Automatic Power-Resumption Alarm

By N. Pugalum Perumal


Most power-resumption alarm devices produce non-stop alarm when power resumes. To overcome this problem, here is an automatic Power-Resumption alarm circuit that sounds an alarm for a few seconds only after power resumes. Author’s prototype is shown in Fig. 1.

Fig. 1: Author’s prototype for automatic power-resumption alarm

Circuit and working

Fig. 2 shows the circuit diagram of the automatic power-resumption alarm. It is built around quad NAND gate CMOS IC 4011 (IC1), timer NE555 (IC2) and transistor BC547 (T1). IC 4011 has four NAND gates but in this circuit, we have used only two. A 9V power adaptor may be used for this circuit.

Fig. 2: Circuit diagram of the automatic power-resumption alarm

Whenever power failure occurs this circuit does not sound any alarm. But when power resumes it produces an alarm sound for about 11 seconds. This sound duration is determined by NE555 timer (IC2) configured in monostable mode. One-mega-ohm resistor (R4) and 10µF, 25V electrolytic capacitor (C2) are the timing components. Duration of the alarm can be increased or decreased by changing the values of R4 and/or C2. This is determined by the relationship:

T = 1.1×R4×C2 in seconds

Whenever power resumes, input pins 1 and 2 of NAND gate N1 become high, so NAND gate N2 output pin 4 also becomes high. This high output switches on transistor T1. IC2 receives a trigger pulse from T1 for a short time. Hence, the monostable circuit is switched on to produce an alarm.

Construction and testing

Fig. 3: PCB pattern of the automatic power-resumption alarm

A single-side PCB pattern for the automatic power-resumption alarm is shown in Fig. 3 and its component layout in

Fig.4. After assembling the circuit on the PCB, enclose it in a suitable box.

Fig. 4: Component layout of the PCB

This is an easy-to-assemble circuit. Its alarm sound can be increased by using an additional amplifier circuit.

Download PCB and component layout PDFs: click here

N. Pugalum Perumal holds a diploma in electrical and electronics. He is a design enthusiast.

This article was first published on 28 October 2016 and was updated on 7 February 2020.


    • Sure! You can use comparator as well, but you will need an extra components for it. Lets say you have comparator you will need ref. value in one of the inputs. Your ref value should be enought to trigger the timer 555. So you have 2 options:
      1) resistive voltage devider
      2) zener diode

  1. I’ve printed the pcb and made the circuit, but it does not stop the alarm, I used bc546 instead of bc547, could this be the reason, please help me as I spend a lot of money till now on this circuit and I want it to work.

  2. I’ve printed the pcb and made the circuit, but it does not stop the alarm, I used bc546 instead of bc547, could this be the reason?


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