Touch detecting Switches

Posted on Jul 29, 2012

One of the unusual features of the out-of-production Programmable Drum Set was its use of touch switches for control. Like most touch switches, these detect the difference in the capacitance of a plate when it is being touched by a finger versus the parasitic capacitance of the plate alone. The capacitance added by touching the plate is detected using a square wave oscillator. In the diagram below IC1:A and IC1:B, two gates from a 4001 CMOS Quad NOR IC, form an astable multivibrator with a 50kHz frequency as determined by the time constant of R2 and C2. The square wave output is buffered by IC1:C and used to drive a group of touch switches. Only two switch plates - TS1 and TS2 - are shown in the illustration, but in the PDS, 16 touch switches were excited by this one clock.

Touch detecting Switches
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Touch detecting Switches - image 1
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When the clock waveform is high, the capacitance of the sensing plate (TS2, for example) quickly charges through the forward biased diode D4 whether the pad is being touched by a finger or not. When the clock is low, the capacitor discharges through R5 and with no finger present the voltage quickly falls below the threshold required to switch IC2:C. Because of the inversion of the gate, it's output is a series of positive pulses at the clock frequency. The pulses quickly charge C4 through D5, but the C4 - R7 time constant is long enough that C4 can't discharge between pulses, so the output of IC2:D (which is the "switch" output) is low when there is no finger on the pad. A finger touching the pad dramatically increases its capacitance and the larger capacitance cannot discharge through R5 fast enough to allow IC2:C to switch, so its output pulses go away and its output stays low. With the charging pulses gone, C4 soon discharges through R7 and when its voltage falls below the threshold of IC2:D, this gate's output switches high, which represents the switch being "on". Some touch switch designs tend to be "bouncy", which is to say that they may toggle rapidly between "on" and "off" when the pad is in that gray area between touched and, uh ... untouched. This design gets rid of the "on" bounce with short delays before responding - a finger has to be on the pad for 1mS. or so before the output changes state. To...

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