Re: Why doesn't my touchlamp work when I touch it slowly?

Date: Fri Jun 2 13:52:14 2000
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Engineering
ID: 959312446.Eg

Message:

Hi Lee. Good question!

How does a touch lamp work?
http://www.madsci.org/posts/archives/may98/893276774.Eg.r.html

My above explanation ignores several issues which make the circuit more complicated. The circuit needs filters which reject radio waves (so that CB radios and cellphones won't turn your lamp on and off.) It needs spike suppressors on the AC power line and on the antenna, so that electrostatic sparks won't trigger the lamp or even destroy the sensitive electronics. And it also needs some way to adjust its own triggering level.

Suppose the circuit is used with lamps having different sizes of metal case. Will the large metal case fool the lamp into thinking a human is touching it? Or suppose the touch-lamp is placed very close to a metal filing cabinet, computer case, etc. We don't want these large metal objects to turn the lamp on or off, or to overload the antenna circuit so that it can't "feel" the touch of a human body.

To accomplish this, the designer would insert a "high-pass filter" between the antenna circuitry and the flipflop. This filter lets the flipflop ignore the signal, yet still lets it respond to CHANGES in the signal. That way the touch-lamp doesn't care if the lamp's metal case is large or small, or if large metal objects are nearby. It should only respond to fairly sudden increases in the antenna current.

I don't know if all touch lamps act this way. Yours may, but lamps from other companies may not.

Here's an experiment to try. I'm not sure if it will work, because it depends upon the sizes of the people touching the lamp, and upon the dryness of their skin. It might work better with the small bodies of children.

Put your hand upon the lamp. It turns on (or off).
Have a second person touch the lamp. Does it still turn off (or on) while your hand remains touching it?
If a second person can affect the lamp, then have the second person touch YOU. The lamp should respond. Let them touch your face, hair, clothing, tongue, etc. Warning: this might trigger a tickle-fight!

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