Fish4Fun
So long, and Thanks for all the Fish!
Nothing complicated here, just a simple circuit to turn a liquid fill solenoid on/off depending on liquid level with an adjustable amount of buffering to minimize pump cycling .... 
While virtually any standard float switches will work, the current/voltage/power capabilities required are nominally 12mA | 12Vdc | 144mW allowing the use of inexpensive sensors like the ZP4510 China-Cheap sensors readily available on e-bay/Amazon etc.
Operation Explanation:
When the system is NOT full Switch "Full" connects R1 to the gate of mosfet IRLZ44 via +12V turning the mosfet on. Despite the LED named "Fill_On", this does NOT actually engage the H2O_Solenoid, but rather places the circuit in "ready mode".
Once the float switch "Lower_Limit" makes contact it triggers the gate of SCR BT151 which engages the H2O_Solenoid and (as long as the solenoid current is sufficient) the SCR latches until the "Full" Switch opens and the Mosfet IRLZ44 turns off. SCR BT151 could be replaced by a "Latching Relay" with some minor modifications, but the SCR configuration should have a virtually unlimited MTBF while relays inevitably eventually fail. (If there were only a semiconductor replacement for the H2O Solenoid ;-) )
Physical Configuration:
In my application the cistern being filled requires roughly 20 gallons of H2O per inch, so I used a single piece of 1/2 inch HDPE with two 5/16" holes in it to attach a pair of ZP4510 Float Sensors, one pointed up ("Full") and the other pointed down ("Lower_Limit") making the "on/off buffer" roughly 2 inches (40 gallons). This 2 inch buffer prevents excessive pump cycling while maintaining the cistern nominally full.
In my application the "Fill Pump" is a well pump that only has a nominal output of ~2 gallons per minute. The 550 gallon cistern is emptied by a high pressure/high volume pump that provides water for household use at 60psi and up to 25 gpm. While 2 gpm is insufficient to supply transient demands, 120 gph is more than sufficient to keep the cistern from being completely drained and even in extreme cases 25 gpm transient supply is plenty.
Other Applications:
Might include automatic watering systems for livestock, ponds and other water features...
I use a similar circuit to fill a tank used to supply water for a live bait system in my retail store. In this case the "Lower Limit" is a photo-cell that only enables the "Fill Cycle" when the store is dark. Why? The water needs to be aerated prior to entering the live bait system, water straight from the tap has near zero oxygen and needs to be de-chlorinated. A couple hours or so of aeration achieves both without the addition of chemicals, or undue stress to the bait. So each night when the lights are turned off the tank fills up and an aerator runs for a couple of hours. Every morning there is a full tank of de-chlorinated, oxygen rich water to start the day.
Hope this helps someone!
Fish
While virtually any standard float switches will work, the current/voltage/power capabilities required are nominally 12mA | 12Vdc | 144mW allowing the use of inexpensive sensors like the ZP4510 China-Cheap sensors readily available on e-bay/Amazon etc.
Operation Explanation:
When the system is NOT full Switch "Full" connects R1 to the gate of mosfet IRLZ44 via +12V turning the mosfet on. Despite the LED named "Fill_On", this does NOT actually engage the H2O_Solenoid, but rather places the circuit in "ready mode".
Once the float switch "Lower_Limit" makes contact it triggers the gate of SCR BT151 which engages the H2O_Solenoid and (as long as the solenoid current is sufficient) the SCR latches until the "Full" Switch opens and the Mosfet IRLZ44 turns off. SCR BT151 could be replaced by a "Latching Relay" with some minor modifications, but the SCR configuration should have a virtually unlimited MTBF while relays inevitably eventually fail. (If there were only a semiconductor replacement for the H2O Solenoid ;-) )
Physical Configuration:
In my application the cistern being filled requires roughly 20 gallons of H2O per inch, so I used a single piece of 1/2 inch HDPE with two 5/16" holes in it to attach a pair of ZP4510 Float Sensors, one pointed up ("Full") and the other pointed down ("Lower_Limit") making the "on/off buffer" roughly 2 inches (40 gallons). This 2 inch buffer prevents excessive pump cycling while maintaining the cistern nominally full.
In my application the "Fill Pump" is a well pump that only has a nominal output of ~2 gallons per minute. The 550 gallon cistern is emptied by a high pressure/high volume pump that provides water for household use at 60psi and up to 25 gpm. While 2 gpm is insufficient to supply transient demands, 120 gph is more than sufficient to keep the cistern from being completely drained and even in extreme cases 25 gpm transient supply is plenty.
Other Applications:
Might include automatic watering systems for livestock, ponds and other water features...
I use a similar circuit to fill a tank used to supply water for a live bait system in my retail store. In this case the "Lower Limit" is a photo-cell that only enables the "Fill Cycle" when the store is dark. Why? The water needs to be aerated prior to entering the live bait system, water straight from the tap has near zero oxygen and needs to be de-chlorinated. A couple hours or so of aeration achieves both without the addition of chemicals, or undue stress to the bait. So each night when the lights are turned off the tank fills up and an aerator runs for a couple of hours. Every morning there is a full tank of de-chlorinated, oxygen rich water to start the day.
Hope this helps someone!
Fish
