Hello James,

You don't need an oscilloscope to determine what is wrong with
your project. A simple function generator (LM555 circuit) and
simple frequency counter is all you need. Shouldn't cost your more
than $2.00 for the parts. Feed the output of the function
generator (LM555 circuit) into the 89C2051 while you hook up the
simple frequency counter. You can even provide feedback from the
89C2051 to stop the function generator when the proper amount of
pulses have been counted. Once you get this part of the project
working then you can turn your attention to the possibility of
noise from the tachometer. Below are some web site which have
circuits you can build to construct a function generator and
simple frequency counter.

Regards,

Charles Bannister
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Simple Frequency Counter
http://www.hanssummers.com/radio/sfreq/index.htm

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LM555 and LM556 Timer Circuits
http://home.cogeco.ca/~rpaisley4/LM555.html