Dave,
Looks much better. Still a few problems however.
With 3.3V drive from the PIC, the emitter of Q2 can only reach 2.7V, the
emitter of Q23 about  2.1V. The 220ohm resisor will limit the maximum total
LED current to (3.3 - 1.5) /220 = 8mA max or about 3mA per LED. But you may
get also current limiting due to the 47k base resistor on Q23

Your best bet with this circuit is short out the 220ohm resistor and the
47k resistor. LED current will be about (2.1-1.5)/47 = 13mA per LED or 40mA
total. For more current, reduce the 47ohm resistors. (Down to maybe 1.5 ohm
for 380mA  or 1.2 ohm for 500mA - it will depend on LED froward voltage
drop - I have allowed 1.5V for this
).
If you are really likely to run down to 2.3V for Vcc, then for constant LED
brilliance,  some sort of constant current supply for the LEDs would be in
order. Not a real easy task at  such a low voltage.

(Or if you have a spare PIC pin you could squarewave, then you could use a
boost circuit to produce a higher voltage & make life a lot easier - or
maybe just introduce a complication somewhere else!)

 Richard P




                    Dave King
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                    20/02/02 11:02
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Hi Richard (and anyone else that looked at my mess)

What I had put up was a non working schematic, I've tried so many
many variations that I just put up the section that I though needed to
be sorted out. I was hoping it might allow someone the idea of what
I was trying to do. Ie using the available current from an output pin on a
pic to
drive a 2n2222 that in turn switches a larger "power" transistor which
hopefully
switches off and on with my desired pattern..

I think I'll correct it to the point where it might make sense and get rid
of the
schematic portion I have on there. That might eliminate a lot of the
confusion.

If you can look at it again (give me an hour to correct and upload) it
should make
more sense.

Thanks!

Dave

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