I've been using a float switch wired directly to a small pump for make-up water.  A description is here:  http://www.utahreefs.com/forum/display_topic_threads.asp?ForumID=11&TopicID=277&PagePosition=1&ThPage=1#1442">http://www.utahreefs.com/forum/display_topic_threads.asp?ForumID=11&TopicID=277&PagePosition=1&ThPage=1#1442

After about 6 month's use, the reed switch in the float began to stick (sometimes open, sometimes closed).  After talking with the manufacture and other's here at work, I think the problem was due to surge currents occuring when the pump is first turned on and back surge high voltage across the reed switch when the pump shuts off.  This causes the switch to stick.  Here's an asci drawing of a fix to this problem:


                          0.1u      100
                      +----||------/\/\-----+
                      |  AC 250V   1/4W     |
                      |                     |        +--------+
              ________|_____________|<|_____|________|  PUMP  |
                      |      TRIAC  |>|              |        |
          120VAC      |      400V    /               +--------+
              ---+    |             |                     |
                 |    | 1K          |                     |
                 |    +-/\/\--o-o---+                     |
                 |     1/2W   float                       |
                 |            sw NC                       |
                 |                                        |
                 +----------------------------------------+

Here's a picure of the assembled circuit:

And here's the final end product.  All the electronics fit nicely into a small piece of 3/4 inch PVC pipe:

$12 for the float switch.

$3 for cord, plastic, misc.

$2 for the electronic parts.

Jon,
You told me once where to get the solenoid valve online but I have since forgotten.  Maybe you could post it here for everyone to see.
Thanks!!
Marcus

I need to build/buy a float switch for my refugium so it does not overflow the main tank if one of the u tubes stops.  And I do not want to run the pump dry either.

Any suggestions anyone?

Mark,

I would love for you to come out and help/guide me.  I live in South Jordan, so I know that would be quite an inconvience (sp?).  I would have to have my wife's permission also.  Can you/will you?

Rodney,  I agree 100% with mark.  You shouldn't need a float switch for the application you're concerned about.  A better engineered overflow is the answer.

Utahtaper, I'm usually a big fan of mechanical over electronics, but this switch needed to power a refill pump and I didn't have room to elevate the make-up water.  And it's turned out so nice, I'm putting another in the sump for my new tank!

Mark,

Here is a very rough sketch of what I have.  I am concerned that the overflow will stop for some reason, the pump will continue to pump until it runs out of water, then continues to run dry until it burns up.  Meanwhile the main tank has overflowed by one or two gallons or ??

Why are you concerned that the syphon will break?  Are you getting large air pockets in the upper bend?  If so, tap a small air line fitting into the upper most spot in the bend and get a powerhead (or use one already in the tank) and attach the "venturi" hose to a the air line fitting.  This will prevent air from accumuating in the bend.  If there's no air there's less of a chance that the syphon will break.

The other issue I see from your drawing is the overflow box.  Maybe yours is ok and your drawing just doesn't show it, but you should have another "weir" in the outside overflow box to help maintain the syphon during a power outage.

Switches can be wired to help protect against some of the things you're concerned with, but it's most likely more work then it's worth.  You could wire your return pump through a normally closed float switch (which is submerged normally).  If the water level were to drop down to the switch, it would kill the power to the pump.  If the switch is located at the proper depth, it might keep the tank from overflowing if the syphon were lost.

Although you can do what Jon mentioned above to avoid bubbles in your overflow tubes, I think this is a risky fix.  If the air tube get damage, cracks, or comes off the air pump then you will lose your siphon and most likely have water on the floor.

If you are accumulating bubbles in your overflow tubes then the tube is probably to big for the amount of water that you are pushing through it.

  That's why I don't like non-drilled overflows.  Jake's right, a smaller u-tube is probably a better fix.

My first plan to fix the sticking float switch was to wire it through a RELAY.  Power the switch with only 12v DC and run the pump through the contacts on the relay.  In my quest to find a "simple" 12 volt DC power supply circuit (I didn't want yet another "wall wart" for this), I came across the TRIAC.  The triac works just like a relay only a separate power supply for the switch is not needed and it's a very small component.  I could have used an AC relay and not worried about a separate power supply for the float switch, but the AC relays I've seen are all much bigger then the triac.  So, a small current is sent through the float switch which activates the triac which then sends line current through the triac to the pump.  The resistor in series with the float switch drops the voltage across the switch which helps to keep it from sticking together.

The capacitor and resistor (wired in parallel to the switch) are there to filter out the surge currents when the pump starts.  Hope this helps.

I noticed that a number of people are looking for float switches, we
have the mini float switches. They are $6.00 each not $12.00 or $18.00

 http://www.floatswitches.net - www.floatswitches.net