Improvements relating to direct voltage transistor trigger circuits

1,035,478. Transistor switching circuits- ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Sept. 2, 1964 [Sept. 3, 1963], No. 34828/63. Heading H3T. A circuit for monitoring a D.C. signal comprises a first transistor to which the D.C. signal is applied together with a square wave so that the transistor is conductive when the combined signal on its base is above a predetermined level and non-conductive when the combined signal is below another predetermined level, and two other transistors forming, with the first transistor, a bi-stable arrangement to accelerate switching of the first transistor between its two states. In Fig. 1 the signal to be monitored is applied between it1 and it2 to the base of Tr1, and the square wave is applied via Tr4 and a limit-setting resistor R5. Initially Tr1 is non-conducting and Tr2 and Tr3 are conducting, but if the combined signal becomes sufficiently positive it causes Trl to conduct and cut off the other two by bi-stable action across the common emitter resistor R1. If the combined signal then becomes sufficiently negative Tr1 will be cut off and the other two will conduct. The voltage levels at which switching occurs are set by R5 and R10. If the input D.C. is too high or too low, so that the combined signal never comes between these two levels, no output will be obtained at ot; the same will occur if there is a failure in the trigger circuit or the supply thereto. The A.C. signal at ot can be detected by passing it to an amplifier, transformer and rectifier bridge to which a relay is connected (Fig. 3, not shown). The voltage at it3 can be sinusoidal if it is squared before reaching the base of Tr1.