Here is another audio annunciator used in factories, industries, hospitals, auditoriums, theatres, etc. to indicate the power failure and resumption. The circuit will give a long beep on power resumption and an intermittent beep when power fails. The power failure alarm circuit that I posted earlier will activate the alarm (continuous tone) only for a few seconds based on the capacitor used. Here in this circuit, it will give an endless intermittent beeping sound during power failure until the push button is pressed or the power resumes. Apart from the audio alarm, it will give a visual indication. The LED flashes until the power resumes even after the alarm is stopped by pressing the push button.
This is a very simple circuit without using any microcontrollers. The circuit is built around the popular timer ICs. 555 and 556.
The circuit is powered by a 12-15v regulated power supply. We can also use an SMPS. The IC U1 is wired as a monostable multivibrator. Once the mains power resumes, it triggers the IC U1. At the same time, one part of the dual timer U2 is triggered by the bipolar junction transistor Q1 at pin 8. Once triggered, it gives a long continuous beep. The duration can be adjusted by varying the values of R1 and C1. Diodes D4 and D5 are used to feed only the positive pulse to the gate of the MOSFET Q2. The output pin 9 of the dual timer is connected to the drain of the MOSFET Q2. One part of the dual timer is wired as an astable multivibrator. The output of the astable multivibrator (pin 5) is connected to an LED through a current limiting resistor R10. The threshold pin 12 of the dual timer is connected to the negative rail via a resistor R7 and the positive rail via a push button switch. The push button switch (PB) is used to turn off the intermittent beeping sound during the power failure. LED (power off) flashes continuously until the power resumes even after the beeping sound is stopped by pressing the push button. The 9V battery supply is fed only to the dual timer U2 when the power fails whereas the 12V is fed to both U1 and U2 when the AC mains power is present. Diode D1 is used to prevent the battery supply to the timer IC U1 when the power fails. The on/off switch in the battery holder is used to turn off the power failure alarm permanently.
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