I can remember the big pound notes before they brought out the smaller versions, and the old pre-decimal pennies and sixpences. Not the white fivers though!
This is a way simpler problem and you just need to stick an opto isolator, a couple of diodes and resisters on the tach signal. It will pick up the pulses from the coil that would release the magic smoke from an arduino and output a nice safe 5V/3.3V pulse to the arduino. Then its just a bit of code to turn the pulses into a digital rpm output for your excel sheet.
Probably about £1 in parts!
I dont know tons about it but an opto isolator is a Schmitt Trigger. You wont need to mess about with anything massively complicated to iron out the spikes other than the conditioning achieved with the resistors.
Never tried it though so if you invest in all the parts to test it and it doesn't work I'm afraid you have lost an entire pound note!
Thanks Russ, as ever, good advice! I think you’re right, a bit of signal conditioning and a couple of diodes would do the trick. I was thinking the hysteresis of a Schmitt would clean up the secondary “bounce” pulses. I guess with a large enough pot/resistor and a zenner you can “tune out” the secondaries. Good food for thought. Thank you.