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How Touch Switches Work

Touch switches only need to be touched to operate

By Gabriel Gache, Science News Editor

12th of June 2008, 13:26 GMT

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Touch switches work by harnessing the properties of the human body

Most of the switches used today in electronic devices are flipped through a mechanical action. Even the almighty digital computer is still relying on one. Mechanically activated switches however have the disadvantage that they are easily damaged by dirt, excessive moisture or physical mistreatment. So in the last two decades or so, touch sensitive switches started to become even more popular, since they can be completely sealed off from the dirt and moisture in the surrounding medium and physical damage is mostly impossible.

There are multiple types of so-called touch sensitive switches, albeit only two are touch switches in the true sense of the word - the capacitance touch switch and the resistance touch switch. Other types of touch switches may work by detecting variations in the temperature of the human body or in the radio-wave reception pattern when the switch is touched.

One of the most exploited properties of the human body in touch sensitive switches is capacitance. Capacitance is the ability of a certain object to retain electrical charge. A touch sensitive switch has a certain capacitance when untouched, meaning that a certain number of electrons are charged inside it, although not enough to flip. When a person touches the switch, part of the electronic charge stored in the body is passed to the switch. An electrical circuit detects the rise in capacitance and then acts accordingly to the previous state, turning a certain device either off or on. Such switches are currently commercially available in integrated circuits as well and are mostly used in touch sensitive lamps and public computer terminals.

Resistance touch switches however work by detecting the difference in the electrical resistance between two metal plates. They are much simpler in construction than capacitance touch switches, although they are probably less popular. As the metal plates of the switch are touched, the switch is flipped to one of the two possible states. Simple resistance touch switches can be built with the help of two transistors coupled in a Darlington configuration, where one of the metal plates is connected to the base terminal of the first transistor.

MOS-FET transistors can also be used to build resistance touch switches by connecting the gate of the transistor to one of the metal plates, while the other plate is powered with a positive voltage through a resistance. This type of switch is called N-Channel, or enhancement-mode.

TAGS:

touch | switch | resistance | capacitance

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