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open circuit to run things? Electronics people?

Soothing Shrimp

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Is there something cheaply manufactured that would make things run when turned OFF?


My idea- hack into cheap 9v battery air pump(s) to run when electricity goes out.


The only way I can figure is when electricity is turned on there's an open circuit, and when off the circuit closes to operate the air pump.


Perhaps some kind of solenoid?


Some kind of relay?


Edit: Made an assumption they were powered by 9v, but I see in a later post there are some on 2 D batteries.

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I got you thinking huh. I was also going to invest in some of those battery air pumps, that way when I come home if the power is off for an extended time I would just drop one of those in the tanks.

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Following - and FYI I will be setting up one of my battery powered air pumps this weekend. I'm going to put it in a bucket, hooked to a sponge filter and see just how long it will last. It takes 2 D batteries... so at least I will have some idea of how long I would have with them.

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Hi Soothing,

You need an electro magnet - the thing that creating magnetic field when you plug it into the main 110V-220v, then next to it you need to place Reed switch it has to be normally open (NO) and connect your pump via reed switch to the battery and plug you electro magnet. So when you have the power the switch is open and the pump is off when you have power cut off your magnet is off and switch is on and pump is working until your power is back on. Easy - busy mate.

Here the ling for your reed switch - http://www.ebay.co.uk/itm/230958928195?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT%C2'> uk ebay, I'm sure you can find one on US ebay, don't buy Normally close switch or your pump will be working when the power on and not working when is off.

I don't thing is going to be a problem to find an electro magnet, you can serch on ebay and buy one for few dollars. All should be cost you 10$ or so.

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  • 1 month later...

This is a easy problem to solve with a SPDT 5V relay.


Use a cellphone charger adapter and wire the 5VDC output to the relay coil.

Connect the relay Common pin to the pump positive.

Connect the relay Normally Closed pin to the battery positive.

Connect all the DC grounds together.


When the AC power is on, the relay is energized and the common is connected to the Normally Open pin which has nothing on it.

When the power is lost, the common pin is connected to the Normally Closed pin which completes the circuit and the pump turns on.


I can draw you a diagram if it will help.

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Any 5VDC SPDT relay will work as long as the current capacity is greater than your pumps draw. These ones are good for up to 10A and are reliable. I've used them many times before.



When the AC power is good, the relay is energized and the pole (arrow) will be pulled to the Normally Open contact. Nothing happens.


When the AC power fails, the relay is not energized and the pole (arrow) goes back to it's natural resting position Normally Closed. The circuit is completed and the pump turns on.


To power the relay coil, just use a cell phone charger. Cut the cable and split out the red and black. The relay coil may or may not be polarized depending if it has a built in protection diode.


If you have any questions, let me know.

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  • 1 month later...

This is all over-complicated. If you are still considering this idea, let me know. You could do it without buying anything extra and it would be more reliable and safer.

Sent from my XT1049 using Tapatalk

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Thought I posted my reply, guess I forgot


You could always just use the DC pump 24/7.

The battery/wall wart would essentially work as a UPS.


This uses everything you have AND will keep running exactly the same no matter what. It does have the disadvantage of using the DC pump as your main air pump.

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