In http://www.straightdope.com/mailbag/mtouchlamp.html, we get the SDAS report on touch lamps.
While the report is unlikely to lead to any casualties, it was rather off-the-wall in may respects. Like highly misleading and basically incorrect in many respects.
“Nearly all touch lamps work because of a property of the human body called capacitance, which is the ability to store electrical charge.”
Misleading: Capacitance is not a property specific to the human body, in fact any object that conducts electricity has an intrinsic capacitance.
“Most touch lamps employ an oscillator, which is a type of tuned-circuit amplifier that produces an AC current.”
Quite a bit off: I don’t know were they got that definition of an oscillator, it sounds like seomthing from a bad glossary in the Boys Book Of Science. In this case it’s neither correct nor helpful.
One can make an oscillator from a tuned-circuit and an amplifier, sure. That’s how it is described in some of your more pedantic texts, which are trying to explain the two major requirements for
an oscillator-- a frequency-determing subcircuit (the tuned-circuit), and a gain subcircuit (the amplifier).
But this description isnt helpful here, for three reasons:
#1: A typical touch-lamp oscillator doesnt use a tuned-circuit,.
#2: Very few oscilators use an explicit amplifier.
#2: The touch-lamp oscillator, as 98% of oscillators do, supplies an AC voltage, not a current.
" When you touch the lamp’s metal housing, you introduce your capacitance into the circuit. Now the oscillator has to pump charge into a much larger surface area-- yours plus the lamp’s. That causes the oscillator to detune, or change frequency.
The lamp detects this change and toggles the light on or off."
Basically NO: Well, that scheme would work just fine. Only problem, nobody would design it that way as there are much more reliable and cheaper ways to do it.
Most touch lamps use a simple R/C oscillator, which is about 90%
cheaper than a tuned-circuit oscillator.
The detection method you presented, capacitance detuning and detuning detection would also work, but again it would be way too expensive to do it that way. That would require some sort of frequency discriminator, maybe another tuned-circuit ($$) or a phase locked loop ($).
Why do it that way when there are simpler methods?
Here’s a link to an actual schematic of what’s inside a touch lamp controller.
Note: no tuned circuits, no frequency discriminator. It probably works by applying some of the oscillator voltage to the lamp body, then noting the change in current when you touch the lamp.
I’m sure the answer writer meant well, but this answer just wasnt up to the typical Cecil standard.