Digital Fuel Reading

[letsrelaythat]

Hi All,

After filling my car with fuel, the gauge took a good half hour to move. Obviously older cars will do that, but I am looking to upgrade and install a digital fuel meter.

One of my mates had his fuel sender unit swapped, cause his wouldn't read any values near the middle of the gauge, i.e it would read always empty when he had either very low fuel or fuel right in the middle of the range.

Anyhow I borrowed his sender unit, and established that the reading when the tank is full, is just over 1500ohms, and empty is 1760ohms.

I want to make a 5 LED display, using tri-colour LEDs, i.e full tank is all 5 LED's green, just over 2/3 tank is one green LED. Just under 2/3 is all amber LED's, down to just over 1/3. Under 1/3 lights the red LED's up, right down to the last one.

I was thinking maybe a PIC, but I've never used one before and I want to keep costs down. If anyone has a good idea or a schematic/link that would help it would be much appreciated.

Thanks in advance,

Tim.

 

[McGuinn]

Yes, maybe a PIC with an ADC and a wheatstone bridge to give an output voltage.
As I am finding with PIC's, if you haven't got a pre-bundled kit of programmer, software and cables and such, it's a pain to get into.

 

[gerty]

Before you design your circuit you might want to verify the resistance values on your car. The values vary according to manufacturer and model of vehicle. One vehicle I worked on was 33k empty and 1k full.

 

[Oznog]

I don't see where you need a Whetstone bridge at all. The sending unit is going to be either a potentiometer or a rheostat. A pot would need a voltage divider since it will send more than 5V, a rheostat would use a pullup resistor.

In either case the output impedance of the sender circuit will probably be too high to feed into the PIC's ADC to get full accuracy, you will want a low offset op amp to buffer it first.

Be aware that the lack of useful readings may be in the sending unit. If installed incorrectly, it may be sticking against the side of the tank. Or the pot may be scratchy.

Don't listen to McGuinn- for a PIC, you just need a good programmer (under $100) and a serial cable. It's not rocket science, but it performs like rocket science. It can make building just about anything 10x-100x easier.

 

[Nigel Goodwin]

you just need a good programmer (under $100)

That sounds rather expensive! - under $25 would be more like it!.

 

[letsrelaythat]

Hmm well at this stage I still want to stay away from a PIC, just keep it to simple soldering. I thought that electricitiy follows the easiest path, so if I had a resistor value for eac stage of the tank, and had a resistor which has less resistance than the sender unit reading, couldn't i light up an LED by making it choose a path that has less resistance than the sender unit?

thanks in advance

 

[Nigel Goodwin]

Hmm well at this stage I still want to stay away from a PIC, just keep it to simple soldering. I thought that electricitiy follows the easiest path, so if I had a resistor value for eac stage of the tank, and had a resistor which has less resistance than the sender unit reading, couldn't i light up an LED by making it choose a path that has less resistance than the sender unit?

It's not as easy as that, you can do it using zener diodes, but it's pretty crude.

A far better idea is to use the LM3914, a chip designed to drive 10 LED's from a variable voltage - exactly what you need!.

 

[letsrelaythat]

Hmm I might try that, anyone know an electronics program that has IC's like this that I can use?? I normally use crocodile clips but that doesn't have any IC's, well like 6 of them...

Preferably free ones are good

Cheers.

 

[mataston]

pic micro programming

pic micros are easy to program, there are loads of software out there on the net to do it, pic basic and picbasic pro being 2 of them, uses basic language.
program is written using just a text editor, all compiling is taken care off.
check out www.asamicros.com here you will find programmer and software and schematics should you wish to build one yourself. i bought one of these and it works a treat. hope this helps.

thanks