Audio Circuit DiagramsFire Alarm

Mains Failure Alarm Schematic Circuit Diagram

Critical Alarm System for Power Interruptions

This circuit has been meticulously crafted to serve a vital purpose. Triggering an audible alarm in the event of a mains power failure. Such an alarm is indispensable, especially for individuals relying on refrigeration to preserve perishable goods. The circuit’s power source is a 12-V mains adapter. LED D5 serves as an indicator, illuminating when mains voltage is present. However, if the mains voltage vanishes, so does the +12 V supply voltage, leaving the voltage regulator IC1 and the relay driver T1-T2 devoid of power. Noteworthy is the relay driver’s energy-saving design, which reduces coil current to approximately 50% after a few seconds, ensuring efficiency. A comprehensive discussion on its operation and circuit dimensioning is available in the article titled ‘Relay Coil Energy Saver.’ It’s worth mentioning that the capacitor’s value at the output of voltage regulator IC1 signifies an alternative use beyond the conventional noise suppression.

Mains Failure Alarm Schematic Circuit Diagram

Effective Power Management for Continuous Alarm Operation

In the absence of mains power, Re1 is deactivated, and the 0.22 F Goldcap situated at position C4 serves as a power source for IC2. When mains voltage is present, C4 gets charged to approximately 5.5 volts. IC1 acts as a 100-mA current limiter, ensuring stability, and D10 prevents any reverse current flow into the regulator output when mains voltage is lost. Despite the Goldcap manufacturer’s assurance that current limiting isn’t necessary during charging. It’s implemented here for added safety precautions. The CMOS 555 in astable multivibrator mode conserves power, allowing the audible alarm to operate for an extended period. Resistors R5 and R6 establish a brief ‘on’ time of just 10 ms, sufficient for the active buzzer to emit a loud warning. If the pulses are too brief, increasing the value of R5 is an option, although it will result in a higher average current drawn from the Goldcap.

Understanding the Significance of CMOS Batteries

CMOS, an acronym for “Complementary Metal Oxide Semiconductor,” plays a crucial role in powering the BIOS firmware in laptops. Even when the computer is unplugged, the BIOS needs to remain operational. This is where the CMOS battery comes into play, ensuring the BIOS functions seamlessly without interruptions, thereby maintaining the laptop’s essential functionalities.


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