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Mathless LED Driver Tested
Broken SD Cardreader replaced, picture taking capabilities restored. Most will probably agree by the quality of the pics that soon is the time for a new camera.
The circuit was changed, replacing the R2 of 1K for a 10K. This drops the current required for the reference transistor, Q1, to under 200uA, an insignificant level compared to the LED current draw.
Testing was done with a LM317/LM337 based cigar box PSU, and a Fluke 110 DMM. The assembled circuit acted as expected, and as simmed by LTSpice. The Vbe of the reference transistor, Q1, was tested as being .65V, giving a forward current of slightly under 10mA with a 68R resistor for R1. Current, and thus brightness, was consistent from 3V to 18V, althought higher voltages are possible while still keeping the LED driver transistor, Q2, within its absolute maximum power dissipation spec.
Possible Mathless LED applications include dynamic input LED lighting for people with no clue about electronics or even basic math, or in an academic lab as a current source experiement.
Posted by rmrubin 
September 2007