Interlock circuit

1,082,426. Transistor switching circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. May 5, 1966, No. 19903/66. Heading H3T. [Also in Divisions B6 and G4] An electronic latching circuit suitable for use in a touch-operated keyboard comprises a number of bi-stable circuits each having a " set " input terminal, the " set " input signal for each circuit causing the other circuits to be reset. In Fig. 1 (not shown), capacitive type buttons (Ka-Kk) when touched feed a " set " signal to the corresponding bi-stable circuit (Ba-Bk) and light a corresponding lamp. The " set " signal also passes through an OR gate (G) to generate a " reset " signal (at R) which is effective to reset any of the bi-stable circuits that are in the " set " state except that or those receiving the " set " signal. All circuits may be reset by touching button (K R ). The buttons (Ka-Kk) are one group of a number of such groups, a message being set up by pressing one button in each group and transmitted if the correct lamps are lit. If two buttons in a group are touched two lamps light and if this is not corrected as by leaving one finger on longer than the other, a fault condition may be signalled. In Fig. 2 which shows one button circuit A and one bi-stable circuit B the button K forms part of a bridge R 1 , R 2 , (C 1 and C 5 ) and Cin. Touching the button unbalances the bridge so that pulses applied at 10 pass via the bridge to transistors T 1 and T 2 to cause a capacitor C 2 to progressively charge. This causes the bistable circuit to be changed to its " set " state and lamp L to light. The lowering of the potential at the input of B causes the diode D2 of an OR gate G to conduct and so to cause transistor T7 of the reset pulse generator R to conduct. The resulting reset signal is fed in parallel to the other bi-stable circuits and also through D3 and R4 to the input of the gate where it constitutes negative feedback. As such it is smaller than the original " set " signal and accordingly does not reset bi-stable B.