[AMRadio] "Magic" lamp QRM

[Peter A Markavage]

Here ya go Don, for what it's worth, by doing a Google search. Also check
the ARRL web page, http://www.arrl.org/tis/info/touchlamp.html  for real
great info on touch lamp problems, radiation, cures, etc.
Pete

Also several explanations off the web:
How does a "touch lamp" work? -- LAM, Enosburg Falls, VT
A touch lamp detects your touch by looking for changes in the electric
properties of the lamp's surfaces. It monitors these properties by
putting a fluctuating electric charge on them. As electric current flows
toward the bulb through the lamp's wires, it passes through an electronic
device that places a high frequency (about 60 kHz) alternating current
onto those wires. This added current causes the lamp's surfaces to take
on a small fluctuating electric charge--first positive, then negative,
then positive, over and over again. This surface charging involves
electrostatic forces, which extend long distances between charged
objects, and occurs even though the lamp's surfaces aren't directly
connected to the lamp's wires. The more surface the lamp has, the more
easily it can hold that electric charge--the greater the lamp's electric
capacitance.
When you plug the lamp in, the electronic device uses its fluctuating
charge to determine how easy it is to add or subtract charge from the
lamp's surfaces. In other words, it measures the lamp's capacitance. It
then begins to look for changes in that capacitance. When you touch the
lamp, or even come close to its surfaces, your body effectively adds to
the lamp's surface and its capacitance increases significantly. The
electronic device detects this increase in capacitance and switches the
lamp's state from on to off or from off to on. The fact that you don't
have to touch the lamp to affect its capacitance means that a touch lamp
can have insulating paint on its metal surfaces yet still detect your
touch. You can also buy touch lamp modules that plug into the wall and
turn the lamp that's connected to them into a touch lamp. These modules
are so sensitive to capacitance changes in the lamp that you can trigger
them just by touching the lamp cord.




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. 
On Sat, 05 Oct 2002 22:05:51 +0000 "Donald Chester" <k4kyv at hotmail.com>
writes:
> 
> I recently noticed a very rough, broad signal that wipes out 30-kc 
> portions 
> of the spectrum from the middle of the broadcast
> band through 10 m. It is especially bad on 160 and 80m. I did a 
> little 
> sniffing with a portable radio and found the source to be a
> table lamp my daughter just bought. The thing doesn't have a 
> conventional 
> switch, but a metal base, and you turn in on and off
> by touching the base with your hand. The thing must have some sort 
> of 
> oscillator somewhere in the 450 kc range, and works by
> detuning the oscillator when you touch it. What kind of rubbish is 
> someone 
> going to think of next to garbage up the radio
> spectrum!
> 
> I have never seen one of these things before. Is this a novelty or 
> speciality item, or is this the way all table lamps are made these
> days? (This was probably the first time in over 20 years since 
> anyone here 
> brought home a new table lamp.) Surely, this must fall under Part 
> 15, so I 
> wonder why the FCC even allows those things to be sold on the 
> market. The 
> noise is far worse than the hash from my computer monitor, and I 
> thought 
> that was already bad enough. I'm surprised the AM broadcast industry 
> isn't 
> raising bloody hell about stuff like this. I can't imagine polluting 
> the rf 
> spectrum with rubbish because someone might be too lazy to flip a 
> switch on 
> a table lamp.
> 
> I'm wondering if anyone else has had problems with interference from 
> those 
> things, and what might be the simplest solution. I tried an in-line 
> rf 
> filter, but it made no improvement whatever. Unless there is 
> something 
> simple to by-pass, the only solution I can think of is to go into 
> the thing, 
> rip out the electronics, and mount a conventional swith with a knob 
> on the 
> base. I'd like to hear form anyone who has successfully fixed one of 
> these 
> things some way other than clip-leading the power cord across the HV 
> 
> s of the plate transformer and throwing the big switch.
> 
> Don K4KYV
>

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