Fuel Gauge/Sender Variants

[bigherb]

Different senders - possible differences in fuel tanks, profile, etc? Pretty sure I have seen fuel tanks listed for pre 85 vans and they do look different.

Tanks are the same apart from the filler hole size.

[CJH]

I think you might want to feed the circuit from the 10V regulator too.
How is the circuit going to effect your gauge reading too? it'll be adding resistance in parallel with the sender.
I'm not dissing your idea, just throwing a few things in the pot.

Two very good points. I had already twigged that the 10V regulated supply will be better because the LED switch point won't then vary with the state of battery charge. So my sums to date have assumed a 10V supply. I had already ordered the PCB before this dawned on me, so the connection says IGN, but I can just as easily take a feed from the regulator into that connection.

I'm not sure that the warning LED circuit does add any more resistance to ground in parallel to the sender, does it?. The only other route to ground from the sender wire would be via the transistor base - does the base connect to the emitter? I don't know - I'm rapidly reaching the limits of my knowledge here!

Thanks for the offer of the trim pot - that could be useful depending how I get on with the ones I've ordered. For my sums, I took my cue from the circuit layout that I posted above, and I calculated that that circuit aims to put 2V on the base of the transistor to turn it on. So 2V is my target. Using a 10V supply, and an 'early' (37-288) sender, if I aim to trigger the LED at, say, 200 Ohms (66% of the range) then I calculated that the sum of the fixed resistor and the trim pot should be around 810 Ohms. With the 'late' (34-168) sender, 66% would be about 120 Ohms, so the fixed+variable resistor needs to total about 490 Ohms. If I choose 470 Ohms for the fixed resistor, then the 'early' sender needs a trim pot of about 340 Ohms, and the 'late' sender needs a trim pot of about 20 Ohms. So I've ordered a few: A 100 Ohm, a 500 Ohm and a 1000 Ohm, but they're all less than a full turn for the full range. So although the 500 Ohm would suit both the early and late senders, it might not have the resolution I want. Let's see.

[bigbadbob76]

I'm not sure that the warning LED circuit does add any more resistance to ground in parallel to the sender, does it?. The only other route to ground from the sender wire would be via the transistor base - does the base connect to the emitter? I don't know - I'm rapidly reaching the limits of my knowledge here!

Yes it will, but it's hard to calculate with your circuit.

First the basic requirements-
To turn the transistor on fully, you need 0.7v between base and emitter.
to turn the led on you need approx 2v accross it.
to limit the current through it you need a resistor in series with it.
So 10v-2v= 8v
assuming 20mA through the LED, 8v/0.02A= 369 ohms.

normally you would have the led and resistor in the collector line of the transistor and just supply the base (via a current limiting resistor) with the voltage dropped accross the sender.
I'm not sure it will work as you have drawn it.

If you have the led and it's resistor in the collector and the base driven with 2V via a current limiting resistor then-
2V-0.7V= 1.3V.
Depending on thetransistor you chose/have handy, you then look up the base-emitter saturation current, lets call it 5mA which is typical for a small transistor.
so 1.3v/0.005A= 26 ohms.
Then add the base-emitter impedance of 50 ohms (typical) gives you 76 ohms in parallel with your sender which is pretty low and will effect it a lot.
Also the resistance of the gauge will effect your sender biasing resistors so you won't get the voltage you expect anyway.

I think you need to re-design the circuit using the gauge to bias the sender as it is normally connected.
then I'd suggest using a FET to switch the LED. FETS have high input impedance and will not effect your gauge/sender resistances.
hope that helps more than it confuses.

[CJH]

hope that helps more than it confuses.

Hmm - not sure about that! But that's my lack of knowledge rather than your explanation. I followed some of it. I'll re-read it and see if any more sinks in. The LED and limiting resistor are on the dash PCB, so ideally I'd like the output of the 'warning' PCB to be just a switched live, which is what the emitter of the transistor appears to be. You say the LED and limiting resistor would normally be in the collector - is there a problem with them being in the emitter?

Thanks for the input - all gratefully received. Maybe I should just let you design the circuit, and I'll implement it in a better PCB.

