My senior colleagues and I designed a circuit to sound an alarm when mains goes off and sound another alarm when main resumes. I found two different circuits online: one for power failure detection and the other for power resumption. I merged the two circuits together to get this power failure/resumption circuit:

power failure/resumption

I have not built it yet. Will this circuit work? What are some tips for best practices with circuits like this one?

  • \$\begingroup\$ It's way more complex than it needs to be. Plus, the 555 connections are gibberish. You have pins 2, 6 and 7 connected directly to ground, and pin 1 connected to a capacitor. I assume you're aware that this uses a non-isolated power supply, and it can't be connected to anything else. \$\endgroup\$
    – Dave Tweed
    Apr 13, 2015 at 21:10
  • \$\begingroup\$ Will this work? Maybe but precisely what are you expecting it to do in detail? Please don't bother saying it sounds an alarm - that isn't detail enough by a long shot. I'm thinking how long the alarm sounds for and at what thresholds the alarms trigger in both directions. \$\endgroup\$
    – Andy aka
    Apr 13, 2015 at 21:17
  • \$\begingroup\$ Thanks @DaveTweed for your comment. so you are saying that the 555(its actually 7555) IC is superfluous. the pins 2, 6 and 7 are connected to the neutral. you said it can't be connected to anything else, so i should use a separate power supply? \$\endgroup\$
    – cooljay
    Apr 17, 2015 at 5:47
  • \$\begingroup\$ thanks @Andyaka When the charge in C5 raises to 1/3 of Vcc pin 2 supplies voltage to the capacitor which powers the alarm(k1) for for 30 seconds. for k2, when mains fails transistor Q1 is turned ON and C1 discharges through Q1 to drive the k2 buzzer for a time am yet to calculate. your help will be welcomed. \$\endgroup\$
    – cooljay
    Apr 17, 2015 at 5:58
  • \$\begingroup\$ A good start is to get a clear specification for what the thing is meant to do. "Sound an alarm when mains goes off and sound another alarm when main resumes" is a bit vague. An alarm sounding 24/7 when the power is working fine is going to annoy a lot of people very quickly. \$\endgroup\$
    – Simon B
    Jul 22, 2015 at 8:29

1 Answer 1


Best practices? At your apparent skill level, give up.

I'm not trying to be rude or anything, but:

  1. You're dealing with lethal amounts of energy from a wall plug (true for any country).
  2. You copied/pasted gibberish for us to proofread, presumably because you didn't recognize it yourself. How are we to know that you won't be similarly sloppy with your physical wiring?

Now, if you really want to continue this, I STRONGLY recommend that you at least ditch the custom power supply and use a small prefab wall-wart / power-brick / DC supply / plug-pack / whatever-you-guys-call-them instead. That'll prevent people from getting zapped.

Then, and only then, you can slowly redesign the circuit from scratch, learning as you go. As a hint, I would be tempted to use an RC circuit for both functions that you're looking for. (ask a different question to learn what that is; don't use the comments here for that) I would expect the supply to turn on and off relatively quickly while the RC circuit reacts more slowly; the difference tells you which alarm, if any, to activate. (ask another question about detecting that difference)


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