Power outage failure alarm - 555 vs relay

I am looking to add a power outage alarm to my 5v Arduino Uno. The alarm will be powered off a second 12v transformer that will sound if the Arduino loses power. I was originally thinking of using the same electromechanical relay that I am using to switch some 120V loads http://pewa.panasonic.com/assets/pcsd/catalog/js-catalog.pdf

I was thinking of wiring the alarm to the NC contact, then holding the contacts open while there is power, so when power falls out, the circuit will close. I then saw some 555 timer ICs that look pretty good, except that the alarm voltage has to be equal or less than the microcontroller voltage, so this wouldn't work.

Should I go with the electromagnetic relays, or would the 555 be a better option (if I can use 12v alarm)? I thought of using a latching relay, but there wouldn't be power to unlatch it during an outage, correct?

Thanks in advance!

pekasus:
I am looking to add a power outage alarm to my 5v Arduino Uno. The alarm will be powered off a second 12v transformer that will sound if the Arduino loses power.

And if the mains supply goes off, that transformer will have no power either.
I’d use a battery to power the alarm. Use the 12V supply or the power supply to the Arduino (before it goes into the power jack) to hold in a relay when the power’s on and when it’s off, turn on the alarm.
Also, have some method of switching off the alarm, even if the power isn’t restored. A loud alarm gets very annoying, very quickly!

Also, have some method of switching off the alarm, even if the power isn’t restored. A loud alarm gets very annoying, very quickly!

Spoken from experience, I presume? :slight_smile: That’s a good idea.

The reason I don’t want to do the battery is because if it is a general power outage, I don’t want the alarm to sound. I was going to power the 12v buzzer and light off of a wall wart on another circuit in the house. That way, if the Arduino blows out OR the breaker flips, I still get the alarm off another breaker.

I asked a similar question before, and a EM relay was recommended and I was told that it will hold up over years with the coil being energized all the time. I just found this thread http://forum.arduino.cc/index.php?topic=14208.0 which talks about using a Fairchild MID400. That looks much easier than the 555 circuit. I would be limited to 7V with it, though.

I think the easiest thing is to make a 5v alarm system. I could then use that to power the arduino if the first system fails so at least I can just run the alarms through the Arduino.

Thanks for the input!