Here's a schematic of the Sonalert circuit.
It adds a diode and bleeder resistor, such that the bleeder resistor current does not go through the Sonalert after the capacitor is charged, to avoid the problem Tony mentioned.
The value for the capacitor and bleeder resistor may need to be adjusted, depending upon how much current the Sonalert you select requires, and how fast you want it to recover after the power goes out.

View attachment 178100

Actually, there could be a resistor or even a diode across the power supply so that the capacitor could discharge around the loop. But as I think about it the polarity would be wrong, so it would have to be a resistor across the supply. The diode will need to be across the sonalert device to protect it from reversed polarity. A 1K resistor would waste 12 milliamps with the supply on, and you could replace the resistor with a 12 volt filament light bulb for a faster discharge and also a visual reminder that the power was back. But still this concept will be much simpler than the one with the timer chip. RC time constants can be very useful even in some funny circuits like this one.

 

Thanks all

I tried @crutschow circuit. The moment I plugged in the 12 power supply, I heard a little click like something blew. The buzzer beeped but didn't stop.

NOTE : I did something different, I couldn't find the 450k resistor in my local electronic store so I took 470k which I thought is closer. Every other thing was same.

To try out the other circuit by @MisterBill2 , I don't have the 5uf Cap presently (have to check the store another time) Also only have the IN4007. And for the "Sonalert" you mean a generic buzzer? Attached below is the one I have for this project. I also have 4.7uf 50v in my small part box.

Thanks

 

I couldn't find the 450k resistor in my local electronic store so I took 470k

That's fine.
If the circuit doesn't work, then I suspect a wiring error.
Double check the 555 pinout and all the connections.

I don't have the 5uf Cap presently

I used a 5mF (5,000μF) cap in my simulation.

And for the "Sonalert" you mean a generic buzzer?

That should work if its current draw isn't too high.

 

Thanks all

I tried @crutschow circuit. The moment I plugged in the 12 power supply, I heard a little click like something blew. The buzzer beeped but didn't stop.

NOTE : I did something different, I couldn't find the 450k resistor in my local electronic store so I took 470k which I thought is closer. Every other thing was same.

To try out the other circuit by @MisterBill2 , I don't have the 5uf Cap presently (have to check the store another time) Also only have the IN4007. And for the "Sonalert" you mean a generic buzzer? Attached below is the one I have for this project. I also have 4.7uf 50v in my small part box.

Thanks

Neither the capacitor nor the resistor are critical to the functioning of my circuit, and the 1N4007 will work just as well, it just costs more.
In the circuit where "something blew" was that the post#11 circuit or the post #20 circuit?
AND if you really need to know how long the power was off, and exactly when it was off, you need two extra clocks, one an AC powered mechanical clock and the other a digital clock that resets to "12:00" when the power fails and starts from that number when it returns. The difference between correct time and the reading of the mechanical clock will tell you how long the power was off, and the time on the digital clock will tell you how long ago it returned. Really simple math at that point.

 

Neither the capacitor nor the resistor are critical to the functioning of my circuit, and the 1N4007 will work just as well, it just costs more.
In the circuit where "something blew" was that the post#11 circuit or the post #20 circuit?
AND if you really need to know how long the power was off, and exactly when it was off, you need two extra clocks, one an AC powered mechanical clock and the other a digital clock that resets to "12:00" when the power fails and starts from that number when it returns. The difference between correct time and the reading of the mechanical clock will tell you how long the power was off, and the time on the digital clock will tell you how long ago it returned. Really simple math at that point.

The circuit I tried was the one on Post #4 by Crutschow. I don't even need to know the exact time when the power goes off. What I actually need is this... When I'm on Generator and I'm maybe watching TV in the living room, the connection where I change the home power to either Utility or Generator is far from the living room, I just need a notification to alert me Utility power has been restored so I can go switch the changeover and shut the generator down.

 

Big thanks to @crutschow My 555 chip was faulty was why it didn't work the first time. I took a new chip and it worked. The buzzer went off for 5secs, exactly how I wanted it.

What do I need to do if I need the buzzer to go for 10secs? Like how can I calculate the components. This is actually my dream, how to sit and calculate the ratings of components required for something I would like to make. I wish I started this from tender age as I seem be obsessed with anything Battery/Wire/Electronic/DIY stuffs

Attached is the image of the module with the 12v input wires, I would buy break open the adapter, connect the wires, and put them all in a small box which would then have a AC input wires I will connect to my mains from the feed box in my kitchen where it is installed.

I just love the small and simple unit.

Once again, thank you @crutschow

Thanks to @MisterBill2 @bertus @Zaishens @Tonyr1084 for the contribution. You guys rock.

 

What do I need to do if I need the buzzer to go for 10secs?

As I stated in my post #4, the pulse time is about 1.1 times the R2C1 time-constant.
So you increase the value of R2 or C1 (or both) so that 1.1*R2*C1 = 10 sec.

The 555 turns off when the THRS voltage reaches about 2/3 the voltage at the Vcc pin, as determined by the R2C1 charging time (red trace in the simulation).

 

Thanks @crutschow

Can I send you a direct message please, I want to request a circuit diagram. Or should I simply open a new thread?

 

Thanks @crutschow

Can I send you a direct message please, I want to request a circuit diagram. Or should I simply open a new thread?

Click on the poster's name and follow the trail to "start a conversation." Usually that is how it works.

 

Thanks @crutschow

Can I send you a direct message please, I want to request a circuit diagram. Or should I simply open a new thread?

I prefer that you start a new thread if it's unrelated to the subject of this one.