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Yeh Homebuilt (I did the math, after all the screwing around, I should have just bought from you - but I'd have the same issue!).

Back to it, the 47K resistor was chosen cause it was best suited to 50K suggested in the circuit diagram.

I might clamp a multimeter to that jumper and see how it without the resistor (watch for peaks above 5V at all).

Is the zener necessary? I understood it to clamp down the voltage from 12 to 5 - I thought it might not be needed, but wanted it to work so followed the circuit to the letter.

Would the resistor really explain the pulse differences?

Oh - the code is the Metric version with smooth numbers that was published on the forum - I'm sure the code is correct for others use it, and it's not too indifferent from your work.