Fault signalling circuit: The present circuit was developed to make it possible for different sensors to be added to an existing alarm installation. These sensors may be gas or smoke detectors, door switches, 1 infra-red detectors, and others.
In quiescent operation, the level at all 44 inputs must be zero. When the relevant sensor is actuated, pin 3 of IC1a is made high via R5. Since this opamp inverts, its output, and thus the cathode of D9, is at 0 V. Since the anode of D9 is at +12 V, the diode lights to indicate an alarm condition. Across both D2 and D3, a potential drop of about 1.2 V then occurs and this results in T1 being switched on and T2 is switched off. Relay Rei is then deenergized and opens the contact via which the alarm installation is controlled. Since that contact is closed in quiescent opera pion, a supply failure is also signalled.
When the cause of the alarm has been removed and the installation has been reset, all inputs return to zero volts, T1 is switched off, T2 is switched on, and the relay is de-energized. This condition is indicated by the lighting of D5. Since D5 and R4 are in series with the relay coil, the load on the relay is then slightly less, SO that the relay draws a smaller current. Capacitor c2 ensures that at Switch-on R4 and D5 are short-circuit-guaranteeing that the relay is energized.
Where the current must be kept low, the standard LEDs may be replaced by low-current types. The value of the relevant bias resistors (R7, R10, R40) must then be increased to 8.2 kΩ.
Networks C3-R5, C4-R8, and C14-R38, form low-pass filters that prevent noise voltages actuating the alarm. That is important, because the cables between sensors and inputs may be very long. The circuit is protected against voltage peaks by zener diodes D6, D8 and D28. This makes it possible for the control voltage to be higher than 12 V.Although regulations prohibit the use of voltages above 42 V.
Diode D1 protects the circuit against polarity reversal.
Capacitor C1 decouples the supply voltage. The current drawn, dependent on the relay, is about 200 mA1.
Type 4050 ICs may be used in the IC1 and IC2 positions, but it should be noted that these are non-inverting devices, so that Part of the circuit action is then reversed.