Question of the Week Podcast

Question of the Week episode

Mon, 17th Dec 2012

How do touch sensitive switches work?

Shadow Dexterous Robot Hand holding a lightbulb (c) With permission of Richard Greenhill and Hugo Elias

We find out why touch sensitive switches are sensitive to skin, soap and potato but not everything else!  Plus we ask how do drinks & exercise affect the taste of your chewing gum?

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  • How do touch sensitive switches work?

    Hi Naked Scientists Team! I hope your fantastic show will go on next year and the decision makers from the BBC will be changing their mind. There was an intriguing thing that happened today and we couldn't explain it to ourselves. Every time you touch a touch desk lamp it goes...



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The common touch-sensitive lamps work by sensing the small AC current which flows through the capacitance of your body when you touch the metal part of the lamp (I can sometimes fell a small tingle, if I touch it lightly).

Your body, potatoes, and soap contain slightly-salty water, which allows more current to flow between the mass of your body and the capacitor of the switch, triggering the light.

This signal is used to control the AC phase at which a triac fires, adjusting the intensity of an incandescent light globe. However, it is not recommended for compact fluourescent or LED lamps, which have internal electronics to keep the lamp brightness constant.

Experiment: Try holding the soap or potato on a dry stick, and see if that draws enough current to trigger the lamp.

Also see evan_au, Mon, 10th Dec 2012

Capacitance-type touch switches don't require electrical contact, some can even be activated at a distance (aka proximity sensor). The presence of AC mains-electricity in the person isn't required for capacitance-type touch switches to work either, e.g. a touch-screen on a (battery operated) tablet used out-of-doors (away from mains electricity). 

There are (indoor) touch switches which are triggered by the tiny mains AC signal in the person's skin, but these are not the capacitance type switches and electrical contact is required, (skin has to touch metal). RD, Mon, 10th Dec 2012

Why does my touch lamp blink sometimes when itís off? Static-electricity isnít a problem in our house. We do have ~58 year old wiring and two plug outlets. (but I made sure the wide plug terminal is plugged into the neutral.) It happens even when Iím in bed and the lamp is across the room. Itís not a consistent interval between blinks Ė but on the order of 3-4 minutes. David, Thu, 15th Aug 2013

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