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Capacitive touch switches boost automotive interface options
With no mechanical parts, as well as conformance to contoured surfaces, cap sense switches provide reliability and lower costs for automotive infotainment, control, and security applications.
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By
Dave Van Ess, Cypress Semiconductor
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Page 1 of 3
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Automotive DesignLine
(02/10/2006 2:37 PM EST)
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It isn't easy being an electrical or electronics engineer in the automotive industry. Mechanical engineers that focus on cams, compression, and horsepower are more the norm, and often times electrical engineers are considered to be glorified wire harness designers. More importantly, higher expectations are placed on automotive designs, above and beyond most any other industry.
Last weekend I went with a neighbor down to the computer store to buy a new keyboard. His computer is two years old and the keyboard no longer functions well. We got in his car and he became irate that one of the radio control buttons broke. Mind you, the car is 12 years old and the keyboard is only two years old, but he has higher expectations of his automobile. We all do.
Switches and buttons for automotive infotainment applications have design constraints not even imagined by non-automotive engineers. They have to contend with:
A wide range of temperature.
A wide range of humidity
Constant contamination from the driver and passengers
Basic technology
Today’s cars have far more switches and buttons than earlier models. These controls also need to be easily installable into increasing more contoured control surfaces. They also have to be cost effective, ruling out hermetically sealed switches. One approach gaining momentum is to convert to capacitive touch switches (cap sense). With no mechanical parts, as well as the ability to conform to contoured surfaces, cap sense switches provide the reliability and cost point required by the automotive industry.
View a full-size image
As seen above, a capacitive switch is essentially a capacitor formed from two adjacent traces; physical laws dictate that capacitance exists between them. If a conductive object, such as a finger, is brought in close proximity to these plates, a parallel capacitance couples with this sensor. Place a finger on the capacitive sensor, and the capacitance increases. Remove the finger and the capacitance decreases. Add circuitry to measure the change in capacitance and it is possible to determine the presence or absence of a finger.
All that is needed to make a capacitive sensor is a trace, a space, and another trace. These traces can be made part of a circuit board with an insulated overlay placed directly over them. They can be built onto windows using glass printed-circuit technology developed for rear window defoggers. Traces can also be screened on the back of an insolating decal and made to conform to curved surfaces, making them workable for almost every application in a vehicle.
To construct a capacitive switch requires:
A capacitor
Capacitance measuring circuitry
Local intelligence to translate capacitance values to a switch state.
A typical capacitive sensor has a value of 10 to 30 pF. Typical finger coupling capacitance to the sensor through 1 mm of insulating overlay is in the range of 1 to 2 pF. For thicker overlays the coupling capacitance decreases. To sense the presence or absence of a finger, it is necessary to implement capacitance-sensing circuitry that can resolve better than 1 part in 100 capacitance change.
A relaxation oscillator is an effective and simple circuit for measuring capacitance. A typical topology is shown below.
This circuit consists of four components:
A synchronous comparator
A current source
A discharge switch
A capacitive sensor.
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