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Break circuit when connection made? Answered

Hi,I want a circuit which deactivates(breaks)when a connection is made in the circuit.I'll tell u in details-I am going to make an automated water pump(to a tank)system,which can recognise when the tank is full and stop the motor from pumping water.For your information this is just a model version for school exhibition and what I have in mind is,When the water in the tank reaches a particular height,the water completes a circuit between two open wires(I may add some salt to the water to make it conducting).As soon as this happens the pump which is pumping water to this tank should stop working.How can I accomplish it? 

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Personally I would attach a microswitch on the top of a tube.

Connect the Common and Normally Closed contacts to the pump and power source

Put a plastic ball in the tube to activate the switch when the water level rises.

Rickharris,I like your idea of the plastic ball's and is perfect for my project but just one question,is the upthrust exerted by water on the ball high enough to push and activate the microswitch?

A micro switch needs very little force to operate it.

Try to get one with a reasonably long lever on it.

This works we made one at school as a water level detector in a mock up of a process control system.

If you prefer you could do this with a tilt switch instead.

You could do that, it's just shorting the circuit but then the electricity is flowing through the water which would be dangerous and lead to expensive electricity bills or drain your batteries if it was part of the main circuit. I'm with Jayefuu and Rick, use a float, which is exactly how toilets and sump pumps (typically) work. I'm sure you can find a free toilet float on craigslist.

If you must do it by shorting through the water then you need a separate low voltage circuit that pulses instead of running continuously, at least once the switch is tripped the first time. Also yo udon't need to add salt, water is already conductive and that will just help corrode things, like your pump and lines.

Which is what I posted. Its AC coupled.

Pure water is a damned good insulator, not a conductor.

The key being pure. If he uses distilled water yes, add salt. If he uses tap water or rain water or any water besides distilled it will be conductive enough to make the connection.

Our water is incredible soft here (so soft it dissolves metals easily). Last time I measured it was <<1mS.

Thanks to everyone for your sincere help,using a float-My model is really small i.e;a 2 litre water bottle as the tank and a 3volt water pump is used.I feel that the float may not fit in the container and could not be applied in this situation,in which I'm being specific because next time when I make a bigger model,I'll use a float.Rickharris,I like your idea of the plastic ball's and is perfect for my project but just one question,is the upthrust exerted by water on the ball high enough to push and activate the microswitch?

A float switch would be simplest....

I was envisaging a long armed limit switch with a ping pong ball glued to the end ;)

meh. I suspect the project is probably to use some electronics to do it - that circuit's very cute.

Look for a simple water detector circuit !
http://electroschematics.com/218/liquid-detector-water-sensor/