Touch switch
Types
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 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:
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: QTouch series of semiconductors including the Model QT 113.
Omron: Model B6TS04LT (4 channel) and B6TS08LT (8 channel).
Freescale Semiconductor: Model MPR083 and MPR084 can be configured as an 8-touch-pad sensor.
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 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.
References
^ “Freescale proximity capacitive touch sensing technology”
External links
HowStuffWorks.com answer to question: How do touch-sensitive lamps work?
New Electronics: That Special Touch
Example schematics
Capacitance touch switch
Resistance touch switch
AC Shield Enhances Remote Capacitive Sensing
Providing an Edge in Capacitive Sensor Applications
AD7147 CapTouch Programmable Controller for Single-Electrode Capacitance Sensors
Capacitance-to-Digital Converter (CDC) Technology
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