This is a very basic alarm designed to let you know when the electricity supply fails. The alarm is powered from a battery which uses no current consumption at all in standby, a battery should therefore last its full shelf life. In use, as long as the electric supply is healthy, relay RLY1 remains energized. Under this (standby) condition, the relay contacts are open and the buzzer will not sound. The battery drain is zero and should last its full shelf life of 2 years or more.
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. Electronics Circuit Application
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