Power Failure Alarm Circuit

This minuscule power failure alarm circuit is a power supply monitoring device that will set off a piezo-speaker when the ac mains mains supply cuts off. It is very helpful to indicate the loss of power supply to some power-critical instruments such as a lifecare device installed in a hospital. Alarm activation at the right time is helpful as it signals that there is a power outage and urgent action should taken to recover the situation by providing an alternative power supply.

This is a 9V battery operated, low component count, and light weight circuit which can be safely connected to any AC230V outlet. At its heart is the famed little timer chip LM555 (IC1) configured as an astable multivibrator. Another important component is the “home-baked” opto-coupler (PC1). No bulky power transformer is used here, hence the circuit is “noise-free” (no electromagnetic interference). Further, this makes the enfolding task very simple!

Power failure alarm prototype

Although a variety of opto-couplers are available in the market but it may so happen that you may not have one ready at hand when you get a spark and want to check the idea as soon as possible. Fortunately, it is easy to construct one at home using an LED and a phototransistor/photo resistor. Just take a small piece of common circuit board and solder the components as indicated here. Keep an eye on the spacing of the components; it should be as close as possible with a slight gap between them. In the prorotype one 5mm white LED + 5mm LDR combination was used. After the construction, enclose the optocoupler in a suitable opaque tube, or cover the whole unit using short-length of heatshrink tube.

Home made optocoupler

The circuit is straight forward. As stated, IC1 is here wired as a gated-astable producing an audio-frequency (AF) output (~1KHz) to drive a standard ordinary/piezo-speaker. Switching of the astable is controlled by the power status detection circuitry built around PC1 and associated components. Switch S1 (SPST) is the system on/off switch, and LED2 (5mm Red) is an optional visual indicator.

Power failure alarm circuit

Parts: IC1: LM555, T1: BC547, T2: BD139, D1: 1N4007, LED1: White 5mm, LED 2: Red 5mm, R1: 100K, R2: 5mm LDR, R3: 1K, R4: 10K, R5,R6: 4K7, R7,R8: 1K, C1,C2: 100nF, C3: 470uF/16V, S1: SPST switch, BATT: 9V battery, SPKR: Ordinary/Piezo-Speaker (not active buzzer).

Lab Note: Value of R3 test selected. Use theoretical/empirical method to find the exact value suitable for your home-made opto-coupler.

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