Analog MPG meter.

[jim-frank]

Here's a block diagram of a pretty simple concept for an mpg meter. The concept is based on the soundcard mpg monitor program- divide the fuel injector pulsewidth into the mileage and use a scaling factor to get mpg.

I've built and tested the prototype circuits for the duty cycle to voltage converter, the frequency to voltage converter, and the signal processors, which are simple comparators. So far, so good; I get voltages proportional to duty cycle and frequency of signals generated on my workbench.

I have a schematic for a simple PWM/analog division circuit that I plan to prototype in the medium future. The whole circuit should be easy to implement with a quad opamp chip, a 555 timer, and a voltage regulator chip plus a handful of discrete components.

If anyone's interested, I'll post schematics of the circuits as I build and test them. It would be nice to be able to read mpg on a simple tachometer style meter without having to lug my notebook computer around...

[dcb]

Hi Jim, have you had a chance to check out the mpguino? It could be assembled bare bones for less than $7 sans LCD and programming support, and only has a handful of components, and can drive an rpm type gauge with just some more code.

I've been entertaining adding a tach like mpg display to it for a while now.


You can think of the mpguino as a really tiny laptop

[jim-frank]

Sure, I've been following the mpguino thread for a while. I just wanted to do an analog design as a design excercise. I didn't realize a guino cold be that cheap- I was under the impression that it would be somewhere over $60 or so...

But I haven't checked back there in a while, either.

[pasadena_commut]

Where and how do you plug in a lead to pick up the fuel injector drive voltage?

[Gregte]

Originally Posted by pasadena_commut

Where and how do you plug in a lead to pick up the fuel injector drive voltage?

I did it on my GMC sonoma 4 cyl. by finding the correct wire on the ECM under the dash. You can just use one of the two wires of any fuel injector. One of them will be hot when the engine is running and the other will be switched, i.e. pulled to ground by the controller. It will be a square wave with a varying duty cycle that varies depending on how much fuel is being asked for. It will not be a varying voltage in the direct sense but can easily be turned into a varying voltage for feeding to a comparator or anything else you wish to do with it.

[dcb]

re: guino costs. The chip itself is like $4, add a couple zener diodes and a regulator/caps/resistors. There is a schematic in this thread. You can leave off the LCD and Buttons but it will be tricky to calibrate.

Open source PCB design

[pasadena_commut]

Originally Posted by Gregte

I did it on my GMC sonoma 4 cyl. by finding the correct wire on the ECM under the dash. You can just use one of the two wires of any fuel injector. One of them will be hot when the engine is running and the other will be switched, i.e. pulled to ground by the controller. It will be a square wave with a varying duty cycle that varies depending on how much fuel is being asked for. It will not be a varying voltage in the direct sense but can easily be turned into a varying voltage for feeding to a comparator or anything else you wish to do with it.

I was hoping to patch in right at the injector, since it is much more accessible on my cars than is the controller. However, I have not had any success in locating a test "tap", that could be plugged in between the cable and the injector. There are places that sell the "cable" side connector, but nobody seems to carry the "injector side" connector. The only other way I can see in is to strip a small section of the cable to gain access to the wire within, and for future reliability reasons, I would rather not go that route.

[jim-frank]

I used a tap splice connector. It's a plastic doo hickey that you clip over the wire to be tapped into, and insert the tapout wire into a little hole. Then you crimp it down with a pair of pliers and you're done. I did this at the wires to injector number one.

[dcb]

There's been some interest in an analog gauge lately here so I thought I'd put an example together. I've only run it on the bench, but it is a couple lines of code to spit out the current mpg (or whatever) on pwm pin 11.

I found a donor meter:



Took it apart, pried off the label plate:


Scanned in the plate and doctored it up in paintbrush:


Printed out the new label and carefully cut it out (used plate as template):



Glued the new label to the original label plate w/rubber glue, and glued the plate back with rubber glue. Hooked it up with a 4.7k resistor to pin 11 and ground.

slapped a test program together and played with maxv till I found a good value for full scale deflection. (had to swap meter leads when needle tried to move backwards)

void setup(){
  pinMode(11,OUTPUT);
}
#define maxv 120
void loop(){
 analogWrite(11,0);
 delay(1000); 
 analogWrite(11,maxv/2);
 delay(1000); 
 analogWrite(11,maxv);
 delay(1000); 
}

Confirmed that half scale deflection looked good also:


Making the new labeling was the hardest part

[ac7ss]

Once calibrated, you could easily use just the meter to show instant MPG. I think I will do that on one of mine.