Stacked Bar Fuel Gauge

[CRayD]

The attached schematic displays a auto fuel gauge that employs 8 flat LEDs super glued together to make a 8 step bar gauge to report fuel tank levels. Also attached is a JPG image of a word file providing some of the theory employed in the schematic. Comments or questions?

 

[philba]

this seems overcomplicated to me. why not just use a 3914? You would get LED 10 segments instead of 8 and get rid of the comparators, led drivers and at least 28 resistors (the lm3914 has them all in one chip). I'd use a low pass filter on the input with a really low cutoff value (like single digit hz) to minimize the sloshing effects.

Also, you can find stacked LEDs with 10 elements so no need to superglue.

 

[audioguru]

The complicated circuit is from 1962 before the LM3914 was invented. It doesn't use vacuum tubes (valves)?

 

[Dan Jones]

I was trying to compose this behemoth ten years ago, halfway into it, I ran out of time to putz with it. I have since tried it with the LM3914, but its not going so hot, I can't get good results. Maybe I should have stuck with the quads? I would have more control over the individual LEDs in the bar, at least. Good for fine tuning or tweaking.

 

[audioguru]

Each LED driven from an LM3914 lights at 10% more of the full-scale input voltage.

 

[CRayD]

LM3914 Stacked Bar Fuel gauge

Dan see attached Sch. that shows a LM 3914 Stacked Bar fuel gauge. After the comments to my earlier 8 Stack gauge using a pair of quad comparators I revised to the LM3914 and very pleased with result. But in acutal application I noted that the signal input from OEM fuel tank still bounces quite a bit, suggesting that the RC network at input (C1+R1) needs tweeking. In addition I noted that the input signal voltage jumps about 10% when engine is running and allternator is feeding a charging voltage. I haven't figured out a solution to this problem yet. Finally for those readers waiting to show off their vast technical expertise I add that I continue to use more expensive glued stacked leds as they are smaller (.2"x.08") and also this permits the use of multi colors (I used red+yel+grn).

 

[audioguru]

The LM3914 has a very accurate voltage reference on its pin 7. But the fuel sensor's variable resistor is fed from the fluctuating battery voltage so the reading jumps when the alternator increases the voltage to the variable resistor.

 

[lasvegasmale31]

Not that I am trying to "show off my technical expertise" , but HP does have available multicolor bargraph led clusters.

 

[adamey]

What I'm wondering is how accurate these circuits could be since they don't seem to have any provision for the different types of fuel level senders. Basically a fuel level sender is just a potentiometer, but vehicles come with a wide range of resistance ranges for their gauges. Not only that, but some companies use a high resistance to equal a full tank while others use a low resistance to show a full tank. To make things worse, not all sensors are linear in their resistance changes.

I'd think finding out which type of sender your car uses would be pretty important to making an accurate fuel guage.

As to the output fluctuating with charging voltage, simple supply your own power to to the sender instead of relying on the voltage supplied by the car. Regulate your circuit and this reference voltage to 5V and you'll never have a gauge that fluctuates with battery/charging voltage.

As to fluctuating as the fuel sloshes around, you'll have to experiment with RC values to average things out over time. In your car this is often called a "slosh module" (no kidding).

 

[CRayD]

You will work your brain into a sweat trying to improve the accuracy of the crude OEM fuel gauge system. None of the tanks are prefect geometric shapes (flat ended cylinders or 6 flat surface cubes) instead they all have bumps, concaves, rounded shoulders, etc. etc. and lastly you never know until you run it out of fuel how much fuel is below the intake point, (lost and not usable). So the 80 YO heated bimetal current hog OEM fuel gauge is better than a poke in the eye with a sharp stick and the goal I have is creating smaller replacements for the OEM systems. You are correct in noting that the increased voltage with running engine could be managed by regulating the input to fixed 12 vdc, but the regulator would have to be relatively close physically to OEM input to minimze voltage drop in the line to the relatively high current draw of the OEM system and it (regulator) would have to be able to manage that current draw (WAG at 1A).

 

[audioguru]

My new car warns me when the fuel is low. I hardly ever look at the fuel gauge except before a long trip.

My first car frequently ran out of fuel but its starter motor was strong enough to move the car to the next gas station. I never replaced the battery nor starter motor.

 

[Pommie]

If you use a voltage divider from the cars supply to feed Rhi then that will fix the running/not running difference.

Mike.

 

[audioguru]

If you use a voltage divider from the cars supply to feed Rhi then that will fix the running/not running difference.

Mike.

No.
The cars supply voltage is not regulated so a voltage divider will also have the same difference.

 

[Pommie]

No.
The cars supply voltage is not regulated so a voltage divider will also have the same difference.

Exactly, the problem is caused by the supply and therefore the signal voltage varying with respect to the fixed reference voltage. If the reference voltage is made proportional to the supply voltage by a resistive divider then there will be no discrepancy. The reference voltage and signal voltage will increase by the same percentage.

Mike.

 

[audioguru]

Oh. You are talking about feeding the Rhi of the LM3914 with a voltage divider from the car's supply. Yes, it will work.

 

[xrotor]

Fuel Gauge signal latching

Hi,
I have built this circuit discribed using the display driver and a resistor chain for a fuel gauge I am building. It uses a float switch to activate reed switches to remove resistance from the chain as it moves down ( 10 in all ). I need some help in working out a way of latching the output till another switch is activated, it would remove the need for very carefull reed positioning
Can I feed the output of the display driver to a latch ic or ic`s and then to a uln2003 to drive the led output. As the new signal from the reed is received it would need to the reset all the other outputs and wait for the next signal.
Any help would be appreciated its a long time since I did any electronics do, they still do oc71 transistors !!