Re: How does a touch lamp work?
Date: Tue Apr 21 15:11:47 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Engineering
Touch-lamps are misleading because they seem so simple, but the electronic circuitry
inside them is pretty complicated. It's not as complex as a television or computer,
itís something like an inexpensive radio. Get ready, because the explanation
isnít going to be brief! (grin!)
The touch lamp has several main parts:
A touch-lamp is interesting because it uses a combination of
"static electricity" and electric current to sense your touch.
- A metal antenna
- touch-sense circuitry
- flip-flop memory bit
- low-voltage power supply
- high-voltage electronic switch
- Light bulb
The metal shell of the lamp is the antenna. The touch-sense circuitry works by giving
the antenna a positive charge imbalance and then a negative one. It does this
over and over very rapidly, so positive and negative voltages appear on the metal shell of
the lamp. In other words, the outside of the lamp has vibrating static electricity on its
surface. ( Itís not "static" and unmoving. Yet itís the same stuff as "static electricity.") The
vibrating charge is very feeble, itís far too weak to make sparks. But it
can be measured.
As the touch-sense circuitry moves charge into and out of the antenna, it measures the tiny
flow of charge in the conductor leading to the antenna. As long as nobody is touching the
antenna, this flow of charge always is less than a particular value. (It's probably a few
microamperes of alternating current, millionths of amps.)
If you touch the metal lamp shell with your finger, the touch-sense circuitry has to work
harder. It isnít just sending charge in and out of the metal lamp anymore. Now
it has to electrify your whole body too. Your body has a much larger surface than the lamp,
so it takes a much larger amount of charge. When you touch the lamp, the circuitry detects
the higher current going to the anteanna. It then sends a signal to the memory circuit below,
which causes the lamp to switch from off to on (or vice versa.)
The lamp contains a pair of electronic switches which control each
other. Their function is to "remember" whether the lamp is supposed to stay
on or off. Together they are called a binary flip flop, and they act as a
single memory bit just like that in a computer. When the touch-sense
circuitry gives them a signal, they "flip" one way and send a signal to
turn the light bulb on. When the touch-sense circuitry gives them a second
signal, they "flip" the other way and tell the light bulb to turn off.
High voltage, High current switch
The lamp has one big transistor in it which controls the light bulb.
This electronic switch can withstand dangerous amounts of voltage (120volts from the
wall, plus surges from distant lightning storms.) It
can pass several amperes of current through itself when turned on.
The flipflop memory circuit gives the main transistor a tiny signal, and this
makes the transistor act like a closed switch. This turns on the light bulb.
If you touch the lamp again, the touch-sense circuitry will detect it, and
send a signal to the flipflop memory circuit. The memory circuit flips,
and stops sending its signal to the main power transistor. The power transistor
turns into an open circuit, and the light bulb turns off.
All this circuitry would be expensive, but itís all reduced to microscopic
size and printed into the surface
of a silicon chip about 1/8 inch square. Transistors, wires, and insulators
are formed on the silicon, and the chip costs less than 1$. But just because
itís small and cheap, doesnít mean itís simple!
I never had my own touch-lamp to play with, but I can think of some
things you might try.
- Hold a metal object in your hand and touch it to the lamp. The
lamp should turn on or off. Metal is a conductor. It is full of movable
charge, and it offers a path for charge flow. It electrically connects your body to
the metal shell of the lamp, so the lamp "knows" that youíve touched it.
- hold a plastic object in your hand and touch it to the lamp. Nothing
happens. Plastic does not contain movable charges, it is an insulator. Plastic provides no path for charges to flow, so your body has not touched the lamp electrically.
- Dip your finger in oil, then touch your finger to the lamp. Nothing
should happen. Oil is insulating. It interrupts the electrical connection
between your finger and the lamp. (Donít get in trouble for getting the lamp
- Tape a strip of aluminum foil to the metal part of the touch-lamp. Now
touch the foil. The lamp should go on or off. Aluminum (as well as all metals) is conductive, and it electrically becomes part of the antenna.
- Wet a strip of paper towel, hold the wet part in your hand, and let it
touch the lamp. The lamp should go on or off. (If it doesnít work, try
using salty water on the paper towel.) The water makes the paper become
conductive. If you do this with a dry paper towel, nothing should happen.
Try wet string. Try wet thread.
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