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Mike, thank you for the help!
I see now the importance of the logic output level - I hadn't realized it might vary with input level.
I mocked up the voltage divider and did some bench tests. However, my sound card signal generator is voltage limited. The a/c voltage does not increase like the actual reluctor.
Even with the divider, I was still limited to about 240hz. And if I push the input frequency higher, the mpguino reported pulse rate rapidly drops.
So I'm still puzzled. Are my peak pulses too low, my lows too high, or is there another issue with pulse detection at that rate, perhaps software?
I may try and go back to using a spare arduino to generate logic pulses in order to verify that the mpguino can actually detect at the necessary rate.
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