Should the power fail, the relay de-energizes, its contacts close and the alarm is sounded. BZ1 is the audible device and can be a loud piezo type sounder,bell or buzzer. The battery should be chosen to match the sounders operating voltage.
Although drawn with a full power supply; (transformer, bridge rectifier and smoothing capacitor) this can be substituted with a readily available DC adapter. In this case just chose a relay whose operating voltage matches the adapter.Once power fails, there is no way to stop the sounder, this is why switch S1 is in circuit, to mute the alarm.
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