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Apr 09, 2011, 12:53 PM
Registered User
Cope's Avatar
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Water activated switch?

Hi folks I was thinking is it possible to make some type of switch that turns on my 12V bilge pump when it (The switch) gets wet? I could just put in a servo actuated switch but I would like the pump to just come on automatically. This way I reduce the chance of it running dry or sinking.

Thanks for the time and advice.
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Apr 09, 2011, 01:05 PM
Tachikaze's Avatar
From our combat web page. We use these to activate our bilge pumps on our combat warships.
Apr 09, 2011, 01:10 PM
Registered User
Cope's Avatar
Thread OP
Thanks. That's just the kinda thing I was looking for. I hope our local Radio Shack has the stuff.

I will be running in fresh water, How long before the plates need cleaning?

The thing in the bottom center of the pick is the pump right?

Thanks again.
Apr 09, 2011, 03:03 PM
Registered User
Check at the Radio Shack. Years ago they sold a water alarm, battery powered, that was the same idea all built. Used on my sump pump for years. Also Google high water alarm under shopping. Many, many.
Apr 09, 2011, 06:37 PM
Registered User
they sell things called float switches specifically for bilge pumps at boating stores.
Apr 10, 2011, 01:24 AM
1/2 a bubble off
Apismelifera's Avatar

Ram Bilge Pump Boat Saver

Try a web search on that. The first one I found was:
Apr 10, 2011, 10:02 AM
Retired for now
Two strips of metal with wires soldered to them between the power source and the pump will work. One strip is placed vertically a little higher ( the strip to the pump) than the other verticle strip and both about an inch apart. When the water gets to both strips it makes contact and starts the pump. Shuts off when the water gets below the higher strip. Used that on a bunch of real boats over the years. Pete
Apr 10, 2011, 10:30 AM
Registered User
Cope's Avatar
Thread OP
Thanks, That was my first idea. It cant be that simple, Can it? I'll have to solder up some wires and start playing around. This sounds like a great way to get a few 12V wake ups. Thanks again.

Funny thing is, The boat that I thought was leaking from the stuffing tube was really leaking from a hole in the hull. I had to tear everything out to find it tho......So Now I don't really even need the switch but I still like the idea.

Thanks again.
Apr 10, 2011, 02:30 PM
Tachikaze's Avatar
We battle usually once a month, so, in prepping the boat for combat these systems are all checked. So, clean once a month.
Apr 10, 2011, 05:08 PM
Thermite + ice = Big boom.
boaterguy's Avatar
Look at either the Century or Flying Fish RC boats, they are juvenile but they have a water safety system, I do not know exactly how it is wired but It only runs the motors in water. Another way is to make two contacts, one with a hinge and metal strip protruding, and a float, so when the water ascends the float comes with, and touches the second contact.
Apr 10, 2011, 09:35 PM
fredspak's Avatar
I have built and used the circuit described by Tachikaze. It is simple to build, inexpensive, and reliable. It's easily tested before each use by immersing both electrodes in a cup of water (just don't let them come in direct contact.)
Apr 11, 2011, 05:22 AM
woodybob's Avatar
Harbor Models has them in stock.

Apr 11, 2011, 07:12 AM
Damp and Dizzy member
Brooks's Avatar
Not being an electronics guy, I wondered why Tachikaze's schematic included a transistor.

From my reading, it appears that the TIP 120 transistor allows one to switch on a 12 volt motor using only a 1.4 volt activator (the current running through the bilge water). Since pure water is a insulator, it does not conduct electricity well unless there are some ions (salt) dissolved in it. You can't know how salty your bilge water will be if you run in fresh water. Thus, the voltage across probes dunked in freshwater will likely be much lower than the 12 volt battery input. The pump would likely run slow, if at all.

As the "alternate water sensor" on Tachikaze's diagram indicates, you have to have the probes very close to ensure getting enough voltage flow to run the 12 volt pump.....unless you use the TIP 120 as the sensor/switch.

Combat guys like Tachikaze probably know more about the practical end of bilge pumping than most modelers :-)
Apr 11, 2011, 12:35 PM
Tachikaze's Avatar
The TIP 120 is the activating switch for the circuit. Most of us are running on 12V as the motors we use for the most part are running on 12V. The motor is a little Mabuchi motor set into an acrylic base with the copper tabs attached to the sides. Depending on how you have it set in your boat, anything above 1/8" of water will activate it. I will try and get a photo of one of our units posted tomorrow.
Apr 11, 2011, 01:21 PM
1/2 a bubble off
Apismelifera's Avatar

Water Level Sensor

The circuit in Post #2 does use a transistor. A TIP120. Looking up the specifications for it the only ones of real importance here are; how much current the transistor can carry, how much voltage it can safely tolerate when it is off, what its current gain is, and the maximum input current it can handle.

Maximum current rating is 5 Amps. So if your pump motor is rated at 5 amps or less you're OK there. Not sure what your pump draws when it is pumping water? Put it in cookie sheet on the kitchen counter. Pour in about a 1/2" of tap water into the cookie sheet. Run the pump from the same battery you will use in the model, and measure the current the motor draws while pumping water into the sink.

Maximum voltage rating is 100 volts. Obviously 100 is more than 12, but here that only gives you a safety margin of 88 volts. Again, this is OK.

The current gain of this transistor is 1000. Lets us assume for a minute that your pump draws 5 amps when running. So 5 amps of pump current divided by a input current gain of 1000 is .005 amps. This means that you only need .005 amps of input current minimum, to get the transistor to turn on enough to carry 5 amps. More is OK to a point as long as the input current does not go over the maximum input current.

Maximum input current is .12 amps. Looking at the circuit, when the plates are in water, current goes from the positive plate through the water to the input plate. If that current is .005 amps or more you're OK there. But I notice the design calls for a 2K Ohm (2000 ohms) current limiting resistor. In this circuit configuration the input of the transistor needs to be 1.4 volts higher than battery minus. Or it needs to measure 1.4 volts, which it will once the input current goes up to .005 amps. what does the resistor do? It limits the input current from going too high. 12V minus 1.4 volts equals 10.6 volts. If the sensor plates were shorted together you would have 10.6 volts divided by 2000 ohms equals .0053 amps. A tiny bit more than needed. But still less
than the maximum allowable input current . So again, that is OK.

Pure water is an insulator you say? I believe that you are right sir. But when was the last time you went out to the pond you run your boat in and drank some of the pond water? Pond water contains ions, dissolved metals like iron, zinc, copper, minerals, and a host of other chemical and biological ingredients from fish foul and other sources. All of those things tend to make it a virtual liquid short. If you were a poor unfortunate who had to run in a very clean pond you would just make the sensor plates bigger to trigger the pump. One could also reduce the value of the resistor to 1K Ohm if bigger sensor plates did not work.

If anyone who as actually built this circuit could please tell us how big the sensor plates were that they used, how far apart and so on, even better a picture or two of them would be nice.

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