Why cant you mount a probe in the timing hole itself for mag prove then set in offset of 20deg?
That's the plan! Just have to take measurements and find a sensor that'll fit with the proper output.
I'm looking at a NO inductance sensor... give it 12v off the vehicle, mayyybe use an NPN sensor to pull a 5v PNP fet to get the output arduino friendly. Ideal scenario would be to generate a positive pulse everytime the timing mark goes by, but I know it'll likely result in a dip in an inductance sensor output (based on tach sensor testing). That's where a threshold would have to be set so the NPN could pull down a PNP 5v fet.
Or I keep it all positive voltage and see what the output is like from a PNP NO sensor. Use a voltage divider to get it to that 0-5v friendly range and have arduino watch for the voltage drop as the timing mark whizzes by. Surely it'll have a fairly steady voltage until that mark goes by.
Another random thought... at idle, say 10 rps, the timing mark on the harmonic balancer is moving at about 282 inches per second, and if the mark is (generously) 1/8" wide, it's only 1/226th the size of the wheel, so from start of groove to end of groove it'd take .0044 to pass by the sensor. The chip on the Trinket can run at a maximum of 16khz, so about 0.000625s per cycle, not counting instructions being executed... so yeah, as long as the code re-runs fast enough we should still have good resolution on the RPM. I was worried the balancer may outrun the microcontroller!
UPDATE
Well my first filter didn't work. A little zener diode, 1uf cap, and 1k resistor wiped out the signal. Without the diode it was down to a few mv. I need to take another approach, maybe half wave rectification with a pass filter to wipe out the low end noise.
UPDATE2
Bridge rectifier inbound Saturday. It'll get all the juice that the stock sensor puts out onto the positive side of the spectrum and then I can re-eval the waveform and maybe run a high pass filter to isolate the peaks.
UPDATE3
Test mule is down... banks air box being disposed of and turbo plumbing being reworked. Got a full and half bridge rectifiers in, and soldered them up on test boards with alligator leads.