GB931639A - Electric alarm circuits - Google Patents

Abstract

931,639. Transistor switching circuits. WHITE & RICHES Ltd. Jan. 8, 1962 [Jan. 9, 1961], No. 846/61. Class 40 (6). An electric alarm circuit includes a two-state transistor trigger T1, T2; two resistancecapacitance networks R1, C1 and R2, C2; a cyclically-operated change-over switch RL2/1, RL2/2 in a first position connects the junction of R1, C1 to the input of the trigger, and the capacitor C2 to a discharge means R8 and vice versa in a second position; and indicating means L responsive to the state of the trigger circuit; the arrangement being such that a steady D.C. applied to the resistors R1, R2 increases the voltage on the capacitor connected to the trigger input to a value at which, should the change-over switch fail to operate, the trigger circuit changes its state operating the indicating means L. As described, cyclicallyoperated valve of an oxygen breathing apparatus controls, via switch SW1, the relay RL2 which operates the pairs of change-over contacts RL2/1, RL2/2. If, due to a fault, the contacts remain in the position shown for an excessive length of time, capacitor C1 charges via resistor R1 until the potential on the base of transistor T1 is sufficient to switch the trigger circuit to its other state. Relay RL1 then releases, completing the circuit to indicator lamp L. A temperature-sensitive resistor X1 is included to compensate for changes in relay resistance with temperature. The circuit of the Provisional Specification (not shown), relates to a delay circuit having only a single capacitance resistance network connected to the trigger circuit input. Closing a switch allows the capacitor to change until the trigger circuit switches operating the indicator lamp. The relay RL1 is provided with a second contact which then disconnects the capacitor from the trigger input and connects it to a discharge circuit to prepare it for a subsequent operation.