Unravelling the Mystery of Touch-Activated Lamps

[T@P]

you've all seen one before, a lamp that turns on when you touch a metal part of it. i was wondering how they worked...?

Here are my ideas on how it works: well it can't be sending an electric current through you, that would just be dangerous and wouldn't relly serve any purpose. then i noticed it didnt work when i touched it with some object (conducting and not) and that really proves that it isn't a current, but also shows that it could be by heat. now i know that heat spreads itself out evenly on metal objects, so i was thinking that if you touch any metal part of it, you would heat it up a little, and thus if you have a heat sensor somewhere in the lamp it could turn on. only problem is the light bulb gets hot and invariable would turn on (or actually off) the lamp were it built that way.

anyone actually know why/how they work?

 

[Chi Meson]

A tiny charge is indeed sent into you (or taken from you) but at such a low current that it is not harmful. Without getting into the details (which I'd have to look up) it would be nearly impossible to get a dangerous shock from a properly designed switch box.

The tiny charge causes a voltage drop across a tiny capacitor; an IC reads this drop as an "on" switch.

Correct me if I am wrong: I assume that you cannot be harmed because there is a transformed dc circuit that is wired through a capacitor which is connected to the conducing body of the lamp. Only a limited current can "flow through" a capacitor in a dc circuit.

 

[T@P]

im not sure i understand; if there was a current flowing through the metal frame then there would have to be a resistor in it or it would short out. if this resistor is you, you would have to hold it to keep it from shorting out. if it isnt, you need another resistor in there and an entirely separate loop (if its the loop with the lamp, then its an AC circuit not DC and its at 120v...), meaning there has to be a battery in it.. and i never heard of one of these lamps actually running out. not to mention they don't have that third plug in the outlet as a safety in case it goes wrong and since you are essentially touching a live wire the whole time, it would make a lot of sense.

another thing though, which is that the old lamps made in the 30s ish like this were made such that as you held it it became steadily brighter. I am not *completley* sure that's true, but reasonably so, and I am not sure that would work with your model.

perhaps some clarification?

 

[Integral]

Inside the lamp is a solid state module which is attached to one side of a capacitor in the module. Essentially the frame of the lamp becomes one plate of the capacitor. When you touch the plate you change the capacitance of the capacitor, thus a small current will flow. This current is detected by the solid state module, which causes a relay (probably a solid state relay) to change state, thus turning on the lamp. Since the plate of the cap you touch is ground and insulated by a dielectric from the hot plate you are never in danger of electrocution.