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How does a touch lamp work?
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!
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.
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 antenna. 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 (120 volts from the
wall, plus surges from distant lightning storms.) It can pass
several amperes of current through itself when turned on.
The flip flop 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 flip flop 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. |