Touch switch

A touch switch is a type of switch that only has to be touched by an object of greater capacitance 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.

Overview

There are two types of switches called touch switches:
*Capacitance touch switch:A capacitance switch needs only one electrode to function. The electrode can be placed behind a non-conductive panel made of for example wood, glass or plastic.

*Resistance touch switch:A resistance switch needs two electrodes to be physically in contact with something electrically conductive (for example a finger) to operate.

Capacitance touch switch

A capacitance touch 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 among others:
*Analog Devices: Model "AD7147" (12 channel).
*Cypress Semiconductor CapSense: The "CY8C21434" can be configured as an 8-channel capacitance sensor, as used in the Apple iPod
*Quantum Research Group, a division of Atmel: Model "QT 113".
*Omron: Model "B6TS04LT" (4 channel) and "B6TS08LT" (8 channel).
*Freescale: Model MPR083 and MPR084 can be configured as an 8-touch-pad sensor [ [http://www.freescale.com/webapp/sps/site/taxonomy.jsp?nodeId=0112698268 "Freescale proximity capacitive touch sensing technology"] ] .
*AD Semiconductor: Model "TSM12" (12 channel) Door Lock SHS-5200.

These devices can also be used as a short-range proximity sensor.

Resistance touch switch

A resistance touch switch works 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.

References

External links

* [http://home.howstuffworks.com/question42.htm HowStuffWorks.com answer to question: How do touch-sensitive lamps work?]
* [http://www.newelectronics.co.uk/article/7628/That-special-touch.aspx New Electronics: That Special Touch]

Example schematics

* [http://www.discovercircuits.com/DJ-Circuits/5vmom1.htm Capacitance touch switch]
* [http://www.aaroncake.net/circuits/touch.asp Resistance touch switch]
* [http://www.analog.com/library/analogDialogue/archives/42-05/AD7147_captouch.html AC Shield Enhances Remote Capacitive Sensing]
* [http://www.analog.com/library/analogDialogue/archives/41-12/capacitance_sensor.html Providing an Edge in Capacitive Sensor Applications]
* [http://www.analog.com/en/prod/0,2877,AD7147,00.html AD7147 CapTouch™ Programmable Controller for Single-Electrode Capacitance Sensors]
* [http://www.analog.com/en/content/0,2886,760%255F788%255F66102,00.html Capacitance-to-Digital Converter (CDC) Technology]


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