Got bored and it was nice outside so I went out and tested it. It works good until the pulse width gets pretty long. The voltage drop on the injector decreases past 2.5V drop towards the end of the pulse. So I put a potentiometer in place of the two resistors so I could vary the voltage drop it is looking for. Turns out it works perfectly 100% with a voltage drop of ~0V to 0.5V. I can turn every accessory in the car on at once and the numbers don't change other than when the computer responds to the extra load on the engine and opens the injectors slightly more. There is no jitter or variance from what the laptop is reporting from the ALDL port.
So it might be possible to do away with the zener and potentiometer and just go straight for 12V on the - input. I think it needs a voltage drop of 0.1V on it though just in case there is a tiny difference in voltage on the MPGuino compared to injector voltage. Maybe just a standard silicon switching diode could give a 0.7V drop and skip a lot of the mess in the circuit. I will pick up another op amp this week and build a breadboard interface circuit for the official MPGuino.
Maybe use a 1N34 diode if I can find one, I think they had a pretty small voltage drop of like 0.3V or something and very little capacitance. A bit more experimenting and I should have the simplest circuit I can get on there figured out. Right now it is 3 components a zener, resistor, and pot. It might also be possible to just connect - straight to 12V and use a potentiometer on the offset null to tune the circuit. Once I get one on a breadboard I can try a few things and see what I can come up with that is reliable and simple now that I know the basic idea works good.
Also the ascii drawn circuit has the + and - reversed if anyone wants to build one
