> A resistor from the opto emitter to ground. Exact value depends on current
> transfer ratio of your opto, and how much you are driving it with. Then
> tie the base of an NPN (like a 2N3904 or 2N4401) to the emitter of the
> opto. Both collectors (opto and NPN) go to your +12V. The emitter of
> the NPN goes to your speedometer.
>
> Let's see, 80mA of collector current. Make sure the transistor is rated
> for that much. A 2N4401 should have a minimum current gain of 100
> at those voltages and currents. That means the opto has to provide
> 80/100 = 0.8 mA of current, plus whatever is going through the
> resistor. e.g. if you use a 12K resistor, there's ~1mA through the
> resistor. So the opto should provide a minimum of 2mA
> (0.8 + 1.0, rounded up). More is better. Look on the data sheet
> for your opto, it should tell you the minimum current transfer
> ratio (ignore the 'typical' value, use the 'minimum')

Spec sheet says 7% CTR at 16mA If. If I read their graphs right, I can get
around 10mA output current with a 10v output when I'm pushing 40mA If.
Sheesh. Reminds me of the famous two trains leaving a station, one from
Philly...and so on. Even though I'm driving the hall-sensor with 12v, I'm
really getting close to 10v out of it back to the speedo.

> That will tell how how much you need to drive the opto to guarantee the
> proper output. But you probably knew that...

Yup. Already rastled with that one just to make it work in the first place.
However, I'll take that as a compliment. Thank you. Times like this I'm
pining for the binary.

-DO