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 NXP Semiconductors


  • Touch Switch

  • Created: Mar 03, 2014

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Summary

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.


The circuits main component would be the NXP 74AUP1G175 which provides a low-power, low-voltage positive-edge triggered D-type flip-flop with individual data (D) input, clock (CP) input, master reset (MR) input, and Q output. Its master reset (MR) is an asynchronous active LOW input and operates independently of the clock input. Information on the data input is transferred to the Q output on the LOW-to-HIGH transition of the clock pulse. The D input must be stable one set-up time prior to the LOW-to-HIGH clock transition, for predictable operation. Schmitt trigger action at all inputs makes the circuit tolerant to slower input rise and fall times across the entire VCC range from 0.8 V to 3.6 V. The NXP 74AUP1G175 also ensures very low static and dynamic power consumption across the entire VCC range from 0.8 V to 3.6 V.


The near infinite input impedance of CMOS makes it ideal for use in touch and proximity circuits. Usually a touch sensitive circuit needs physical contact, while proximity circuit needs only the presence of an object such as the human body. Touch sensors rely on three features of the human body. Skin resistance is usually a few hundred thousand ohms, the body has a capacitance to earth of around 300pF and the human body acts as an antenna, picking up 50Hz power line fields. The circuit is a proximity switch based on human coupling of the 50Hz power line. A hand very near the plate will induce hum onto the plate and this will be passed to the circuit. The first gate is a XC7SH02 high-speed Si-gate CMOS device, which provides a 2-input, NOR function with both inputs strapped together its sensitivity will depend on the size of the plate. The output of the 74AUP1G175 low-power, low-voltage positive-edge triggered D-type flip-flop can be connected to a relay via a transistor. It could then be used to turn on a light or other piece of electrical equipment.

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