# Circuit: Water Sensor: Works when one probe is in water while the other is not

Hi, just wondering how I could make a circuit so that it would only work when one water sensor if touching water while the other isn't. Here's a quick http://i539.photobucket.com/albums/ff356/AnneFTW/Random/WaterPump.png" [Broken] and replace the buzzer with a pump, but how do I use probes with two with different functions? Would it be very complicated? If so I probably won't make be able to make it, just wondering if there's a simple solution to this.

(I don't have much circuit knowledge so sorry if it's a stupid question)

-Thanks

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Think of it as two sensors each gives a high signal when water is present
You want an output when A (the tank on the left) is higher than the signal from B (tank on right)

The normal water sensors measure resistance between a probe stuck into the tank and the (metal) wall of the tank - water has a higher conductivity than air.
It the tank walls aren't metal you can have a pair of contacts close together in each tank and have the water make a short circuit between them.

Probably easiest to buy a water level sensor for each tank and then think about the compare circuit

vk6kro
You can get float switches which work like the float in a toilet cistern.

These come as normally open switches, or normally closed, or both.

If you had a normally open switch on the left tank, it would close when the water level reached it.

Then you could have a normally closed switch on the right tank which would stay closed if the water level was below the switch.

So, you put the two switches in series with each other and the coil of a relay (or with the motor if it was a small one) and a power source.
The relay contacts can be arranged to switch the motor on and off.

As long as the water was above the left switch and below the right one, the relay would pull in and operate the motor.

You can get float switches which work like the float in a toilet cistern.

These come as normally open switches, or normally closed, or both.

If you had a normally open switch on the left tank, it would close when the water level reached it.

Then you could have a normally closed switch on the right tank which would stay closed if the water level was below the switch.

So, you put the two switches in series with each other and the coil of a relay (or with the motor if it was a small one) and a power source.
The relay contacts can be arranged to switch the motor on and off.

As long as the water was above the left switch and below the right one, the relay would pull in and operate the motor.

Thanks I'd never heard of a float switch! I think that might work. I have an adapted a wet sensor circuit (just replace the switch with a float switch) Do you think it would http://i539.photobucket.com/albums/ff356/AnneFTW/WetSensorFloatSwitch.png" [Broken]?
(this is a model I'm making btw - the pump is tiny - it's a very basic circuit)

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vk6kro
Yes, that sensor would work OK.

For a model, you could use a home-made float switch made of a microswitch and something that floats on the end of a hinged lever. A pill bottle with a good seal on a snap-on lid might be OK.
Film cannisters for 35mm film would be ideal, but getting rare these days.

If you did use switches, you wouldn't need to use any transistors as the switches could handle adequate current to run a small motor.

Rainwater is a fairly poor conductor as it is similar to distilled water, so sensors depend on dissolved Carbon Dioxide. You could get a tank of water that doesn't conduct much, so depending on conductivity may not be a good idea.

Using a float switch also means the actual switch can be placed well away from the water and preferably outside the tank.

Yes, that sensor would work OK.

For a model, you could use a home-made float switch made of a microswitch and something that floats on the end of a hinged lever. A pill bottle with a good seal on a snap-on lid might be OK.
Film cannisters for 35mm film would be ideal, but getting rare these days.

If you did use switches, you wouldn't need to use any transistors as the switches could handle adequate current to run a small motor.

Rainwater is a fairly poor conductor as it is similar to distilled water, so sensors depend on dissolved Carbon Dioxide. You could get a tank of water that doesn't conduct much, so depending on conductivity may not be a good idea.

Using a float switch also means the actual switch can be placed well away from the water and preferably outside the tank.

Would the micro switch not be dangerous in the water?
(Thanks for all the info btw, it's really helpful)

vk6kro
Would the micro switch not be dangerous in the water?

Depending on the voltages involved, yes, it could be dangerous.

So, the trick is to make sure the switch does not ever get submerged in the water.

A float can provide a lot of force to operate a switch remotely. It can push a rod upwards to operate a switch or it can tilt a a see-saw arrangement to operate a switch at the side of the tank.
Or, it can pull a wire (via pulleys) to operate the switch like that.

[PLAIN]http://dl.dropbox.com/u/4222062/tank%20switch.PNG [Broken]

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Whether it would be better to use mechanical switches or electronics may depend on what experience you have, and / or on what you are trying to learn about. There may also be other issues, especially if you are a student or a young person. For instance, what tools you have access to or are allowed to use (soldering irons etc.).

Making your own float switches for a model may require fairly good craft skills, particularly if the floats need to be very small. Immersed-electrode sensors might be easier to make, but as another poster has explained they have their own problems. Sensing and combining their outputs would also require more electronics.

Finally, whatever you do, please make sure that it will be safe. As you are aware, using electricity close to water can be dangerous. You should only use low-voltage batteries to power your project.

vk6kro
For a model project, you could just glue a drinking straw (end on) to a table tennis ball using hot glue. You need the type of plastic drinking straw without a bend in it.

Then have the drinking straw pass up through a piece of rigid tubing (to guide it) and then have a microswitch at the top of the tube. Glass or plastic tubing would be OK.

These switches take very little force to operate but you could protect the top of the straw with a thumb tack if you like. This would stop the ball and straw dropping through the tube to the bottom of the tank if the thumb tack is wider than the tubing.

The table tennis ball floats on the water and the drinking straw rises with the ball when water rises in the tank.

hi,
i need some idea on doing my final year project.
its is about car digital temperature display based on the water level..
so here i hope you can give me an advise or suggestion to guide me.