How do touch sensitive switches work?

Some modern lamps are touch sensitive - rather than having to flick a physical switch, the light comes on with a gentle touch from a finger. But only certain materials seem to be...

16 December 2012

Part of the show The Science Behind Broadcasting

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Credit:

S Mundasad

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Question

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,
Martin

Answer

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. It's 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 circuit's 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 isn't 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 it'll probably work. On the other hand, materials like paper, plastic, wood, they don't generally really store electrical charge, so they won't 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, you'll 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 Martin's lamp is enough to switch it on or off.