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
ID: 891020878.Eg

Message:

Hi Luke!

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, it’s something like an inexpensive radio. Get ready, because the explanation isn’t going to be brief! (grin!)

The touch lamp has several main parts:

A metal antenna
touch-sense circuitry
flip-flop memory bit
low-voltage power supply
high-voltage electronic switch
Light bulb

A touch-lamp is interesting because it uses a combination of "static electricity" and electric current to sense your touch.

Antenna

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. ( It’s not "static" and unmoving. Yet it’s the same stuff as "static electricity.") The vibrating charge is very feeble, it’s 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 isn’t 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.)

Memory bit

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.

Silicon chip

All this circuitry would be expensive, but it’s 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 it’s small and cheap, doesn’t mean it’s simple!

Experiments

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 you’ve 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. (Don’t get in trouble for getting the lamp all oily!)
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 doesn’t 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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