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Touch Sensing Circuit

IP.com Disclosure Number: IPCOM000081501D
Original Publication Date: 1974-Jun-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Williams, JA: AUTHOR

Abstract

The figure illustrates a touch operated capacitive sensing circuit, in which a new method of operation for sensing capacitive changes brought about by the touch of a human body is set forth. Briefly, the method involves charging a reference capacitor in one leg of a differential amplifier and simultaneously charging a human body, which is capacitively connected via a finger touch plate to another balanced leg of a differential amplifier, and then measuring the discharge characteristic of the human body relative to the fixed internal capacitance in the opposite leg of the amplifier. The use of a differential amplifier to perform the sense and output function, converts the analog discharge characteristics to a digital output of "on" or "off" or "1" or "0", as desired.

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Touch Sensing Circuit

The figure illustrates a touch operated capacitive sensing circuit, in which a new method of operation for sensing capacitive changes brought about by the touch of a human body is set forth. Briefly, the method involves charging a reference capacitor in one leg of a differential amplifier and simultaneously charging a human body, which is capacitively connected via a finger touch plate to another balanced leg of a differential amplifier, and then measuring the discharge characteristic of the human body relative to the fixed internal capacitance in the opposite leg of the amplifier. The use of a differential amplifier to perform the sense and output function, converts the analog discharge characteristics to a digital output of "on" or "off" or "1" or "0", as desired.

In the figure a differential amplifier 1 having a low-input bias current serves as a monitor to two matched diode resistor and capacitor networks. A charge is placed on the capacitances in the network in each leg, one in the reference leg, and the other in the touch-sense leg, as labeled. Whenever the reference capacitance is exceeded by body capacitance of the human touching the touch plate, an output is created which is available to drive logic circuits. The input waveform of a typical sort is shown in the figure consisting of twelve microsecond square-wave pulses applied through matched diodes 2, to charge a reference capacitor 3 and to finger touch face plate and body capacitance 4.

The diodes 2 at the input allow the circuit capacitance to charge up to the full square-wave voltage level, typically five volts. As the square-wave input goes to zero level, the diodes 2 are back biased and allow very little flow of current in reverse. The circuit capacitances then discharge through one megaohm resistors 5 in each leg of the network. If the touch-sense circuit is not touched, then the reference side of the network discharges at a slower rate than the touch- sense side of the circuit, and the output of the differential amplifier 1 is considered to be off....