[bigbadbob76]

The emitter of the transistor is not a simple switched live, you need to take the transistor base biasing current into account, you'd need a higher base driving voltage than the 2V you designed in.
A FET is more like a simple switch.
Could the LED and resistor not be connected to ign + at one end and the transistor/FET be a switch to ground?
I reckon a simple FET switch and probably a zener diode will do it but I'll need to have a think and get back to you.

[CJH]

Could the LED and resistor not be connected to ign + at one end and the transistor be a switch to ground?
I reckon a simple FET switch and a zener diode will do it but I'll need to have a think and get back to you.

Yes, the dash PCB allows the spare LEDs to be configured as 'switched earth', so that's possible. The supply would be IGN rather than 10V, but I think the 10V is only important for the comparator part of the circuit, not the LED supply.

If you can come up with a suitable circuit that allows the gauge to work as before that would be great. If you can additionally incorporate some sort of smoothing (e.g. a capacitor) to stop the light flashing too much, that would be even better.

[bigbadbob76]

No problem, I'll work on it.
I'll need to know the voltage between the sender terminal and earth when the fuel level is where you want the LED to come on.
This will be different with different senders but I'll build in some adjustment.
My tank is full at the moment so I can't measure mine.

[bigbadbob76]

Ok. Here's a quick fag packet drawing. This should work.



Values still to be calculated but basically the zener doesn't conduct until the sender voltage is high enough to trigger it so the circuit doesn't effect the gauge.
once the sender voltage exceeds the zener voltage it conducts and current flows in VR1, this current means you get a voltage at the wiper of VR1 which can be used to switch the transistor via current limiting resistor R1.

C1 and R1 combined form a damping circuit that keeps the transistor switched on for a time after it has been triggered.
If you put C1 on the transistor side of R1 it will delay the light coming on, you choose.

VR1 can be sufficiently high resistance not to effect the gauge reading too much when the LED is on and not at all when it's off.

If you really must have it as a switched live for the led, you need an extra transistor (pnp) following the first one.

[CJH]

That's great, thank you for doing that so quickly. I'll have a go with that in veroboard, and if it works out I'll make up a PCB to include when I next place a PCB order. Ultimately it would be good to include this extra circuit on the dash PCB so that it can be used by just placing a jumper.

As an add-on board I'd need ZD1, VR1, C1, R1 and the transistor, with connections for the fuel sender, +10V, GND and the LED (earth side).

To measure the sender voltage, I think I can do that on the bench with a spare gauge, a variable resistor (to simulate the sender), and a multimeter to measure the voltage. If I set the 'sender' resistance so that the needle is entering the red zone (like Bigherb's 10l picture) then measure the voltage that should be a good starting point.

[bigbadbob76]

Good plan with the vr and spare gauge.
The gauge is a hot wire meter so it's resistance varys with it's reading so you can't just measure its resistance when cold and use that.
If you can do voltage checks at low, very low and totally empty and report back we can do the sums. The gauge draws quite a bit of current (350mA when full) so don't use a weedy little vr or it will smoke. Haha.
Depending on the voltage readings you measure, you might get away with a resistor instead of zd1.


Sent from my iPhone using Tapatalk

[CJH]

The gauge draws quite a bit of current (350mA when full) so don't use a weedy little vr or it will smoke. Haha.

Thanks for the tip - I don't think any of the ones I have on order will be up to the job. I'll get a general purpose potentiometer from RS in that case.

I had a quick go at implementing your circuit in Eagle (ready for when we know the values of all the components), and I realised that of course the board won't need a 10V connection, so just GND, sender and LED.

[bigbadbob76]

as a starter for 10, try using-
10K for VR1
47K for R1
470 ohm for Rled
22uF for C1
replace Zd1 with 1K

yes, just sender, gnd and led connections needed. the supplies come from the gauge and the led.

[CJH]

Thank you. Am I right that the value of RLED has no impact on the rest of the circuit - it just controls the current through the LED and therefore its brightness? With my dash kit I've found that some of the LEDs are very bright, so I've tamed them with higher resistor values. That won't affect the function of the circuit will it?

[diabolov]

God! If I only had a small part of what you two know about electrickery....I'd be happy.

Regards to you both.

following....from a distance.

Mark

[bigbadbob76]

Each to their own skill diabolov, I'm glad to put back to this forum as it's helped me a lot.

Yes Chris, you're right that Rled has no effect so higher values will be fine.