On Mon, 10 Aug 2009 00:45:03 +1000 cdb wrote: > I'm wondering why the backlight is turned on via the emitter of the > pass transistor instead of switching by base. In other words, why > would the designers have chosen a common base configuration for this > type of application? I think you'd better do a screenshot (PrtScr in Windows, then paste in Paint) of the circuit, as several important connections seem to be missing. Common base circuits are frequently used to do voltage translation. Which probably is necessary, as I'm guessing the cathode of the backlight LED is connected to the +supply. That means the anode must receive an even more positive voltage from somewhere? John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist