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Fuel level switch

Devon24

Oct 26, 2023
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I was wondering if there is a way to trigger a relay at 40 ohms and turn it off at 90 ohms, any ideas are appreciated.
 

AnalogKid

Jun 10, 2015
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Yes there is, but your question is missing a *lot* of necessary detail. How large/small a relay? Coil voltage / resistance / current / power - ? What is the device that measures 40 to 90 ohms? Someone turning a pot or volume control? A photocell sensing the sunrise/sunset? A fuel level sensor in a car? Are you talking about measuring a stand-alone resistor, or a component in an operating circuit?

What power source is available to run the sensing circuit?

etc.

ak
 

AnalogKid

Jun 10, 2015
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OK, for reasons that completely escape me, I saw the question but somehow missed the title (unless it has been edited). So . . .

Is this for a vehicle? Can we assume a 12 V to 14 V system? Is the sensor already powered by the vehicle?

If all of that is true, then you don'y have to measure the resistance because there already is a current going through it; you just have to measure the voltage. Because you are interested in only two of the sensor values, this can be done with two comparators and a flipflop.

One comparator sets the flipflop when the sensor voltage is below its trip point. The flipflop output energizes the relay. The other comparator resets the flipflop when the sensor voltage is above its trip point, de-energizing the relay.

There is a way to do this with one comparator and no flipflop, but the circuit design math is a bit more complex.

Does this sound like something you are able to build?

ak
 

Bluejets

Oct 5, 2014
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Most fuel guages are a current meter driven via a variable resistance ( fuel tank sensor) via a fixed voltage regulator.
 

crutschow

May 7, 2021
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Is the resistor by itself or is it connected to a fuel-level gauge or other connections?

A window comparator should also work.
 

crutschow

May 7, 2021
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Not without a latch or some other steering mechanism.
A window comparator using comparators with open collector outputs, such as the LM339, can have the outputs connected together with one resistor pull-up to generate an output wired-AND function, for a simple steering mechanism (example below):
So, within the window, both comparator outputs are high to generate a high output.

1698345627883.png
 
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AnalogKid

Jun 10, 2015
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Yeah, I'm aware of how they work. My point is that the reqs in #1 sound like this to me:

Assuming the resistive sensor is biased with a fixed resistor to either +12 or GND. This changes the direction the resulting node voltage moves in as the tank empties, but it still is a simple fixed/variable-resistor voltage divider. As such (and because of the voltage direction ambiguity), I'll phrase things in terms of resistance value rather than voltage.

1. Below 40 ohms >> relay is "triggered" (continuously energized)

2. 40 to 90 ohms >> Relay stays on

3. Above 90 ohms >> Relay is turned off (de-energized)

4. 90 to 40 ohms >> Relay stays off

5. Below 40 ohms >> Repeat

Maybe i'm reading in things that are not intended by the TS, but to me it sounds like either one comparator with a large hysteresis window or two comparators and a flipflop. If you scaled the voltages just right, you could use a 555.

Note that because we don't know what the relay does, we do not know if there is any feedback between the relay contacts and the sensor, such as a fuel pump. I'm assuming that there is, call it an experience-based guess.

ak
 

AnalogKid

Jun 10, 2015
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Depends on the nature and characteristics of the signal being switched.

ak
 

Delta Prime

Jul 29, 2020
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What we've got here is... failure to communicate.(with the thread starter). Some men you just can't reach. So you get what we had here last week, which is the way he wants it... well, he gets it.
;)
I love setting you guys up! you're so predictable.
 
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Delta Prime

Jul 29, 2020
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You really need to sober up so we can understand what you are saying. :rolleyes:
Quoting:Cool hand Luke!
If it was anyone other than you Sensei... I would say."Keep on looking at the back of your head that's normally where the brains are kept." But I wouldn't say that to you. In fact I'll join you. :rolleyes:
And I have told you N number of times now N+1.
I am sober...Court Ordered!
As stated before I'm two steps ahead you're low hanging fruit so predictable and knew you're going to comment :D
 
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Delta Prime

Jul 29, 2020
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Then your anticipating this...
Go BIG BLUE!
We're going to kill the buckeyes!
 
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