>> use the opto to turn on a transistor? What exactly is the application?
>> i.e. what are you trying to turn on?

>It's an electronic speedometer simulator. I can drive the speedo fine but,
>once the turn signal self-cancelling module is plugged in, no output. The
>speedo is all that the opto can power (no load resistor necessary). I was
>able to determine that about 80mA is necessary to drive the self-cancelling
>module.

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