# The Naked Scientists Forum

### Author Topic: How does touching a lamp alter the brightness and / or turn it off?  (Read 2563 times)

#### dentstudent

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##### How does touching a lamp alter the brightness and / or turn it off?
« on: 06/03/2009 09:42:25 »
At home we have a ceiling light that is a row of six lights on a bar, which is suspended from the light fitting. You can dim or lighten the bulbs by holding the bar, and you can turn off the lights by quickly touching and letting go of the bar.

So how does this work? How does the unit know how to dim the lights or when to turn them off?

Here's a picky:

#### lyner

• Guest
##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #1 on: 06/03/2009 22:38:01 »
You know how, when you touch the pin of a microphone jack, you can get a God awful humming? That's because of 'what we in the trade' call "stray pickup".  It occurs because of stray electric and magnetic fields due to the mains etc. which are connected through your body.
Your lamp circuit detects this when you touch the bar and activates a switch in a certain sequence to give you the lights you want.
« Last Edit: 06/03/2009 22:39:36 by sophiecentaur »

#### LeeE

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##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #2 on: 07/03/2009 22:58:58 »
I suspect it uses a capacitance switch.  The metal is slightly charged, turning it in to a capacitor, and when you touch it, it changes the capacitance.  The change and duration of the change in capacitance is is detected by the switch logic to brighten/dim the light, or turn it on and off.

#### lyner

• Guest
##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #3 on: 08/03/2009 00:15:49 »
That certainly is a possible method but measuring capacitance is more complicated than using a high gain amp and a rectifier.
Have you any details?

#### dentstudent

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##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #4 on: 08/03/2009 10:00:54 »
Thankyou SC and LeeE! Both systems sound a bit complex for my layman's knowledge! I remember having a similar system where there was a light unit with a plant, and you could adjust the light in a similar way by touching the leaf.

This stray pick-up - so the lamp unit is reliant on these other loose systems in order to operate? So if the other electrical systems in the house were entirely insulated, would that mean that the light dimming capacity wouldn't work?

It's still not any clearer to me, but I suspect that's a problem of the student and not the teachers!

#### LeeE

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##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #5 on: 08/03/2009 15:03:14 »
Hi Sophiecentaur,

It was just something I recalled learning about years ago, when I got my first 'touch-dimmer' and wondered how it worked, but the wiki article on touch switches 'touches' on them and gives some links.

http://en.wikipedia.org/wiki/Touch_switch
« Last Edit: 08/03/2009 15:04:50 by LeeE »

#### lyner

• Guest
##### How does touching a lamp alter the brightness and / or turn it off?
« Reply #6 on: 08/03/2009 21:03:07 »

This stray pick-up - so the lamp unit is reliant on these other loose systems in order to operate? So if the other electrical systems in the house were entirely insulated, would that mean that the light dimming capacity wouldn't work?

Insulation doesn't remove the sort of pickup I referred to. There are fields of all frequencies going everywhere in the world - particularly somewhere served by mains electricity (for your lamp).

I imagine that an isolated switch, controlling a DC lamp, fed from a battery, on an isolated planet might have to use the capacity measuring system.