Patent GB-1350785-A (GB)
Title: Switching apparatus
IPC: H03K17/98
Filed: 1971-04-19
Abstract: 1350785 Keyboards CONTROL DEVICES Inc 19 April 1971 [2 April 1970] 27310/71 Heading G4H [Also in Divisions G1 and H1] A non contact making keyswitch comprises two conductors 16 and 122 1 , Fig. 2, respectively connected to an oscillator and an oscillation sensitive switch; a movable conductor 118 can be brought into close proximity with the other conductors by depressing the spring loaded key 110, the increased capacitance between the conductors 16 and 122, being sufficient to pass an activating signal to the oscillation sensitive switch. A grounded conductor 120 screens the conductors 16 and 122 1  when the key is raised. A recess 119 in conductor 118 prevents the signal being shunted to ground when the key is depressed. The body and moving part of the switch may be of plastics, projections 113 preventing the conductor 118 touching the others. The conductors 16 and 122 1  may have separate insulation. The switch may be mounted on a printed circuit board. Keyboard.-A coded keyboard using the above key switches is shown in Figs. 1A and 1B. Keys 10 1 -10 2  are for example associated with different alphanumeric characters while keys 10j are for control purposes such as shift return in a typewriter. When a key is depressed the signal from oscillator 16 is coupled to a transistor switch; in the case of key 10 1  depression will cause the voltage on capacitor 26 1  of switch 20 1  to fall from a value near the voltage of supply 32 to the saturation voltage of transistor 24 1 . This voltage change is coupled to a sense bus 44 by diode 42; the arrangement of the diodes 42 determines the coded output obtained, e.g. while switch 20 1  is shown connected to one sense bus, switch 20i is connected to two busses. The output potentials on the busses are thus high or low depending on which key is pressed. To eliminate errors due to differences in switching times between various keys sense amplifiers 50 1 -50 k  are used, one per sense bus. These amplifiers drive NAND gates such as 64 1 . Each bus is also connected via a diode in OR gate 80 to a strobe circuit 90, in Fig. 1B. Suppose now that key 10 1  is depressed. There will be a drop in voltage at the output of the switch 20 1  and hence on the sense bus 44 1  and any other bus to which the switch 20 1  may be connected as required by the coding used. This drop in voltage on the sense busses will trigger the associated sense amplifiers, but the output of the sense amplifiers will not change because of the NAND gates 64 1 -64 k . The threshold of the strobe circuit is lower than that of the sense amplifiers due to the voltage drop across the diode of OR gate 80. As the voltage continues to drop on the busses the strobe circuit is eventually activated to drive all the NAND circuits 64. Coded outputs thus appear simultaneously on lines 66. No such precautions are necessary for the function keys 10j since, as control devices, they are not connected to the encoded lines 66 but have separate output lines 67. To prevent signals being generated if two keys are depressed together the total current flow through resistor 30 is monitored, and the strobe circuit is disabled by NAND gate 104 when the current exceeds a value set by the inverting input to amplifier 70. The switches may be used as a read-only memory, the output signal corresponding to the bits to be read.