Circuit Schematic Power Failure Alarm using LM555 IC

555Timer. Circuit Schematic Power Failure Alarm using LM555 IC like in figure 1 below is one application of LM555 IC application. 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.

Circuit Schematic

Figure 1. Schematic Power Failure Alarm using LM555 IC(Source:

Component List: 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).


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!

The Original Article : "Power Failure Alarm"

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