Dave,

I think your application is almost certainly entirely &v easily doable with
3.3 V but I am somewhat uncertain EXACTLY what you are trying to achieve.
I have looked at your posts and website cct. While I THINK I know what you
want I've seen enough such things to realise there may be something required
which isn't quite spelt out.

If you could spell out as exactly as possible what you want to achieve -
RATHER THAN how you think you might achieve it, then I and others can almost
certainly help.

Is it

- Turn LED display on and off under PIC pin control.
- Operate from 3V3 supply.
- Operate common cathode LEDS
-  Drive LEDS with up to 350 mA total consumption at any one time.
- LEDs operate OK with <= 2.5V drop across them.

    ie are these LED strings or single LEDS?
    What is the voltage across the LEDs when running at full desired
current.

If this is the whole requirement then consider it done.

Note that you should almost certainly NOT be using an emitter follower as
you are here with such low supply voltages.
The emitter follower has a MINIMUM 0.6v drop across it. Unacceptable in this
application.

In your latest circuit replace the MPS3904 with a PNP bipolar transistor
with suitable rating.
Swap collector and emitter connections.
Drive this transistor's base directly from a PIC pin with a 470 ohm
resistor.
LEDS will be on when PIC pin is low.
A second driver transistor can be arranged if needed when we know answers to
the above questions.

The above PNP needs to be suitably high current, high Beta (current gain)
and low saturation.
A Zetex ZTX749 would be very suitable but there are others.
A BC327 may be marginally suitable but is a good starting point.

A FET may be possible but would need to be a logic FET with an extremely low
threshold (operating) voltage and you don't really need one here.

If there are other requirements please specify fully.

If you copy your response to rusl@paradise.net.nz I will probably see it
sooner.



            Russell McMahon

.


      Russell McMahon
_____________________________


> If you've been looking at this trying to sort it out I've uploaded a
version
> with actual resistor values and such as well as showing a image of the
signal
> that is at the base of the transistor Q23.
>
> I haven't played with the resistor vales on the emitter side of Q23 yet
> (R21, R23, R24)
> so you can ignore these components for now. A few of you have mentioned
the
> problems
> coming there so I'm not ignoring it ;-]
>
>  From what everyone has said It looks like this will be a no go on 3.3v
> unfortunately. I'm going
> to redo the entire board to 5vdc later on unless someone mentions the
> "miracle" transistor
> part number that will make this work. I would still like to have a 3 or
> 3.3v version but it looks
> like i'd have to pretty much scrap the way I'm doing the board and the
> external leds.
>
> If anyone would like to make any suggestions I've upped the images again
to
> http://members.shaw.ca/gnat/led.htm
>
> I think I can see the light here and I really appreciate all the help from
> everyone! THANKS!
>
> Cheers
>
> Dave
>
> --
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>
>

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