Depends on how much current the op-amp can supply.  The op-amps that I used way back then could supply only a small output current.  The emitter-follower fixed that problem.

Tank you Sebelec :)  

Hi Dwane, I don't see this. For me The output of the op-amp (called VRef) does not move as its value only depends on potentiometer P1. It is true however that even when no led are present, all the transistors are forward biased. But when you plug a led Vref does not change (nor VBAtt theorically). Did I messed something? Regards, Michel

Really good project Pascal, quick, cheap and ... usefull. Poor cats!

why not using a LM3409 of TI? It's a PFET buck controller for high power LED drivers. It can be used for constant current sources with a range from a hand full of mA up to 5A !!! Having such a battery of current sources it will be easy to compare low power LEDs and high power LEDs with the same circuitry. The output current is adjustable using only a single constant voltage from 0V to 1,24V. This voltage could be derived from a DAC for each channel. Output is absolutely constant current - no pulsed DC! The IC works with an input voltage range from 6V to 42V. Only reverse voltage protection should be used at the input. For more information see TI-website. I used this IC in many projects. It works very efficient. No extra cooling is needed.  Last but not least: LM3409 with all peripherals might equals the cost of one LT3092 @ Reichelt :-) Regards, Claus