Capacitive Touch Switch for Automotive Applications 2

One wire, spacing, and another wire are all required to form a capacitive sensor. Directly covering these traces with a layer of insulating transparent plastic film can make them part of the circuit board. They can be made on car windows using glass printed circuit technology for rear window defoggers. It is also possible to use screen printing technology to make it on the back of the insulation relief carving and match it with the curved surface, so that they can be suitable for almost all applications in automobiles. Constructing a capacitive switch requires: ● a capacitor ● a capacitance measurement circuit ● local intelligence for converting the capacitance value into a certain switch state. A typical capacitive sensor has a capacitance value of 10pF~30pF. The capacitance of the finger coupled to the sensor through a 1mm insulating transparent plastic film is usually 1pF~2pF. When using thicker transparent plastic film, the coupling capacitance decreases. In order to detect the presence or absence of fingers, it is necessary to design a capacitive touch sensing circuit that can measure the 1% capacitance change amplitude caused by fingers. Relaxation oscillator is an effective and simple capacitance measurement circuit. Typical topology structure of relaxation oscillator circuit  diode