Over the years, I have installed several submerged float switches in the bilge of my boat. They’re supposed to run the bilge pump when the level of water in the bilge exceeds a certain level and then stop the pump after the water level is lower. They usually work well for the first year or two but inevitably fail. Sometimes they fail simply because they’re submerged in water constantly and they had a very small leak. Other times they failed probably because I left them underwater over the winter and they were frozen.

To improve upon this I wanted to switch, pun intended, from a submerged mechanical switch to a solid state, adjustable switch. I wanted to be able to remove as much of the switch from the water, hoping to extend its useful life. Of course there are a few commercial switches available but they’re not always customizable. Also, buying a packaged switch wouldn’t teach me anything about the circuitry required or give me the intimate knowledge on how they work.

Now, giving credit where it’s due, none of the circuits in my implementation are completely my own. I borrowed liberally from others who have gone before me. My design has two components and I used someone else’s circuit designs for both. My “value add” was the combination of the two circuits and the replacement of a fixed resistor with a potentiometer, in order to make the switch adjustable.

Before I dive into the design of the circuits, a bit of background may be useful. The level of water in the bilge of a floating boat is not a stable thing. Most importantly, the level rises and falls, not always predictably. The average level of the water may stay the same over longer periods of time but the water can slosh back and forth. Both of these properties can confuse a simple water level sensing switch.

There is a simple solution to this problem though. If the level of the water is sensed by allowing the water to conduct between two electrical probes, setting the height of those probes above the water is important. In short, the pump must be able to reduce the water level to below the height of the probes. Otherwise, the pump will start as soon as the water level rises to, or sloshes against, the probes. The pump will then run and reduce the water level just enough  to break the circuit between the probes and stop the pump. But then, possibly within a few seconds the water level will rise, or slosh, closing the circuit between the probes and running the pump for a few seconds again. This cycle will likely continue indefinitely.

However, if the probes are well above the lowest possible water level and if the pump can run long enough after the probes are above the water level to reduce the water to that lowest level, then the pump is much less likely to rapidly cycle between off and on. If you’re understandably concerned about the pump stopping when the water is running into the bilge non-stop, have no fear. As long as the probes are submerged, the pump will never stop, at least as long as it has power.

Now we can jump into the actual circuits. The first part is the water level sensor. It’s the simpler of the two circuits but possibly the more important part. I started with a circuit from Gary A. Pizl, described at http://www.mhsd.org/model/autopump.htm. If you look closely at Gary’s design, he has the switch connected directly to the pump. His design has the problem of continuously cycling on and off as the bilge water sloshes. To improve upon that design I disconnected the circuit from the pump and instead connected the output of the circuit to a timer circuit, described in the next paragraph.

The initial circuit for the timer was taken from John Hewes’s monostable design at http://www.kpsec.freeuk.com/555timer.htm#monostable . I made a few modifications to it, however. I didn’t need the reset capability, because I could simply cut the power to stop the timer. I also found that it works just fine without pulling pin 4 of the 55 high with the resistor to positive voltage. I didn’t put the small capacitor on pin 5 either. The major change was to the value for resistor R1. Instead of using a fixed resistor, I used a potentiometer. That way I could adjust the length of the timer.

The assembled timer looks like this:

Assembled board

And the complete schematic is: