> >they look identical, how do you tell one from the other??
>
> There is normally an indicator on the case to indicate + and -
> for the LED, and possibly E and C for the transistor. These
> markings are often moulded into the case rather than printed
> on

I've had the odd one with no markings, in which case it's trial
and error. You can find which is which (and the polarity of the
emitter, a diode) with an ohmmeter. For 5V operation put a
220R resistor in series with the emitter and ground the other
side. The polarity of the detector may not be obvious. Put a
10k resistor in series with it and ground the other side. 5V goes
to the other end of the 10k resistor. If you got it right (50/50
chance), you'll see the voltage at the 10k/detector pin junction
dip when you point the opto at a piece of white paper. If nothing
happens reverse the connections to the detector

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