Electronic Touch Switch

Mechanical contacts have
the disadvantage that they wear out. That is why it is practical to use
an electronic ‘touch switch’ in some situations. With such a touch
switch the resistance of the human skin is used for the switching
action. The schematic shows the design of a circuit that senses the
resistance of the skin and converts it into a useful switching signal.
The touch switch contacts can be made from two small metal plates,
rivets, nails, etcetera, which are placed close together on a
non-conducting surface.

In this circuit a comparator of the type LM393 has been used. In the
idle state there is, via R1, a voltage equal to the power supply
voltage on the non-inverting input of IC1a. Because the inverting input
of IC1a is set with R2 and D3 to D5 at the supply voltage minus 1.8 V,
the open-collector output of IC1.a is, via R3, equal to the power supply
voltage. This voltage is inverted by IC1.b. The voltage at the
non-inverting input of IC1.b amounts to half the power supply voltage
(through voltage divider R4 and R5) and is lower than the voltage on the
inverting input.

Circuit diagram:

Electronic Touch Switch Circuit

Electronic Touch Switch Circuit Diagram

The output of IC1.b is therefore a ‘0’. If the two touch contacts
are bridged with a finger, the voltage at the non-inverting input will
become low enough to cause the comparator to toggle state. The moistness
of the skin results in a resistance of 1 to 10 MR. If this circuit is
used in the vicinity of equipment that’s connected to the mains, then it
can be sufficient to touch only the upper contact to operate the
switch, provided that the circuit has been earthed. The body then acts
as an antenna which receives the 50 Hz (or 60 Hz) from the mains.

This is enough to toggle IC1.a at the same 50 Hz. C1/R3 prevent this
50 Hz from reaching the input of IC1b and provide a useable ‘pulse’ of
about 10 s at the output of IC1.b. Note that a fly walking across the
touch switch conducts enough to generate a switching signal. So do not
operate important things with this circuit (such as the heating system
or the garage door). Do not make the wires between the touch contacts
and the circuit too long to prevent picking up interference. The power
supply voltage for the circuit is not very critical. Any regulated DC
voltage in the range from 6 to 20 V can be used.

Author: Heino Peters – Copyright: Elektor Electronics Magazine

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