The circuit of this project uses a 555 in astable mode producing a frequency of about hertz, by using resistors of 100k as R1, 1M? as R2 and capacitor C1 of 1 micro farad. What makes this circuit different is the diodes connected at pin7 and pin2 from voltage supply. When supply is present at these diodes they become forward biased and supply some extra voltage at pin7 and pin2 and consequently circuit does not work. Pin 7 and pin 2 work only when voltage at them is two- third and one-third of Vcc (here 9Volts) respectively & do not work even at high voltage or low voltage. This concept is used in this circuit. When power supply fails to arrive at these diodes pin7 and pin2 functions normally circuit works and sounds buzzer. Even when power supply fails diodes have some work to do; they become reverse biased and does not allow current to flow in direction of power supply.