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Thanks, Bob.
After bench testing three different circuit types, I finally settled on an lm339 based comparator (see below). Bench testing a breadboard version went well. An audio sine wave produced a TTL signal that the mpguino could track up until 488 hz. Though that is not enough - my truck vss produces 10hz per mph. So that is only 48 mph. I considered that it might be related to the perfboard layout, or perhaps my ttl isn't ideal.
I added the circuit to my perfboard, but the in-vehicle tests were a disappointment. I am apparently getting interference/feedback from within the mpguino circuit. I suspect the lcd driver is sending pulses into the ttl driver. My mph climbs, but then stays elevated at 25 or 40 mph - even after the ignition is shut off and the vss and inj inputs are disconnected from the mpguino.
So I think the next step is to cut the perfboard to separate the sine2ttl circuit from the rest of the mpguino. Any suggestions on other steps, like circuit mods, to otherwise improve the situation would be appreciated.
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