Electrical test instrument

1,060,616. Transistor bi-stable circuits. STANDARD TELEPHONES & CABLES Ltd. Nov. 8, 1963 [Nov. 15, 1962], No. 43278/62. Heading H3T. [Also in Divisions G1 and H1] An electrical test instrument, suitable for making continuity tests, includes a trigger circuit containing PNP transistor 1 and NPN transistor 13 whose stable operating state is determined by the magnitude of the load connected across the input terminals 7, 8. If a low resistance (less than 5 ohms) is connected between the terminals 7 and 8, the current through the transistor 1 increases so that the transistor 13 is switched on. The negative-going voltage produced at the collector of the transistor 13 is fed to the base of the transistor 1, and the opposite stable state to that occurring under no load conditions is established. The negative signal also switches on the transistor 15 and hence the lamp 19, thus indicating the low resistance condition at the terminals 7, 8. The particular resistance value at which the circuit triggers is adjustable by the potentiometer 9, and the circuit is temperature stabilized by the resistance chain 9, 10, 11, 12 (12 having a specified temperature characteristic) and the forward biased diode 21. The circuit and its batteries 4, 5 are contained in a torch-like holder, and the leads 7, 8, 24, 25 extend to a probe unit which contains the lamp 19. Details of those assemblies are given (see Division H1).