electronic touch switch

Posted on Feb 7, 2014

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,

electronic touch switch
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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. 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...

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