You're really making your life a hundred times easier if you just tap into the crankshaft sensor. This will eliminate the need for almost all of the hardware that has been discussed in this thread.
And the crankshaft sensor already offers you a ready to use logic level pulse. All you have to do is find out how many teeth there are on your car's crankshaft flywheel, which induces the logic level pulses in the crankshaft sensor. Then divide your measured pulses per time interval by that number (plus one, for a complete revolution) and extrapolate to one minute.
...and Robert will become a close relative. ;)
No, my friends, I'm not measuring voltage. I'm measuring pulses over time.
I cannot tap into the CPS. I already even have a circuit that taps into the distributor 12 signal wire that they use for tachometers, or tap into the coilpack signal wire. But since the industry is used to clamping a wire over the HT spark plug wires in 5 seconds and go, I'd love to use that method instead of tapping anything.
Also the CPS is the most sensitive sensor any little interference can throw the ECM off.
TomGeorge, I ran your circuit through a simulator. It seems to be frying the parts. But a circuit like that is something i need.