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From: Craig Findlay
Subject: Re: Single Contact Touch Sensitive Switch
X-Newsreader: Forte Agent 1.93/32.576 English (American)
X-Original-Trace: 12 Jan 2003 16:36:52 +1000, 184.108.40.206
Date: Sun, 12 Jan 2003 05:36:58 GMT
NNTP-Posting-Date: Sun, 12 Jan 2003 16:36:58 EST
Organization: Comindico Australia - reports relating to abuse should be sent to email@example.com
Winfield Hill wrote:
>> 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."
There is a (patented) technology which can be used for touch sensitive
switches where there does not need to be actual contact between the
switch "contact" and the finger. The contact itself can be insulated
by a layer of plastic, glass, etc. It is also selective enough to be
used in 16 or 20 key keypads with a pad pitch of maybe 20 to 25mm.
The technology used to be used here in Australia in a particular brand
of PIN-pad used in EFT point of sale terminals. If I remember
correctly the principle involves detection of a slight change in
frequency of an oscillator caused by hand capacitance, but is self
calibrating to take into account local environment capacitance, etc.
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