Question of the Week

How do touch sensitive switches work?

Sun, 16th Dec 2012

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Martin Schaefer asked:

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 on or off. But I accidentally touched the lamp with a piece of soap in my hand that I wanted to put to the side and the lamp went off. We tried out a lot of materials but it didn't work with wood, stones, plastic or clothings. We then found out that you can do the same trick with potatoes. Why does a piece of soap in my hand work so well to switch the lamp on or off as if it was my own hand? Many thanks.


Best wishes,



Why are touch sensitive switches so sensitive to skin, soap, and potato, but not everything else?  We asked Philip Garsed, PhD Electrical Engineer student at Cambridge University about the science behind this home experiment.

Philip -   This effect is all to do with the fact that our bodies, along with many other things are able to store a certain amount of electrical charge.  If you've ever had a static electric shock, you already have some experience of that.  Itís how much electrical charge something in store is known as capacitance.  

In a touch lamp, when an object that can store charge like your hand comes close to the centre circuit, it will influence the circuitís behaviour and usually, this will cause a change either of voltage or the speed of the timer circuit.  And if that changes big enough then the lamp will switch on or off. 

But it isnít just you that can store charge.  Loads of other things can and that includes soap, fruit, vegetables, fizzy drinks in bottles, as long as it can store enough charge to fool and tend them into thinking that there's a hand there then itíll probably work.  On the other hand, materials like paper, plastic, wood, they donít generally really store electrical charge, so they wonít likely to work.  

The technology used in touch lamps is very similar to that that's used in touch screens.  The sensors do vary a little bit in sensitivity zone though.  So, in some cases, youíll need to hold the bar of soap or piece of veg directly with your hand for it to work.  But in other cases, just its presence near the sensor will be enough. 

Why not have an experiment to see what else you can get to work?  I found it really funny that I could scroll through my emails on my phone, using a tangerine from the fruit basket

Hannah -   So, it turns out that anything that can conduct electricity like the inside of a tangerine and also block electricity like the tangerine skin can act as a capacitor and store charge.  Simply putting this capacitor near a touch sensitive gadget like Martinís lamp is enough to switch it on or off.  



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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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