Touch sensor

A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many lamps and wall switches that have a metal exterior as well as on public computer terminals. A touchscreen includes an array of touch switches on a display. A touch switch is the simplest kind of tactile sensor.

There are three types of switches called touch switches:

A capacitance switch needs only one electrode to function. The electrode can be placed behind a non-conductive panel such as wood, glass, or plastic. The switch works using body capacitance, a property of the human body that gives it great electrical characteristics. The lamp keeps charging and discharging its metal exterior to detect changes in capacitance. When a person touches it, it increases the capacitance and triggers the switch.

Capacitance switches are available commercially as integrated circuits from a number of manufacturers. These devices can also be used as a short-range proximity sensor.

A resistance switch needs two electrodes to be physically in contact with something electrically conductive (for example a finger) to operate. They work by lowering the resistance between two pieces of metal. It is thus much simpler in construction compared to the capacitance switch. Placing one or two fingers across the plates achieves a turn on or closed state. Removing the finger(s) from the metal pieces turns the device off.

One implementation of a resistance touch switch would be two Darlington-paired transistors where the base of the first transistor is connected to one of the electrodes.

Also, an N-Channel, enhancement-mode, metal oxide field effect transistor can be used. Its gate can be connected to one of the electrodes and the other electrode through a resistance to a positive voltage.

Piezo touch switches are based on mechanical bending of piezo ceramic, typically constructed directly behind a surface. This solution enables touch interfaces with any kind of material. Another characteristic of piezo is that it can function as actuator as well. Current commercial solutions construct the piezo in such way that touching with approximately 1.5 N is enough, even for stiff materials like stainless steel.

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