I have a fuel tank that will be filled by a 12V fuel pump. The pump needs to turn on when the tank is empty and turn off when the tank is full. As the tank empties, the pump should stay off until the tank reaches empty. There are zillions of circuits that do this, but they all rely on binary switches that sense full and empty conditions. But my tank has a fuel level sender (Moeller makes it) that is a variable resistance driven by a simple swing arm with a float. When the tank is full, the arm is roughly horizontal. As the tank empties, the arm swings down until it hangs vertically when the tank is completely empty. The swing arm drives a variable resistance with 33 ohms meaning empty and 240 ohms meaning full. The fuel level sender is connected to a gauge that sends 12 V through the sender.
My thought was to add a 200 ohm resistor in series with the fuel sender to create a voltage divider. The voltage drop across the sender will then tell me what the fuel level is. My calculations tell me that when the voltage drop is 1.7 volts, the tank is empty, and when it is 6.5 volts, the tank is full.
So -- what I need is a circuit that activates a relay when the circuit's input voltage is 1.7 volts (or less), and keeps the relay active until the input voltage reaches 6.5 volts. It then must turn the relay off and keep it off until the voltage drops to 1.7 volts again. What I don't want is a pump that runs whenever the voltage drops below 6.5 volts, because it would kick on and off constantly. In my application, it takes about three hours to empty the tank, and about 10 minutes to fill it. So my pump only needs to run for about 10 minutes out of every three hours.
I would want to be able to tweak the limit voltages of course -- the fuel level sender's resistances may be slightly out of spec.
I have looked at various parts (comparators, window comparators, etc.), but I can't figure out how to rig them up so that the pump stays off while the tank drains, and only kicks in when it's empty. Again, this is trivial if you have binary "full" and "empty" switches, but I want to use my existing fuel level sender.
FWIW, I'm not concerned about start-up conditions. For example, suppose the pump was running and the tank was rising to about half full when I turn the system off. When I turn it back on again, the pump can stay off -- no need to "remember" that it was in its "fill" cycle previously. The only thing that matters is that while the system is energized, the pump turns on at 1.7 and turns off at 6.5. But it would be a nice feature to have the pump *always* turn on when the system is energized, then of course turn off when the tank is full. That way whenever I start a trip, my tank will be full.
BTW, the fuel tank is a "day tank" on a sailboat. The day tank is supplied from the main fuel tank. The day tank feeds the engine. If you're interested in sailboat auxiliary diesel engines and why I would want a day tank, let me know and I'll explain.
Circuit suggestions?
My thought was to add a 200 ohm resistor in series with the fuel sender to create a voltage divider. The voltage drop across the sender will then tell me what the fuel level is. My calculations tell me that when the voltage drop is 1.7 volts, the tank is empty, and when it is 6.5 volts, the tank is full.
So -- what I need is a circuit that activates a relay when the circuit's input voltage is 1.7 volts (or less), and keeps the relay active until the input voltage reaches 6.5 volts. It then must turn the relay off and keep it off until the voltage drops to 1.7 volts again. What I don't want is a pump that runs whenever the voltage drops below 6.5 volts, because it would kick on and off constantly. In my application, it takes about three hours to empty the tank, and about 10 minutes to fill it. So my pump only needs to run for about 10 minutes out of every three hours.
I would want to be able to tweak the limit voltages of course -- the fuel level sender's resistances may be slightly out of spec.
I have looked at various parts (comparators, window comparators, etc.), but I can't figure out how to rig them up so that the pump stays off while the tank drains, and only kicks in when it's empty. Again, this is trivial if you have binary "full" and "empty" switches, but I want to use my existing fuel level sender.
FWIW, I'm not concerned about start-up conditions. For example, suppose the pump was running and the tank was rising to about half full when I turn the system off. When I turn it back on again, the pump can stay off -- no need to "remember" that it was in its "fill" cycle previously. The only thing that matters is that while the system is energized, the pump turns on at 1.7 and turns off at 6.5. But it would be a nice feature to have the pump *always* turn on when the system is energized, then of course turn off when the tank is full. That way whenever I start a trip, my tank will be full.
BTW, the fuel tank is a "day tank" on a sailboat. The day tank is supplied from the main fuel tank. The day tank feeds the engine. If you're interested in sailboat auxiliary diesel engines and why I would want a day tank, let me know and I'll explain.
Circuit suggestions?