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# Need to turn solenoid on with var resistor

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Hey All,

Basically, I want to turn on an "always off" 24V water solenoid valve. I have a float that has an integrated circuit that puts out an infinite resistance when the float rises to the top. So, what I would like is a circuit that puts out 24V when the float gets to the top (reaching infinite resistance).

Any help would be appreciated.

Does the float include a switch - or is the "output" variable from some much lower value up to infinity? If the output is variable and within a useful range you might plan to flow a trickle of current thru it and use a comparator to toggle when the voltage drop is at an appropriate point.

The next question would be - do you want the solenoid valve just off or just on as the float crosses a certain point? A "feature" that you could add, if the float revealed various positions, is the ability to pulse the solenoid valve on for short period as a way to moderate the flow of water. The "short period" I have in mind is many seconds, a minute or so - but the application and flow rates may not allow for this approach.

Connect the float resistor in series with a fixed resistor to create a voltage divider. Connect a potentiometer across the same voltage that feeds the divider. Compare the voltage at the pot wiper to the voltage at the divider using an IC comparator chip (or an opamp used as a comparator) make a logic signal which switches depending on the two inputs. Use a transistor to switch the solenoid coil.

Wow, I was hoping for more of a simple circuit that used the Infinite resistance as a switch input (sort of an off switch used as an on) to turn on a solenoid. I do appreciate all responces but I would like to keep this circuit as simple as possible.

Maybe I'm not looking at the responces right. I'll check into them incase this is the most simple.

Thanks.

Does the float include a switch - or is the "output" variable from some much lower value up to infinity? If the output is variable and within a useful range you might plan to flow a trickle of current thru it and use a comparator to toggle when the voltage drop is at an appropriate point.

The float is circular and fits around a hollow shaft that has a mini circuit board. The mini circuit board acts like an all or nothing switch, giving no impeadance until it reaches the top then it reads infinite.

The next question would be - do you want the solenoid valve just off or just on as the float crosses a certain point? A "feature" that you could add, if the float revealed various positions, is the ability to pulse the solenoid valve on for short period as a way to moderate the flow of water. The "short period" I have in mind is many seconds, a minute or so - but the application and flow rates may not allow for this approach.

Yeah, it's just on or off. There's no variation in the resistance. I suppose that's the purpose of the mini circuit board.

Ok, I disassembled the float mechanism. I uses a reed switch that the float opens with a magnet when it passes over.

My problem still exists, I need to turn on a solenoid when the power is interupted. Any ideas?

Use a small relay controlled by the float switch. Use the Normally Closed relay contact to control the solenoid.

A transistor is cheaper than a relay. See below. If you care about static power consumption, use a FET and a 1meg or 10meg resistor.

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