GB1288925A - - Google Patents

Abstract

1288925 Touch operated switches PLESSEY CO Ltd 30 Nov 1970 [15 Nov 1969] 56029/69 Heading H3B [Also in Division G4] A touch operated electric device comprises an A.C. generator G which has one terminal grounded and the other one connected to a touch contact TWC via a resistor R. When a finger is placed on the contact, there is a capacitance TC between the contact and ground so that A.C. current flows through resistor R and is detected. In Fig. 1, a diode is connected across the resistor to provide a voltage which switches transistor T on to provide an output signal at the collector. The screen of the lead to contact TWC is returned to the live side of generator G to minimize self capacitance losses in the lead. The generator may operate at 150 kHz. FET.-In place of the bipolar transistor, an FET may detect the A.C. current flow, Fig. 2. No diode is necessary in this circuit. A potentiometer VR1 allows in phase outputs due to stray capacitance to be balanced out. Differential amp.-Fig. 3 shows two touch contacts with a common sensing circuit DA. Resistors RX, RY form two arms of a bridge, the touch capacitances forming the other two. VR2 allows the bridge to be balanced under quiescent conditions. Diodes D1-D4 and resistors R3-R6 protect the inputs of DA. The phase of the output O/P depends on which contact is touched. Coded displays.-Fig. 4 shows 32 contacts TW1-TW32 in a binary coded touchwire display. Each pair of contacts is connected to sensing circuitry SM1-SM16 as above, and the output from these circuits goes to a binary coding strapping field via a high pass capacitor and a pair of diodes which act as threshold devices. The signals from this field pass to binary digit amplifiers BDA1-BDA5 whose outputs are a binary coded indication of the activated sense circuit. Phase detectors PD1-PD5 give O (odd) or E (even) outputs depending on the phase at their inputs, and all the O and E signals are fed to respective OR gates G1, G2. The outputs of these gates go to OR gate G3 to drive a trigger G3 which gives a signal A when a contact is touched and a signal B when the touch ceases. Signal A sets output toggles TB1-TB5 in response to signals from OR gates G4-G7 on lines D2-D5, while bit 1 of the code is set by the output from OR gate G2. Signal B indicates to an associated data processing device (not shown) that a contact wire has been touched, and the processing device then reads the signals at the outputs DB1-DB5 before giving a reset signal RS. In practice, further circuitry is used to ensure that only touches exceeding a certain duration give an output; it is also necessary to ensure that only one contact at a time may be touched.