I have one on my bedside table and last night I tried to turn it off by smacking the base lightly with the paperback book I was holding. No effect. I then discovered that it will neither turn on nor off if the base is touched by anything inanimate. But when touched by me (or by the paw of the very puzzled and slightly alarmed dog), it switches off (or on).
So I’m curious. How do they work? How come they don’t work if you touch the base with something other than your hand?
I believe they work by sensing a change in capacatance. I won’t go into electronics 101 here but you book is not conductive so the lamp never knows that anything touched it. Your skin is moderately conductive. Try this as an experiment. Inert a metal object like a butter knife between the pages of the book. Hold the book which you know will not set off the lamp and touch the metal to the base.
My guess has been that toucing the lamp affects the resistance in a circuit somehow making its way through the shell of the lamp. I think as a child, I was curious about this too, and IIRC I was able to find other objects that would also change the state of the lamp. I’m sure someone will know exactly what it is, but I’m fairly certain that the shell of the lamp propagates some form of electric field that is detectably altered by contact with the skin.
I believe they work by electricity. No, no, not just the bulb; I believe they produce a small voltage difference from ground. People (since we’re filled with water and electrolytes) can allow a small current run from the lamp to ground. Paper, though, and probably the other inanimate objects you used, are very poor conductors, so the lamp could not detect any voltage difference. Have you tried touching the lamp with, say, a penny?
If you have a circuit with a very high series resistance the change in voltage when you touch it is significant enough to be detected and used to operate a switcing amplifier, from there it is a simple matter to use the output of that to operate a switching device.
There are other ways to do this, for example the human body acts like an aerial picking up the magnetic fields which emanate from the electrical equipment and cabling in the surrounding area. These magnetic fields rise and fall with mains frequency.If you touch a circuit designed in the right way it will sense the signals that have been induced into you and operate.
You might be concerned that all these magnetic fields passing through your body are not good for you but at the minute levels we are talking about this is most unlikely.
Circuits can be made incredibly sensitive the average radio is capable of detecting signals in the picowatt range 10[sup]-12[/sup] so it isn’t difficult to make circuits than can pick up mains hum from your body.
Another way I have seen used mainly in induction proximity sensors but can be applied to humans too.
If you build an oscillator circuit you can change its frequency of operation simply by moving differant material close to it, if you have ever prodded around in a radio while it is running you will have an idea of the effect, when you do this you finger (or whatever turns you on) can change the radio tuning and you lose the station you were recieving.
By designing a cicuit that will switch state when the frequency of oscillation changes you can arrange things so that the presence of your finger will be picked up and the light operated.
You could use ultra-sound if you wanted or even a device that recognised the cahnge in temperature when you touch it.
Depends which is cheapest usually.
Which probably leaves it down to body capacitance which is where we started.
Most of the answers have already been given by other posters.
[list]
[li]temperature of the human hand[/li][li]resistance - the human body conducts electricity and so your finger can close two contacts placed closely together[/li][li]radio reception - the human body can serve as an antenna, so look for a change in radio-wave reception when the switch is touched[/li]and of course, the most common method, capacitance. A quote from the site: «Capacitance is the capacity an object has to hold electrons. The lamp, when standing by itself on a table, has a certain capacitance. This means that if a circuit tried to charge the lamp with electrons, then it would take a certain number to “fill it”. When you touch the lamp your body adds to its capacity. It takes more electrons to fill you and the lamp, and the circuit detects that difference.»
There is a musical device based on this principle called a theremin. You may never have heard of it, but no doubt you’ve heard one in operation. They were commony used to make flying saucer sounds in SciFi movies and Jimmy Page played one on “Whole Lotta Love”. That swooshy, dying airplane like sound… that’s a theremin. A favorite project for many junior electronic engineers.
Also, if you’ve ever seen a flourescent lamp that refused to come on until you waved your hand near the balast, that’s the same phenomenon.
As for the lamps, 99% of them use the human body capacitive effect, though I disagree slightly with the “How Stuff Works” web site on this. The reason for my disagreement is that for the capacitive effect to operate the way they suggest, at the very least, you have to actually touch the switch plate. If you’ve ever played with one of these lamps you’ll know that actual contact is not required - sometimes close proximity is all that is required. Slide your feet across the carpet and you can operate the lamp from as far away as half an inch. Also, these lamps sometimes operate themselves due to line fluctuations (power dips). Ergo, the sensor is actually detecting a change in charge, not specifically a change in capacitance. Your body capacitance merely facillitates the storage of this charge.
I think that was a phase shifter or flanger on that track. ELO was notorious for over use of them. A good example of a theremin is the Beach Boy’s Good Vibrations or Ed Wood’s Plan 9 from Outer Space.
I built this thing once with a 1001 Electronics thingamabobby, it was a motorcycle noise maker. It went ‘vrrmmmm’ and the harder I squeezed the wires, the higher pitched it 'vrrmmm’d. I figure it’s because by squeezing the wires harder, I was allowing a stronger current to pass, which higher-pitcheder the noise.
Please accept my apologies, didn’t mean to put things across in less than transparent way.
Please give me another chance.
There are many ways to achieve the same results in electronics just as there are many ways to stack up bricks to build a house.
If you were to touch a live circuit but were wearing rubber boots so that you did not make a path for the electricity to go through you and into ground you would still soak up some electrons and your body would gain a few volts - maybe more if it is house mains.
Now if you let go of the power those electrons would still be there, you have stored them and your body has still got those extra volts as a result.
This is what capacitance is, what is more you will stay charged with those electrons until they can find a way to escape to ground. The better the insulation of those rubber boots the longer it will stay.
When you touched the circuit a tiny current flowed until your storage capacity was filled and then it stopped.
Sometimes using water as an analogy can help.
Imagine the you have a small vessel like a cup and you dip into a large source of water such as a river - the drop of water level in the river would be so small as to be unnoticeable and it would be replenished so quickly you wouldn’t have the time to measure it - in electrical terms that is a low resistance supply.
Now take the cup of water from a jug the is being filled very slowly, the water level drops noticeably and if you put a detector such as a float switch in the right place it would operate.The water level would be renewed eventually so resetting the float switch - in electrical terms this would be a high resistance supply and the float switch is analogous to the electronics that ‘sees’ the fall in voltage and turns the light on.
Actually, an inductive proximity detector probably won’t sense a human, since humans don’t have much of an effect on inductance (only magnetic-type materials such as iron do). A metal detector is an inductive proximity detector- the iron in metal changes the oscillator frequency as you described. The lamp circuits are capactive proximity detectors.
You don’t need to actually touch the plate to affect the capacitance- it’s similar to intefering with TV reception by putting your hand near the antenna, or using a theremin. A theremin senses very tiny capacitance changes produced by moving your hands around it, and produces a varying tone. A lamp switch is about 99% less complicated (=cheap), and really just senses a change in voltage level in the circuit- this change is usually from a capacitance change from the user’s hand, but could also be affected by induced voltage (from 60 Hz power, as others have mentioned).
By the way, the reason people are easily picked up with capactive proximity detectors is because we’re mostly water, which has an unusually large dielectric constant (around 80, compared to 1.00 for air).
I get it. I think.