DAY BROWN wrote in a message to Elvis Hargrove:
 DB> A few years ago, I tried to make a touch switch for a 12 vdc 
 DB> desktop computer I was building.  The 555 oscillator circuit 
 DB> was triggered by a change in the frequency at touch, and it 
 DB> occurred to me, that it would discharge any static in my body 
 DB> even before the relay turned on the computer.
 DB> But despite checking the circuit several times, it would not 
 DB> work.  Well, I drove over to Collins' to hook up to a 12 volt 
 DB> battery charger and brought along the battery to run the new 
 DB> computer.  Worked like a charm, so at first I thought that it 
 DB> was somehow picking up the AC ripple of the battery charger, 
 DB> and how that mighta been affecting the oscillator in some way. 
 DB> Then when I showed it again to Collins, I realized that 1- it 
 DB> worked fine, but 2- I had forgotten to hook up the battery 
 DB> charger, and it was running directly off the car battery. hmmm. 
 DB> took the computer and battery outside, put them on the pickup 
 DB> tailgate, and it was dead again- no trigger. hmmm. took them 
 DB> back into his shop, turned on the lights, and voilla! the
 DB> touch switch boots it up fine.  huh?
 DB> Come to find out, that the 60 cycle AC magnetic waves were 
 DB> being absorbed by my body when in the shop, and triggering the 
 DB> 555 oscillator, but when outside, or in my 12 volt home, there 
 DB> were *no such magnetic waves* and it didn't work.
 DB> Some folks worry about the waves from the VGA that they sit in 
 DB> front of a few hours a day, rather than the electric blanket 
 DB> they sleep next to. Duh?  Is frequency important?
Yeah,  it is.  Consider what microwaves do to meat,  for example,  and what 
they might do to you if you were to continually absorb small doses of the 
stuff...
Or consider that the problem with tv and monitor tubes was supposed to be 
x-rays,  which are way up there in frequency.
I remember the first touch switch I ever countered in a bit of equipment, 
that was in a Panasonic b&w tv set.  They used a small board that had six or 
eight transistors on it,  some of which was an RF oscillator and when you 
touched the contact this detuned things enough to make the circuit work.  All 
this for an on-off switch!
A better deal is detailed in Don Lancaster's CMOS Cookbook,  where the stray 
capacitance of the body does the job.  That one should work anywhere and not 
need any power line frequencies around...
email: roy.j.tellason%tanstaaf@frackit.com 
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