How Do Touch Lamps Work?
My sixth grade students are studying electric circuits and want to
know how a touch on and off lamp works without a switch. The Clapper also
has them wondering. Any explanations?
This is going to involve some hand waving and guessing. I am an ex-
electrical engineer, but it has been a long time since I have done this
stuff. OK, here goes: I think the switch that works in a touch lamp
is actually a "capactively coupled field effect transistor". All
transistors can be designed to act like little relay switches and FET's
are particularly sensitive to residual static charge and subtle
potential differences. Basically the touch lamp has a solid state switch.
My guess regarding the clapper is that it has an audio amplifier that has
a limiter on the input. Any audio signals that are below a particular t
threshold will not get through. When the sound pressure level gets above
a certain level it triggers the amplifier which is set to clip - switch
on. This on signal is the trigger to turn on the lights. The clapper also
might have a hi-pass audio filter on it to prevent mis-firing. A sharp
noise like a hand clap has more high frequency content than low. By f
filtering off the lower frequencies that might be more prevalent in
conversation and allowing only "shrill" noises through one would hope to
eliminate an false starts on the part of the clapper. However, my bet is
that an extremely loud stereo or some serious shouts could confuse a
clapper. You might try leaving this query in the "Ask An Engineer" s
Section of this BBS. Good luck.
Nick P. Drozdoff
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Update: June 2012