This circuit provides an audio-visual indicator if there is a disruption in the mains power supply or when it is restored. The dual timer IC LM556 serves as the primary component of the circuit. When there is power from the mains supply, the bicolor LED gives off a green glow; however, if there is no power, it turns red. The secondary output of the transformer X1 is 12 volts with 250 milliamperes, and it is stepped down from the mains voltage of alternating current. The output of the transformer is fed into a full-wave bridge rectifier that is made up of the diodes D1 through D4, then it is filtered by capacitor C1 and last it is regulated by IC 7809 (IC1) to provide regulated 9V DC that is used to run the circuit. The power source for the red light that indicates there is no power has been determined to be a 9V battery in conjunction with a pnp transistor designated as T1. Simply adjusting the preset value of VR1, the transistor T1 can be readily made to conduct or switched off. When the power supply is first turned on, the pnp transistor T1 is in its cut-off state. As a result, the green light emitted by the bicolor LED1 is visible. When the power goes out, the pnp transistor T1 begins conducting, and the red light from the bicolor LED1 begins to shine. Because IC2's pin 14 does not have access to the Vcc voltage, the output of that chip's pin 9 stays low, and the transistor T3 does not conduct. However, capacitor C7 (4700 F) maintains an appropriate charge, and as a consequence, transistor T4 conducts, and piezobuzzer PZ1 emits sound continuously for around eleven seconds until capacitor C7 empties completely. When power is restored, the bicolor LED1 returns to its normal green glow, and the buzzer continues to sound for approximately 14 seconds. Monostable and astable

operation modes of the dual timer IC LM556 (IC2) portions have been implemented here, respectively. The position of the external timing capacitor in the monostable section is what decides whether or not a positive or negative output pulse is generated. Diode D7 assures that even a brief interruption in power supply will result in the generation of a pulse as soon as the power is restored. A positive output pulse is created when the capacitor C3 is connected to ground, and the following relationship is used to explain how this

T = 1.1×R5×C3. This positive output is available at pin 5 of IC2, where it can be found. Due to the fact that IC2 is a dual-timer IC, the first output of this component is supplied straight to the reset pin 10 of the second section. As a result, the second timer contained within IC2 begins to oscillate. Its frequency of oscillations, shown by the symbol F0, is controlled by the resistors R6 and R11, as well as the capacitor C6. F0=1.4/(R6+2R11)×C6. At its pin 9, the integrated circuit LM556 sends out frequencies in the form of pulses. These pulses are linked to a npn transistor, which, depending on the output at pin 9 of IC2, either conducts or cuts off. In order to show that power has been restored, a red LED2 that has a current-limiting resistor of 270 ohm connected to pin 9 is used. Because the collector output of transistor T3 is supplied directly into the base of pnp transistor T4, the base biasing of T4 changes, which results in the buzzer sounding off for around 14 seconds.