resistance across the touch contacts isequal to the fixed resistance (2 Megs) between the source and gate.
The circuit on the right uses three bipolar transistors toaccomplish the same result with the touch contact referencedto the negative or ground end of the supply. Since the baseof a bipolar transistor draws current and the current gain isusually less than 200, three transistors are needed to raisethe microamp current level through the touch contacts to a coupleamps needed by the light. For additional current, the lamp could bereplaced with a 12 volt relay and diode across the coil.