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Monday, October 8, 2012
Touch Switch_MOSFET_Bipolar designs
Touch Activated Light
The circuits below light a 20 watt lamp when the contacts are
touched and the skin resistance is about 2 Megs or less.
The circuit on the left uses a power MOSFET which turns
on when the voltage between the source and gate is around
6 volts. The gate of the MOSFET draws no current so the
voltage on the gate will be half the supply voltage or
6 volts when the resistance across the touch contacts is
equal to the fixed resistance (2 Megs) between the source and gate.
The circuit on the right uses three bipolar transistors to
accomplish the same result with the touch contact referenced
to the negative or ground end of the supply. 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 level through the touch contacts to a couple
amps needed by the light. For additional current, the lamp could be
replaced with a 12 volt relay and diode across the coil.