OK, that's weird, my copy of the email has text!

Here is what I wrote.

I have a graphics LCD module that I've purloined out of a defunct 
Xerox printer, which I'm reverse drawing so that I can use the module 
in my projects.
 
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?
 
The transistor switch is a PNP type, the emitter is switched by the 
uC. The Collector goes to the base of the PNP pass transistor. Emitter 
of the pass BJT connects to the emitter of the switch, Collector goes 
to anode of the backlight, cathode goes to +5v.
 
Unfortunately my CAD package doesn't like copy and pasting outside 
it's own environment, so my attached diagram will have missing 
connections.
 
Colin
--
cdb, colin@btech-online.co.uk on 10/08/2009
 
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