I'm thinking of using this circuit to drive 5 of these:
http://www.rapidonline.com/netalogue/specs/55-1310.pdf
I think that if I set the pin to an input (high impedence) that I should get my constant current and set it low that should turn the LEDs off.
Do you think it will work?
Also I've shown the LED as 3 LEDs because there are 3 in 1 package. Unfortunately the datasheet isn't really clear but I think they are not internally connected.
Would it be safe to assume that they are closely enough matched that I can use one driver per package or do I need seperate drivers?

 

I decided to just go for it. I didn't bother with the microcontroller and changed the resistors to 1k and 20 ohms. The SOT-23 led measured 1.66V with the 1K resistor across it and the 3 white LEDs in a package were within 0.03 V of each other so I thought that would be fine. Current is near enough to 50mA at anything over 4V which is ideal for 4 NiMH AA batteries.
Maybe not the best soldering job in the world but only my second go at SMT.


Self portrait of 1/5 of ring light attatched to macro lens in mirror:

Result: nice even lighting, not much shadow.

Detail of above:

 

Nice .....

 

Would it be safe to assume that they are closely enough matched that I can use one driver per package or do I need seperate drivers?

For future reference... If you use one resistor per led, instead of the 1 resistor below the transistor shared for all 3 LEDs, then you don't need multiple drivers, and you don't need to worry about voltage matching of the LEDs.

Using different valued resistors per LED, you could even drive completely different type of LEDs with the same output, or adjust different color LEDs for different efficiencies, etc.