TOUCH SENSITIVE ALARM
[attachment=16343]

How it works
Heart of the circuit is a single 40106 hex Schmitt trigger
inverter IC, together with a piezoelectric alarm unit and a
handful of other components. The first Schmitt Inverter
(IC1a) together with its associated 10kW-feedback resistor
and 22pF capacitor forms a simple oscillator with a nominal
frequency around, 1MHz. The oscillator output passes via
a 15pF capacitor to a second Schmitt inverter, IC 1 b,
which in turn is AC-coupled to a full-wave rectifier circuit.
As long as the door handle (or metal plate) is left
untouched, the oscillator output will alternately switch the
input to IC1b above and below its two trigger voltages.
IC1b's output will thus switch alternately low and high at the
clock rate to drive the rectifier circuit. If, however, the door
handle or metal plate is touched, most of the signal from
the oscillator will be capacitively shunted to the


Warning
VISHA kits are sold as stand alone training kits.
If they are used as part of a larger assembly and any
damage is caused, our company bears no responsibility.