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From: Winfield Hill
Subject: Re: Single Contact Touch Sensitive Switch
Date: 4 Jan 2003 04:08:10 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> renowned "Dan McDaid" wrote:
>> Several years back I came across a circuit that was just that, a touch
>> sensitive switch that was a single contact. I.E, it wasnt two contacts
>> that were bridged by the skin on your finger, it was just one.
>> I came across this earlier and it reminded me, its a similar kind of
>> thing - except this works through glass and I think it is a different
>> method to actually having an electrical contact.
>> I've seen this kind before where an electronic light switch was made -
>> a blanking plate put in place of the light switch and on the back of
>> the plastic plate was a square of foil.
>> Can someone please explain to me how this works. I've always wondered
>> and don't have a clue, I just don't see what's going on.
> They (sort of) explain it- the circuit runs "hot" @ 240VAC 60Hz
> relative to earth. Putting your hand near the foil sheet is like
> putting a 240VAC signal with a few pF in series into the base of Q1,
> which acts as a rectifier and amplifier. It depends on the hysteresis
> in the relay operation to make it switch cleanly, there is no
"Note: For proper operation, circuit ground must be connected via
a small value, high voltage-rating capacitor to one side of the
mains supply socket. The "Live" side is the right one."
My folks home has touch-sensing light switches that no doubt work
from a similar principle. However the contact is metal, and when
you touch it you can feel a distinct tingle in your finger. !!
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