resistance mismatch

[pebe]

Hi Ron,

R2, R3, R4 and R5 can all be 1/4W resistors.

R3 is a 1200ohm resistor. It's usually written on a circuit as 1K2 rather than 1.2K because if the decimal point gets lost in printing out the circuit, then it could be read as 12K by mistake.

The power consumed by P2 is peanuts. It is only 2.5milliwatts so anything from 1/8 watt up is OK.

Best of luck.
pebe

[walnutron]

Hi Pebe,
Thank you for sharing your time and your talent; this is something I couldn't have done without your help or the website. I built the circuit and it works! Unfortunately I'm not getting full needle movement at the gauge, but hopefully it's just matter of tweaking. When P1 is at max. resistance, the needle shows 7/8 full and when P1 is at 0 ohm, it shows about 1/8 full. I have some numbers that might help:
*R2 is 0V to 0.76V (rather than 0V to 0.68V).
*P1 is 33.6 ohm (rather than 30 ohm).
*S to - is 0.5V to 4.3V (rather than 0V to 4.3V). You had said that zero volts would not be possible at the input to the fuel gauge from TR1. I was able to achieve as much as 4.5V, but that didn't move the needle any closer to full.
*D3 was installed and it did make the gauge needle move closer to empty (1/8 full).
Please let know if I can supply any other information that might help.

Many thanks,
Ron.

[pebe]

Hi Ron,

We need to know that the meter behaves like the link on Google said it should. Remove D3. Then can you try shorting S to - on the meter and ensure that it does show empty? Then disconnect TR1 and R4 from S and connect a 100ohm resistor between S and - and see if it shows full?

[walnutron]

Hi Pebe,

To get the full spectrum I removed R4 and R5 and made an adjustment to P2; the needle now goes to full. If possible I would like to see the needle go closer to "E" when the sender is at 0 ohm. Thanks again.

Ron.

[pebe]

Hi Ron,

What is the voltage between 'S' and '-' on the gauge when the sensor pot is set to zero resistance?
A FET would be better than the 2N2222. I'll order a suitable one and try it.

[walnutron]

Hi Pebe,

I measure 0.78V when the sender is at 0 ohm. Thanks again.

Ron.

[pebe]

Hi Ron,
I have found a suitable FET was the IRF510 or an IRF540. Here is the revised circuit.

The voltage between ‘S’ and ‘-‘ on the gauge should be zero for the gauge to show empty. These are the results with different types of transistor:-

The original 2N2222 brought it down to .78V (19% of full scale).
The IRF510 brings it down to .07V (1.7% of full scale).
The IRF540 is even better and brings it down to .01V (0.2% of full scale)

Note that the diode in series with the gauge has now been removed.

I hope you find this one OK.

[walnutron]

Hi Pebe,

The second circuit works very well. When I put it together the first time the needle went to full no problem, but I wasn't satisfied with the empty reading. TR1's (IRF 5140) output at S was 0.3V. Having two more in my kit, I changed it out. Now the reading is 0.01V (just as you said it would be) and the needle is on E when the sender is at 0 resistance.

Adding a circuit was the perfect fix because of the flexibility it gives me. Another plus was the learning curve I went through. I'm a gearhead; this was a stretch. I sincerely appreciate the contribution you have put into the project. You have been generous with your time and extremely helpful along the way.

Many thanks!

Ron.

[pebe]

Hi Ron,

I'm so pleased it all worked out OK for you.

pebe