DI-194 (was Re: Data Acquisition newbie question)

[Brian Dessent]

Don DRI05 Ricciardiello wrote:

> My single cylinder engine sparks at TDC every revolution. Max is 9000 so
> lets say 10K upper limit to round it up. That would require a 166.6 samples
> per second which doesn't give me room for much else. If I went with Brian's
> LM2907 suggestion can that method be scaled so that 0 to 10K rpm=0 to
> 10Volts?

First of all, I recommend modifying a channel of the DI-194 so that it
range is 0-5V instead of -10 to 10V.  If you don't do this then you lose
half your resolution even if you make your tach circuit output 0 to
10V.  Because of the power supply issue, it's much easier to make the
tach circuit output 0 to 5V anyway.  Dataq talks about this modification
on their page (url below), basically you just remove two resistors.  See
the questions "Can you show me the front end circuit of DI-194?" and the
question right after it.

http://www.geocities.com/ultimaserial/faq.html

About the tach circuit:

The circuit on the top of page 9 of the datasheet will do that, almost
without modification.  The only components you'll need to change are R1
(which goes from pin 3 to ground) and C1 which goes from pin 2 to
ground.  The governing equation is Vo = Vcc * Fin * R1 * C1.  You want a
max Vo = 5V, and Vcc is 7.56V if you use the Zener regulation method
that this circuit uses (the 2917 has the zener option), and Fmax is
166.7, so that means R1 * C1 = 3.97e-3, but the exact value doesn't
matter because you can't get much better than +- 10% tolerance on
capacitors anyway.  So I recommend you make C1 0.1uf and R1 a 50k pot
(the target value is 40k with C1=0.1uf)  Vary R1 to calibrate the
circuit, I suppose you could use a regular tach for this, just watch the
output voltage at a fixed RPM and turn R1 until it's to your liking. 
Or, just rev the bike to near redline and turn R1 until Vout is near 5V.

Brian
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