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Well, i'm almost there. Took it for a test drive today to check the mph/miles, The result: crap. Mph was severely low & inconsistent. Overall 8.9 miles on the odo were 2.46 mi on the guino.
I jacked the front wheel up & checked the voltage on the vss line. 4.73v with guino disconnected. Connected the guino, and got 4.62v before the 100k resistor & 2.44v after. I figured the voltage was being pulled down too far, making it marginal for the atmega to pickup. I spun the wheel by hand & watched the pulses on a voltmeter - about 30 pulses per 13 rev. Spun it 13 again & ticked off 59 vss on the guino. OK. Spun it 13 revs again - but a little faster - and the guino hit 52/60. 13 more, faster, and 46/60. 13 more the slow way, 60/60.
So I rerouted the vss through a 300-ohm resistor, and now i'm getting 3.6v instead of 2.44. Spun it as fast as possible - 180/180 after 39 spins. Looks good. Took another test drive, and mph looks steady & reasonably accurate. Odo vs Guino was about 2.95 vs 3.00 mi, so that looks decent.
The bad news? Now my speedo has 3-5mph bounce when i get over 25mph -- even though Instant MPH is rock steady on the guino. This bounce may also account for the 0.05 mile discrepancy on the odo.
I guess i'll have to play with different resistors, unless anyone has some better advice ? Is the zener diode supposed to prevent vss from grounding in the guino, or is it there to limit voltage to 5.1v ?
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