I am making my own little power-failure warning system. So when the power goes off a LED and buzzer goes on.

Right now it looks like this: https://i.stack.imgur.com/IzGrt.jpg (Sorry for not making it the right way)

But... The coil is not moving. It's in "off" mode the whole time. And that is kinda strange.

So when I was searching for the error I connected the coil to a 9V battery (even if it's a 5v relay). Then the coil was suddenly moving. So I changed the voltage on the adapter to 9v. But still no reaction.

And YES the current is going trough, because I placed a LED after to see if there was current at all. And it was.

So what could be the problem?

Best regards fiskarn

And well again I connected a LED to the relay-circuit and it was lit. So there's current. Also measured the voltage to 4.9 wich would be enough? But as I said I even tried with 9V and 12V.

Also tried on 3 different relays with the same results.

  • \$\begingroup\$ Different relays have different activation voltages. Are you sure you are providing enough voltage? Is everything connected correctly? Also, please make an attempt at a better circuit. The one you have provided doesn't really clear anything up. \$\endgroup\$
    – Envidia
    Mar 16, 2017 at 23:41
  • \$\begingroup\$ Yes. Well as I said this relay is 5V. And also it works when connecting to a 9V battery, but not when changing the adapter to 9V (or even 12..) Don't really know how to make right circuit examples. But well there's the relay and when OFF I want the LED connected to the battery to light. \$\endgroup\$
    – fiskarn
    Mar 16, 2017 at 23:57
  • \$\begingroup\$ Assuming a standard relay footprint, your drawing only shows one connection to the coil. The other is open. \$\endgroup\$
    – user133493
    Mar 17, 2017 at 0:27
  • \$\begingroup\$ Isn't it the coil terminals (4.bp.blogspot.com/-hJ9A5YS7bsI/TyKZIRsbr3I/AAAAAAAAAyw/…) that are the two pins activating the coil? And the one between them are the ground for the N/C and N/O? Or how should I connect for the coil to activate otherwise? \$\endgroup\$
    – fiskarn
    Mar 17, 2017 at 0:36
  • \$\begingroup\$ Oh one of those. It would have been better to give this information at the start. Moving on, your diagram doesn't show a current-limiting resistor for the LED. Do you have one? \$\endgroup\$
    – user133493
    Mar 17, 2017 at 0:46

1 Answer 1


Solved hehe. Since the LED (wich is a 12v LED) was in series with the relay it didn't work. Changed to parallel and now it works just fine!


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