We have probably studied many types of touch operated circuits, either using transistors, CMOS ICs involving NAND gates, NOT gates etc. Howver these all circuits are based on the priniciple of electrical conduction, i.e. here when the touch pads are touched by our finger allow small electrical current to pass through our skin, switching the coreesponding flip flop. The present design functions entirely on different grounds and employs gt the technique of sensing our finger's temperature rather than the humidity makes it expetionally unique and much foolproof.
The simple circuit described here is able to sense finger touch by sensing thetemperature differences and switches the output on distinguishing the finger temperature from the ambient temperature.
The present touch switch works
on the temperature dependence of
the forward voltage of silicon
diodes. At O C this is about
650mV, but drops by 2mV per C
increase in temperature.
When a finger
D4 the voltage at A will drop below
that at B and the O/P of the
Op—Amp will go high, causing a TTL
compatible pulse to appear at C
D‘l and D2 provide compensation
against ambient temperature
changes. VR1 is initially set so that
VA is greater than VB by about
The system has the intrinsic advantage
that it may be used in
m0isture—pr0ne conditions in which
ordinary touch switches would be
most unsatisfactory due to their
principle of operation.