I actually had doubts about the accuracy of this method, so I put this on the shelf for the summer (actually, in the glove box).
Basically, the injector has 2 leads, one going to 12 volts, the other going to the ECU. The ECU shorts that lead to activate the injector. In saturated injection, it just leaves it grounded for the whole length of injection.
With peak and hold, (as best I can tell from my observations) the ECU shorts the lead to ground for a very short period of time, then switches it to ground and back
extremely quickly, like a light dimmer to limit the current flowing through the injector.
The way that my circuit worked was that whenever the ECU grounded the injector, it also grounded the "trigger' wire on the transistor. The transistor is a PNP (2N3906), so whenever a little bit of current flows from the emitter to the base, I can measure 12 volts at the collector.
The problem that I had with the circuit that did not have a capacitor is that the transistor was switching fast enough that I could see the actual pulses of the "hold" part of injection (where the ECU pulses ground very fast). I was getting a long pulse of 12 volts, then a bunch of short pulses of 12 volts. This wasn't very useful because I wanted to measure the "length" of the short pulses.
The last schematic you see has the capacitor which "holds" the voltage up enough that (hopefully) the guino would see it as one long pulse.
As far as the actual values of the resistors and stuff, most values are not too important, except for the capacitor and the resistor that was across the capacitor.
The resistor between the ECU and the base was 220 ohms, if I remember correctly, the one between the base and emitter was a very high value (to ensure the transistor turns all the way off), something like 10k to 1M.
The capacitor was 4.7 uF, and the resistor I used was 100 ohms, I think.
Something I noticed was that the ECU grounded the lead of the injector when the car was turned off, so that would put constant voltage through the "load" resistor (the one across the capacitor), and because the resistance was so low, it would get kind of hot with the amount of current that would flow through it. The way I solved that was by powering the circuit on a lead that was only hot with the engine running.
Hope that answered your question
