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Touch plates and high resistance

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This is a circuit diagram for a 'Travel touch alarm'

Travel Touch Alarm | Circuit Project Electronic

Q1) The 555 is activated when pin 2 goes to low voltage. But why does it do this when someone touches the touch plate?

In this circuit Audio Booster, with one transistor

Q2) there is another touch plate, what exactly happens when the plate is touched here?

I can understand a touch mechanism where your finger is completing the circuit at a high resistance, but in the two examples above this isn't the case, there is no circuit completion - so what exactly is happening?


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Most Helpful Member
These circuits usually use the capacitance of your body to ground to activate the circuit. But it's not entirely clear in your two examples how that works.

The human body also picks up a significant amount of 60Hz from the building wiring (just touch an oscilloscope probe to see) so that could also be involved.
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