thermal touch switch

This ingenious touch switch is operated, not by skin resistance or capacitance effects, but by heat. The circuit utilises the negative temperature coefficient of silicon diodes. The 741 functions as a comparator with positive feedback and, for the purpose of describing the circuit it is assumed that the voltage drop across each diode is initially the same and that the op-amp output voltage is low. Due to the potentiometer effect of R4 and RS the voltage on the non-inverting input of the op-amp wi1l be pulled slightly lower than that on the inverting input.