Wesley,

I guess a better idea would be to use a single BD679 NPN darlington with
the collector connected to the common cathode of all the LEDs and the
Emmiter grounded.I don't know the current rating of your LEDs
(typically around 25 MA each) @ 25 MA, your whole array would consume about
5.5 Amps. So I'd suggest using a good heat sink. Then, you can directly
drive
the base of the Transistor with a PWM from the PIC.

The Best solution would be to use a MOSFET like BUZ10 (I suspect BD679 would
die out too soon at 5.5 amps!).And moreover the internal resistance of
MOSFETs
is a lot lesser than Bipolars, Which will greatly increase the efficency
with
a matched decrease in the Thermal Dissipation.

My personal philosophy is to reduce the component count without complicating
the circuit. With the above given topology, you might even write software
to switch off the display after a brief span of inactivity or lack of user
input.
Mobile Phones had this feature from ages

Regards,

Jeethu Rao

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Wesley Moore
Sent: Saturday, June 23, 2001 6:36 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Using ULN2803


I am only driving LED's. I have a 22 x 10 matric of 3mm LED's. The reason I
want to vary the voltage is to provide display dimming in the dark since the
unit is destined for my car.

Wesley

On Fri, Jun 22, 2001 at 10:53:02PM +1000, Roman Black wrote:
> Hi Wesley, the ULN2803 does not make output voltages,
> it is a bunch of darlington transistors in a chip
> designed to be either on or off.
>
> If you explain more about how many devices you
> are driving, and what they are, and why you need to
> vary the output voltage then many people can offer
> suggestions.:o)
>
> If you need the ULN2003 datasheet I can email it to
> you.
> -Roman
>
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--
Wesley Moore                      http://www.1moore.org/
RMIT - BEng (Comp Sys Eng)/BApp.Sc. (Comp Sci)  3rd Year

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