Simple Auto Top Off (ATO), inexpensive too!

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CHOMPERS

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Apr 28, 2006
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Sunnyvale Trailer Park
I made an Auto Top Off (ATO) for my saltwater tank and it was surprisingly simple to make. Best of all, it was really cheap too. For those who don’t know what an ATO is, it is a system that replaces water from evaporation. In saltwater tanks, this is very important because it keeps the salinity stable.


Here are the parts that are needed:
Extension cord ($.98)
12v Transformer (free)
Two gang electrical box ($1.88)
Outlet (.68)
Switch (.44)
Float switch ($2)
10A relay ($8)
(not shown) Fountain Pump (free but can be bought at Harbor Freight on sale for $6)
5g bucket (free)

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I started with the float switch bracket. It is made of a scrap of acrylic that I bent with a heat gun. To make things simple, I drilled and tapped the two holes prior to making the last bend.

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Then I made the last bend and drilled and tapped the hole for the float switch and for the fill hose. (I abandoned the hole for the fill hose because I decided that I like to add to the bubble trap. It naturally mixes in with the tank water better there.)

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I then wired the electronics. It’s quite a jumble in the picture, but it isn’t as bad as it looks. There is a high voltage side (110v) and a low voltage side (12v). The low voltage side controls the high voltage using the relay and float switch. The high voltage side has the manual switch wired in as an over ride.

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Just to get everyone caught up on the electronics, here is a run down on what a relay is and why it is useful. In a nutshell, a relay is an electromagnetically controlled switch. In our case it uses the 12v signal from the float switch to throw the 110v switch. Why this is important is because the contacts in the float switch are very small and will quickly be burned out by the current needed to run the pump.

Here is a brief schematic of a relay in operation.

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When leg 86 is charged with 12v, it energizes the coil which closes the switch. This completes the 110v circuit (legs 87 & 30).

Here are a few links that may be usefull
http://en.wikipedia.org/wiki/Relay

http://www.airheads.org/content/view/159/49/

http://www.r1200gs.info/howto/relay.html


The 12v side is wired up starting with the transformer. One wire from the transformer is connected to the float switch and the other to the 12v side of the relay. Then the remaining legs of the float switch and relay are wired together. That’s all there is to the 12v side.

The 110v side is wired starting with the plug. The hot side of the plug (the thin spade) is wired to the manual switch. The remaining lug of the switch is wired to the hot side of the outlet (small slot). The other side of the outlet is then connected to one side of the relay. The neutral wire from the plug is then connected to the other lug of the relay.

My relay is a double pole double throw. You really only need a single pole single throw that is normally open (you can save four bucks).
 
And here is the float switch in the sump. It is in the pump chamber after the bubble trap (baffles) because this is where the water level fluctuates. On the other side of the bubble trap, the water level is always the same (dictated by the top of the first baffle).

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very nice.... i love simple stuff :)
 
And here is the star of the show...well, more like supporting actor...

This is the pump. It is the smallest fountain pump that I could find. It is rated for 66 gph at zero head. The reason for wanting a small pump is because you don't want to change the salinity too fast.

My tank is a 30g with a 10g sump. When I would get home from work, I'd have to dose a little over a quart each day. I had to break it up into a series of small doses during the evening. The ATO only adds about a cup at a time several times a day. Now when I come home, the tank is already full and my critters are much happier.

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I should have posted this picture earlier. It shows the switch and outlet assembled and closed up. The relay is in there under the switches (nice and clean like).

The pump is plugged into the outlet. The switch is there to shut down the operation while sump work or cleaning is going on.


This system draws zero electricity while it is sitting idle. Both the high and low sides are shut down until the float switch closes the circuit. Then the relay turns on the 110v circuit until the sump is filled again. Then both the 110v and 12v sides stop drawing current until the sump needs water again.

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