Pump controller driven by variable-resistance fuel level sender

Thread Starter

leighwebber

Joined Aug 30, 2016
17
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?
 

wayneh

Joined Sep 9, 2010
16,399
Well I only skimmed through but it sounds like what you want is a comparator with hysteresis, like a home thermostat. But you need a lot of hysteresis. I think it's doable. Too tired to make the calculations but maybe you can google the basic setup.

You'll want the comparator to control a MOSFET switch that in turn will control power to the pump, like a relay (which is also a choice).
 

Dodgydave

Joined Jun 22, 2012
9,468
An lm3914 can be used here, its got 10 comparators in one chip, and can be fully adjustable, you need a flipflop to put the pump on and off at the desired levels, like a 555 timer or cmos cd4013.
 
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AnalogKid

Joined Aug 1, 2013
8,608
Since the gauge is a constant resistance and the sender is variable, you already have a voltage divider without adding another resistor. From your description, it sounds like the top of the sender goes to the gauge and the bottom goes to GND. If so, then a simple comparator will give you a go/no go signal that can drive a power MOSFET that can drive the pump. One chip or a couple of transistors, a few resistors, etc.

1. What is your skill set for assembling a small circuit on perf board?

2. Can you post a basic sketch of the poser/gauge/sender circuit as it is today?

ak
 

Thread Starter

leighwebber

Joined Aug 30, 2016
17
I have no problem working with perf boards and such. But the key point is that I don't want the pump to run until the tank is empty. What you are describing would make the pump run whenever the level dropped below completely full. Think of it like this: when the pump fills the tank and shuts off, I want the pump to be disabled until the tank is completely empty. Only then should it run.

As of today there is no circuit. The tank is being built this week, and I'll have the level sender unit in a couple of days.
 

wayneh

Joined Sep 9, 2010
16,399
What you are describing would make the pump run whenever the level dropped below completely full.
You've misunderstood. Think about your furnace. It kicks on at one temperature and turns off at a higher one, usually just a couple degrees more but it could be a deadband of as much as 5° or so. That's called hysteresis, same as what you want for your pump.

The circuit will look a little like this. This isn't done yet and still has problems, but it's getting there and I wanted to share it with you to give you the basic idea. I can refine the details of this, and provide the file, if you want to pursue. It takes time and I'm not going to bother completing it if you're not interested. Note that R6 in this schematic is your pump, U1 is a comparator (I often use LM339, but the one shown is in the LTspice library), R1 and 7 are your sensor, and R5 provides hysteresis.

Screen Shot 2016-08-31 at 10.01.33 AM.png
 
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ericgibbs

Joined Jan 29, 2010
10,835
I think you have the output function backwards. I R5 goes high when Vsender is high. Relay turns on when tank is full.
hi #12,
The plan is to use a relay to switch the pump contacts, [SPCO] that would enable pump operation to suit the user, also give some level of isolation if required.
If he wants to use a transistor pump switch a PNP power transistor could be used.
Eric

EDIT:
PNP Option, used switch relay or pump
 

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#12

Joined Nov 30, 2010
18,210
hi #12,
The plan is to use a relay to switch the pump contacts, [SPCO] that would enable pump operation to suit the user, also give some level of isolation if required.
If he wants to use a transistor pump switch a PNP power transistor could be used.
Eric
I see that you can reverse the output logic at the relay contacts. Just, I didn't think of that because the relay would be energized most of the time and circuit failure would result in a default to the "pump on" condition.
 

Thread Starter

leighwebber

Joined Aug 30, 2016
17
Eric -- thanks for the circuit. To #12's point, can I just add an inverter after o1 to correct this? Should it be a Schmitt Inverter? I'm not sure how o1 behaves as the tank level reaches full-then-a-tiny-bit-below-full. Would o1 oscillate, or would it stay clamped until the tank reaches the next complete state?
 

ericgibbs

Joined Jan 29, 2010
10,835
hi leigh,
Check my EDIT in post #13.
With that degree of hysteresis, the circuit will not oscillate.

E

Note: add a protection diode across the relay coil or pump motor
 

Thread Starter

leighwebber

Joined Aug 30, 2016
17
MaxHeadRoom -- you make a good point. But I need a fuel level sender in any case so that I can monitor the tank while underway. If the pump fails to kick on and that tank goes empty, my diesel engine sucks air and stops instantly. With my boat 100 yards from a rocky shore and a 30 knot wind blowing me to my doom. So I need to see the fuel level on my cockpit instruments. Why not just a two-level float switch? It's one more thing to break down. I can build a circuit and seal it up against the elements -- but it's more difficult to "boat-proof" any mechanical device. OTOH, my design depends on the level sensor never failing -- but the Moeller units have a superb reliability record.
 

AnalogKid

Joined Aug 1, 2013
8,608
As suggested in post #3, a single comparator circuit with a wide hysteresis window will do what you want, since it has two separate switching points. Alternatively, a circuit that is easier to adjust uses two comparators and a flipflop to achieve the exact same function. In a strange twist of fate, this is what is inside a 555 timer chip, although the two trip points would be all wrong for your application.

So...
1. What is the sensor voltage when you want the pump to come on?
2. What is the sensor voltage when you want the pump to turn off?

ak
 
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