|
I think we could do this with a 12c509 if we wanted as cheap as possible, it would be a pain to code and have no expansion possibility but we could build it and a 3 digit lcd display for probably $10. (not that I can remember what i/o pins it has, it has been a while)
I think if we can get the hardware cost around $50 that would be a reasonable point. The ATMega series has a huge possibility for expansion and also we could do away with the *duino and build the circuit with just the 168 chip if someone wanted to have a surface mount compact system built. The development board could be optional for the people that just want a mpg gauge and aren't interested in future expansion.
If I was going to do the injector sensing. I would first cut the voltage down to 5v using a zener diode. That will give you a more or less 0v/5v signal. The signal could be cleaned up with a few additional components if necessary. The easiest way to do it would be set 2 pins to interrupt, one rising edge and one falling. Use those and a counter to figure out the injector open time. True duty cycle could be calculated but it really isn't as important as injector open time. I would probably let it go through 3-5 injector firings adding the open times together to use in a calculation. A metro has 3 injector pulses per 2 revolutions so that would still be a lot of calculations per second. Averaging 30 would still give a pretty fast response for the display.
The VSS signal is always so many pulses per mile. Something like 2000 pulses per mile is typical. Cleaning up the signal so you get a digital 0/5V would be the best in my opinion so the avr could just use an interrupt to process it. That would be much easier than doing an A/D setup and monitoring the voltage for the pulses.
That will save the A/D inputs for future expansion to monitor the analog sensors available on a car. But for V1.0 a good expandable menu system and basic functionality should be the goal. So before any hardware is totally settled on a rough idea of the roadmap through V2 or V3 would be nice.
|