The idea started many years ago.  The original circuit was designed using LT3085 to provide the constant current dimming circuit.  It worked well but could not dim below 700uA and then there was the positive heat coefficient of the pass transistor.   I ended up designing my own JFET current source which offset the temperature coefficient of BJT and allowed me to create a precise analog circuit that reaches to about 250uA.  It was surprising to see that 250 uA can be a useful value for some LEDs and some situations.  I will gather my data and simulations I've saved and post updates. 

This project was challenging in many ways:

1. Originally was designed as 2 PCBs sandwiched with standoffs but my wife said it looked unfinished.  I like the exposed look of it.  This took my usual 2 dimensional design into 3 dimensions and took a few iterations to get right.

2. The analog design is challenging and very touchy.  Many simulations and test circuits brought me to my final design.

3. To get precision from the ADC required some interesting solutions.  The debugging was fun also. 

4. Final design to improve accuracy and make calibration easy as well as put it into a nice case.