Mains Failure Alarm

This circuit was
designed to produce an audible alarm when the mains power is
interrupted. Such an alarm is essential for anyone whose livelihood
depends on keeping perishable foodstuffs in cold storage. The circuit is
powered by a 12-V mains adapter. LED D5 will
light when the mains voltage is present. When the mains voltage
disappears, so does the +12 V supply voltage, leaving the voltage
regulator IC1 and relay driver T1-T2 without power. The relay driver, by
the way, is an energy-saving type, reducing the coil current to about
50% after a few seconds. Its operation and circuit dimensioning are
discussed in the article ‘Relay Coil Energy Saver’. The value of the
capacitor at the output of voltage regulator IC1 clearly points to a
different use than the usual noise suppression.

Circuit diagram:

Mains Failure Alarm Circuit

Mains Failure Alarm Circuit Diagram

When the mains power disappears, Re1 is de-energized and the 0.22 F
Gold-cap used in position C4 provides supply current to IC2. When the
mains voltage is present, C4 is charged up to about 5.5 volts with IC1
acting as a 100-mA current limit and D10 preventing current flowing back
into the regulator output when the mains voltage is gone. According to
the Goldcap manufacturer, current limiting is not necessary during
charging but it is included here for the security’s sake. The CMOS
555 is configured in astable multivibrator mode here to save power, and
so enable the audible alarm to sound as long as possible. Resistors R5
and R6 define a short ‘on’ time of just 10 ms. That is, however,
sufficient to get a loud warning from the active buzzer. In case the
pulses are too short, increase the value of R5 (at the expense of a
higher average current drawn from the Goldcap).

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