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Mismatched Gauge Solution


Guest bigC

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Just a quick one to share my over-engineering of a simple problem. I’ve got an unmatched/inverted fuel sender to my smiths gauge. The sender is the Cortina type with integrated fuel pipe so it will be a bit of a pain to replace. My options were either:

  1. Make do and fill up when my gauge reads full.
  2. Get the reliable, tried and tested, aftermarket gauge matching unit for £40-50
  3. Over engineer my own solution for around £10.

Over engineer it is!!

The Bits I needed:

Arduino nano (chinesium one) £3.50 ish eBay

1k digital pot (SPI controlled variable resistor) + dip convert board £4.00 ish eBay

1k pot (thumb turn) to simulate sender £0.50 ish eBay

1k trimming pot £0.50 eBay

LED - liberated from an old pc. £0

Resistor - found in draw £0

Connector strip - in an old biscuit tin £0

Perfboard cutoff £0

A very basic overview of assembly. The trimming pot with the thumb pot (sender pot) are arranged as a potential divider to an Arduino analogue input to read the sender value. The digital pot is connected to the Arduino via the SPI inputs, the digital pot will be connected to the gauge. LED connected to a PWM output on the Arduino via a current limiting resistor.

This together all looks like this:

POT.jpg

Programming the Arduino:

The basic function of the Arduino is to read the resistance value of the sender, apply a function to give the desired resistance value and set this value to the digital pot which is read by the smiths gauge. I’ve decided to utilise some of the Arduino power and apply an anti-slosh function to smooth the resistance output. This function reads the input value every 100ms for 3 seconds and outputs a rolling average to the gauge. You can also output values from the Arduino to a laptop via USB serial. You can see the smoothing in the graph below. This will also be handy when I get round to setting this up to the actual sender unit. I twiddled the pot to simulate a bit of slosh in the tank (not inverted the output yet):

graph.jpg

The arduino can also activate the led as a low fuel indicator. the first stage turns on the led and the second stage (on fumes) pulses the led. the board has pwm so why blink when you can pulse! This will all be programmed to suit the tank.

 

The next step is to get this fitted to my gauge and make sure it doesn't let the blue smoke out of my digital pot!

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Or do what I do. I have a similar set-up, smiths gauge, sierra sender. I bent the arm on the sender so that when it was empty, it read empty, when full though it only reads 3/4 full. i'm not interested about the level when it's full, but i do need to know when it's getting empty. I've had it this way for years now, no problem.

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I have an inverted gauge as well, although it is very accurate at both (wrong) ends. And like you I don't want to start mucking around with the sender so would prefer an external solution, So would be interested in the circuit diagram when you have proved the system. Or maybe I could do a simpler solution as I don't need to trim I just need to invert.

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Guest Tim Norman

Found the earlier spyda that way Tim, but the later one worked on both the cars I fitted one on. can't speak for all of them of course, just the two I have fitted.

Would love to know what I am doing wrong then. I assume mine is one of the new ones as I only bought it 6 months ago!

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To be honest, I had the spiyda in the basket ready to buy but I thought, sod it lets build it. At least with the homebrew version, I can troubleshoot a lot easier with the graphical output, save settings to my laptop and easily replace hardware with the same settings if things go bandy.

 

alanrichey: To be fair, this would be the bare minimum hardware other than replacing the trimming pot for a 500ohm resistor. The rest of the magic comes from programming the microprocessor of the Arduino. My other thought would be to directly drive the smiths stepper motor from the arduino which would negate the need for the digital pot but increases the risk of ballsing up the gauge. I'll see where things end up and post my findings.

 

Andi: Arduinos can be bloody handy and you can code your way out of a lot of problems. It's worth having one or two to tinker with.

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