I work on a sailboat and I'm trying to recreate the schematic for the vessel's bilge pumps. The typical bilge pump system has one float switch and looks like this (the "Pump and Float Switch" diagram). The drawback to this design in a tradition wooden sailboat is that it only removes a small amount of water and runs at frequent intervals. (This drains the batteries and causes excessive wear and tear.) The existing solution is use two float switches to pump out 5 gallons every hour or so, but the wiring is spread out and doesn't use any recognizable color scheme... No one around here can quite suss it out.

Two float switches are mounted in the bilge with a vertical separation to create an "upper" and "lower" configuration. The bilge pump will be as low as possible.
Here are the basic rules I want:

  1. Turn on the pump when the upper switch floats (or both switches float, like in reality).
  2. Turn off the pump when the neither float switch is floating.

This is how I assume it works:
(I am not an electrical engineer so I appreciate your patience.)


simulate this circuit – Schematic created using CircuitLab

Will this schematic create the system I expect? Don't hesitate with any questions, since my post may not be as clear as I think it is and thanks for your help!

  • \$\begingroup\$ Both your upper and lower switch need to be connected to the battery and the lower float switch should not be connected to the pump. Also is the "upper" float just meant as a backup in case the lower fails to close? \$\endgroup\$ – user6972 Jun 26 '13 at 17:25
  • \$\begingroup\$ Each of the grounds will be connected together (to complete the circuit) and eventually connected to the grounding plate in the hull, sorry forgot to mention that part. But the floats are not intended to be backups, they are intended to be chained together so the pump removes more water and runs less often. \$\endgroup\$ – Sam Jun 26 '13 at 17:33
  • \$\begingroup\$ The vertical separation between the switches is the key to the description. Think of it this way: when the lower switch floats there is only half a pint of water in the bilge, when the upper switch floats there will be 5 gallons of water. I want to pump activate when there is 5 gallons but not turn off until the last half pint has been removed. Any clearer? \$\endgroup\$ – Sam Jun 26 '13 at 17:44
  • 1
    \$\begingroup\$ Ok, I get your conditions. The idea is it doesn't turn on until the upper switch closes and stays on until the lower switch opens. If that's how you want it to work then what you've drawn is correct. Sorry I'm a bit slow this morning :) \$\endgroup\$ – user6972 Jun 26 '13 at 17:44

Here's my thinking

enter image description here

When the bilge starts to fill the upper float is OFF and the lower switch turns ON but nothing happens because the relay switch is open.

When the upper switch operates (lower switch is closed) then 12.7V is fed to the relay turning it ON. As the level of water falls the upper switch opens but there is still a 12V feed from the relay switch through the lower float switch that will keep the relay ON (effectively a latching circuit).

As the level drops the lower switch opens and the relay switch drops out.

Adding a failsafe switch (mechanical override)

enter image description here

  • \$\begingroup\$ Ok, this makes sense to me. Is there any advantage to this design? To play devil's advocate, if the lower switch should fail, then I assume the pump will never run. \$\endgroup\$ – Sam Jun 26 '13 at 17:48
  • \$\begingroup\$ @Sam Yes, the advantage is that the pump is not running on and off in short bursts and empties out a larger volume when it runs. If the lower switch fails then current will run through the upper switch and turn the bilge pump on. It will then turn off and on quite rapidly (just like the one switch system) giving you an indication something is wrong but keeping the level below the upper limit. Worst case would be a failure of the upper limit switch. Pump would not operate but this is no worse than the failure of the single switch system. \$\endgroup\$ – JIm Dearden Jun 26 '13 at 18:03
  • \$\begingroup\$ @sam the likelihood of the lower switch failing is greater because it is going to be submerged more often. I would stick with your original drawing. \$\endgroup\$ – user6972 Jun 26 '13 at 18:10
  • \$\begingroup\$ I see. In your design JIm Dearden, the upper float switch will still trigger the pump for short bursts if the lower switch and / or relay fail. (These things matter between 1am - 6am when I would otherwise have to crawl into a small, dark, and wet place to fix it.) \$\endgroup\$ – Sam Jun 26 '13 at 18:14
  • \$\begingroup\$ @Sam As a failsafe you could also wire an emergency mechanical switch across the relay contacts. This would over ride the circuit and turn the bilge pump ON regardless of the level switches. It would allow you to make repairs by pumping out all the water. \$\endgroup\$ – JIm Dearden Jun 26 '13 at 18:25

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