Here is a very useful project of a power failure alarm or indicator circuit, which will start producing a melodious alarm and light up a bright white LED in the absence of mains power. The circuit is very easy to build and using only few components. The circuit is consist of two stages first is a 

12V power supply circuit stage and second is a UM66 alarm circuit.
A 12 volt relay is connected between the two stages in a way as when it will get 12 volts from the supply it will get activated and disconnect the 3 volt DC supply from the melody circuit and in the opposite condition when mains power is absent it will become in the off condition and connect the battery from the alarm circuit and activate the LED2 and alarm circuit.
The circuit is using two LEDs, you can use red or green LED in the place of LED1 and white LED in the place of LED2. The red one will indicate the presence of mains power and the white one will automatically activate in the absence of mains power and also light up a small area so you can see in the dark. For example at night times when any problem happens in the mains supply then suddenly all lights goes off and the house become completely dark, in that case the white LED will activate and provide light from which you can watch any problem in the mains supply. 
A separate power source / battery of 3 volt is used for the alarm circuit, you can use any type of battery for example two AAA or AA cells of 1.5 volts or you can also use small lithium button cell or coin cell of 3 volt. These batteries will last very long because in the presence of mains the batteries will totally disconnect from the circuit.  

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Power Failure Alarm Or Indicator Circuit