Simple project needing help - Invert Resistance / Ohms

So to start off thanks to any and all who offer up some help I really appreciate it. I'm needing some help to make me look a little more like a rockstar with my son. He recently turned 18 and we got him a 1970 Opel GT (he's always wanted a 60's -70's project car and daily driver) over the past few month we've really done a lot with the car. We're at a point now that we are replacing the old worn out gauges with new Auto Meter ones. Now I know there are tons of threads already on doing this and that with gauges in cars especially temp and fuel systems so I read a lot of them and I promise this is different.

On the Opel GT the fuel sender in the tank does something unusual in the automotive world. Traditionally a fuel sender on a GM vehicle up to the late 90's reads 0 ohm empty and 90 ohm full. The Opel just to be different or some other reason reads FULL at 0 ohm and EMPTY at 90 ohm. The current sender works well and while yes I could swap it out with a new one that's adjustable it's seems overkill. So I'm wondering if anyone knows of a reasonably straight forward way to read the 90 to 0 ohm (I realize it needs to be done via applying voltage etc when reading it) but then I need to output the opposite to work with modern gauges.

In a pinch the gauge should work as is just be reversed i.e. full on E and empty on F. I'm not super knowledgeable on all things arduino but I don't think I'm a moron either. I've built ESP8266 blinds controllers and have several Raspberri Pi's at home in use. The main one on my 3D printer running Octoprint. So I've got a few parts laying around and love to tinker.

I'm hoping for a small reliable dedicated circuit and I appreciate any help that can be offered here.

Steve

A variable resistor typically is half a pot. If you keep the wiper and flip the other contact, you'll get something close to your description. If this is not what you want, read the resistor value and use a digital pot to output the inverse resistance.

DrDiettrich:
A variable resistor typically is half a pot. If you keep the wiper and flip the other contact, you'll get something close to your description. If this is not what you want, read the resistor value and use a digital pot to output the inverse resistance.

Thanks for taking the time, I'll admit right off the bat I learned two things in your short post. first digital pots (used mechanical ones many times before) and second what a wiper is. When I first read your reply I was thinking wiper? I need help with a gas gauge not my wiper lol. So it definitely looks like this is on the right path for me. I found this page walking through the basics https://www.arduino.cc/en/Tutorial/DigitalPotentiometer as well so that should get me started in the right direction. I have 2 followup questions though.

  1. I see a lot of reference to all of these projects using 5v, how will a 12v system play into things with the arduino?

  2. Given the simple requirements needed for this, do you have a recomendation for board to build this from? As mentioned in the OP I've got some ESP8266 experience and setting up ATmega 2560's with my printer. I like the size factor of the 8266 but admit I'm not familiar with many other boards out there.

Thanks again,
Steve

So I was doing some more research and found someone else working on a somewhat related project and got me thinking about applying it to this. Using a yet undetermined board to read the fuel sender resistance and using a micro servo driving a mechanical pot. Those servo steps would then be mapped inversely to the incoming resistance. My immediate concern on this is around the servo getting out of sync so I'd need to build some zeroing out logic in there so the servo would always start over every time it was switched on. Now this is starting to sound like more work than the previous suggestion :slight_smile:

Steve

You'll have to interface the 12V sensors and actors to the typical 3.3V or 5V controllers. See the many threads about vehicle automation.

Can you tell the intended use of the fuel signal? I'd guess that you don't need a digital pot for only driving the needle of a fuel meter by a controller.

Perhaps something along these lines.
Sender should give 0-4v or there-abouts.

Gauge.jpg

Gauge.jpg

bluejets:
Perhaps something along these lines.
Sender should give 0-4v or there-abouts.

Thats a good point, we just got the dash apart last night and the gauge is coming out today so I'll see what readings I can get off of it.