Controlling an analog dash

[DaveH]

You will know if you have the wrong resistors if your gauge won't go all the way up with 100% PWM, or vice versa with 0%.

[Bigpie]

I can make the dial go to empty and full with my resistor pair manually with 1 and then both in series, so should be the right ones.

[muehlpower]

I can make the dial go to empty and full with my resistor pair manually with 1 and then both in series, so should be the right ones.

In my previous cars there was always a voltage regulator for the temperature and fuel gauge to compensate for fluctuations in the on-board voltage. Maybe it works with your car via pwm and your pwm messes up this regulation.

[Bigpie]

How did you control them in your case?
*EDIT* the IR520 did the same, maybe a need much higher frequency PWM? Digipot might be the answer.
**EDIT** Given up and ordered a couple of digipots to play with.

[Bigpie]

Well the digipots work to control the dash. 10k seem to be the easiest to get hold of quickly, but they only give 3 positions on the fuel gauge without paralleling them up. I'll have to get a 1k from China.

[Jack Bauer]

Check out the schematic on this guy :
https://hackaday.io/project/6288-can-bu ... -simulator
Super simple way to drive the vag fuel gauge.

[Bigpie]

Great find, that's a genius solution.

Code: Select all

void Fuel(int amount)
{
  pinMode(2, INPUT);
  pinMode(3, INPUT);
  pinMode(4, INPUT);
  pinMode(5, INPUT);
  pinMode(6, INPUT);
  pinMode(7, INPUT);
 
  if (amount >= 90)
  {
    pinMode(2, OUTPUT);
    digitalWrite(2, LOW);
    return;
  }
  if (amount >= 75)
  {
    pinMode(3, OUTPUT);
    digitalWrite(3, LOW);
    return;
  }
  if (amount >= 50)
  {
    pinMode(4, OUTPUT);
    digitalWrite(4, LOW);
    return;
  }
  if (amount >= 25)
  {
    pinMode(5, OUTPUT);
    digitalWrite(5, LOW);
    return;
  }
  if (amount >= 10)
  {
    pinMode(6, OUTPUT);
    digitalWrite(6, LOW);
    return;
  }
  if (amount >= 0)
  {
    pinMode(7, OUTPUT);
    digitalWrite(7, LOW);
    return;
  }
}

[DaveH]

That's a very cheap kind of DAC (see also https://en.wikipedia.org/wiki/Resistor_ladder). Funnily enough I was just thinking about using a simple DAC yesterday after testing one of my gauges with PWM (will post a video later). The above method is fine if you have plenty of I/O available but I want to control at least 2 gauges and not sure I will have enough I/O on a Teensy 3.2.

[arber333]

That's a very cheap kind of DAC (see also https://en.wikipedia.org/wiki/Resistor_ladder). Funnily enough I was just thinking about using a simple DAC yesterday after testing one of my gauges with PWM (will post a video later). The above method is fine if you have plenty of I/O available but I want to control at least 2 gauges and not sure I will have enough I/O on a Teensy 3.2.

What about using multiplexers?
https://eu.mouser.com/datasheet/2/427/d ... 764700.pdf

Or simply LED drivers?
https://www.ti.com/lit/ds/symlink/tlc69 ... &distId=26

[DaveH]

Here's what I did yesterday. This is using a darlington transistor as that's what I had to hand. It works, but the PWM is audible with this kind of gauge. It's quiet, and it was also raining on the steel roof, so you can't hear it very well in the video.

[DaveH]

What about using multiplexers?
https://eu.mouser.com/datasheet/2/427/d ... 764700.pdf

Or simply LED drivers?
https://www.ti.com/lit/ds/symlink/tlc69 ... &distId=26

I don't think LED drivers would give enough current for most applications, also the max voltage of that particular one is only up to Vcc+0.3. Analog switches also don't allow much current and have a relatively high on-resistance.

[DaveH]

I've come up with a simple low cost solution that should work and still uses PWM.

I made a simplified model of one of my gauges, represented by Rp, Rs and Rn. The maximum current for this is about 70mA.

V2, V4 and A1 are modelling the PWM logic output with a 3.3V supply. This is low pass filtered by R4, R5 and C2 to give about 3mA peak-to-peak ripple in R1.

I'll breadboard this when I get a chance, just need to buy some bits.

[muehlpower]

I removed all of the original mechanics from my analog dash and replaced them with stepper motors. Needs a little more mechanical skill, but now I have full control and low power consumption. I found the necessary information on this page: https://realsimcontrol.com/io_step.html
There is also a special watch stepper that I used to replace my watch.

[clanger9]

Excellent, thanks for that pointer. I'm looking at a stepper motor solution for my (mechanical) rev counter. This seems like the way to go...

[DaveH]

I breadboarded the circuit described earlier. It works great, but the gauge behaves strangely in that when you reduce the current, the pointer doesn't come down to the same place as when it's going up. I can drop it below where I want it to be then bring it back up again, but this isn't ideal.

It's also nonlinear, but I can work with that.