Separate ON-OFF Touch Switch

Providing a touch switch can be a cool way to make your circuit looks sophisticated. Moreover, providing separate touch pad for “on” and “off” function improve the safety, since touching one function doesn’t trigger opposite action to what we expect even at wrong assumption about current state. Here is the schematic diagram of the circuit:

How this circuit works is very simple. The core of this circuit a latch constructed by Q3 and Q3. This latch is a simple nonlinear circuit with positive feedback. If Q3 is “off” then there will be no bias current for Q4  base flowing through 4k7 resistor. In effect Q4 will stay “off”, and Q3 will also stay off since there is no bias current for its base coming from Q4 collector. When we touch the “on” pad, Q2 will be activated and gives bias current for Q3. Once Q3 is biased, Q4 will be activated and provide further bias for Q3 so Q3 will stay active after we release our finger from the touch pad. After triggered, both Q3 and Q4 will stay active until the “off” pad is touched.  Touching the “off” pad will deactivate the Q3, and cut the biasing loop from Q4 collector to Q3 base, so the touch switch circuit will be reset to initial condition: “off”. [Circuit’s schematic diagram source:]