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Fault Signalling Circuit Schematic Circuit Diagram

Fault Signalling Circuit: Expanding Alarm Installations with Sensors

Fault signalling circuit: This circuit was developed to facilitate the integration of various sensors into an existing alarm installation. Sensors such as gas detectors, smoke detectors, door switches, infrared detectors, and others can be seamlessly added to enhance the capabilities of the alarm system.

Operational Logic and Alarm Activation

During quiescent operation, it is imperative that the level at all 44 inputs remain at zero. When a specific sensor is triggered, pin 3 of IC1a is elevated to a high level through R5. As this operational amplifier inverts, its output, and consequently the cathode of D9, maintains a 0 V level. With the anode of D9 at +12 V, the diode illuminates, indicating an alarm condition. A potential drop of approximately 1.2 V occurs across both D2 and D3, causing T1 to activate and T2 to deactivate. Relay Rei is then de-energized, opening the contact that controls the alarm installation. As this contact is normally closed during quiescent operation, a supply failure is also effectively signaled.

Fault signalling circuit Schematic diagram

Resetting and Relay Load Adjustment

After addressing the cause of the alarm and initiating a reset, all inputs revert to zero volts. T1 is deactivated, T2 is activated, and the relay is de-energized. This restored condition is indicated by the illumination of D5. Since D5 and R4 are in series with the relay coil, the relay’s load is slightly reduced, resulting in a lower current draw. Capacitor C2 ensures that during switch-on, R4 and D5 create a short circuit, ensuring reliable relay energization. When using low-current LEDs to keep the current at a minimum, the standard LEDs can be substituted by low-current types. Necessitating an increase in the value of relevant bias resistors (R7, R10, and R40) to 8.2 kΩ.

Noise Filtering and Voltage Peak Protection

Networks C3-R5, C4-R8, and C14-R38 establish low-pass filters, preventing noise voltages from inadvertently triggering the alarm. This is particularly crucial as the cables between sensors and inputs may extend over considerable lengths. Safeguarding against voltage peaks is achieved through zener diodes D6, D8, and D28. This design choice allows the control voltage to surpass 12 V, although it’s worth noting that regulations restrict the use of voltages exceeding 42 V.

Circuit Protection and Component Functions

Diode D1 serves as protection against polarity reversal, and capacitor C1 effectively decouples the supply voltage. The current draw, contingent on the relay, amounts to approximately 200 mA. While Type 4050 ICs can be employed in the IC1 and IC2 positions, it’s essential to acknowledge that these are non-inverting devices, resulting in a reversal of part of the circuit’s functionality.

 

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