AVR-Chat

Thank you.  Yes, I have run out of space more or less.  Most of my I/O
is used, including all the PWM outputs.  Your "controlled resistor"
reference is exactly right.  I needed a brightness control from full
off to full on, and the outputs were going through inverters already
anyways, so current is not a concern.  I wanted to create a simple
passive circuit to control the brightness of the display.  However, I
have limited circuit experience, and could not find a way to control
the brightness with one transistor - the base current of course was
set by the brightness pot, as was the output current.  The problem is
when more segments come on they share the same output current.  

The 7 npn transistors work just fine.  Brightness is constant, and
adjusted with the pot.  Just need an idea to cut components.

The serial-parallel shift idea is interesting.  I could control the
display using 2 I/O pins (clock, and serial data) instead of the
current 7.  As I understand it, the problem with having 2 LEDs sharing
the voltage is eliminated because only one LED segment is on at a
time, correct?  That's very good.  I just don't know how to implement
it.  Do you know of any code examples on the web?  I'm familiar with
most everything, including some assembly.  (Although I don't
particularly like it)

Brit

--- In AVR-Chat@yahoogroups.com, Peter Jakacki <peterjak@...> wrote:
>
> When it comes to dimming leds I would doubt very much that you are at 
> your limit with software. Driving/multiplexing/dimming leds is a very 
> non-critical non-time sensitive low overhead (all that sort of stuff) 
> operation.
> 
> What I can't figure out is why you have run your leds in this 
> configuration? The transistors are doing nothing for current boost,
they 
> are simply acting as controlled resistors. The problem is that the cpu 
> can really only sink an overall maximum per chip so you need to buffer 
> the I/O pins typically with grounded emitter BJTs. I would imagine that 
> your variation in brightness is due to the cpu being overworked (RTM).
> 
> As only one of many ways can I suggest that you drive the leds via one 
> or more  74HC164 8-bit serial-parallel shift registers and then
regulate 
> the led power rail either via your preference of a hardware circuit
(555 
> pwm or DC voltage) or better still via software controlling a PNP. 
> However, if you are simply after regulation and not necessarily
variable 
> dimming then you will probably find that simply buffering the drive 
> solves your regulation problems, assuming that the power-supply is not 
> the problem.
> 
> I have run 64 segment outdoor bargraphs in this fashion with a humble 
> 8-pin pic. If the pic can do it plus receive multiple channels of
serial 
> data in bit-bashed fashion then it does not seem likely that a more 
> efficient avr can't.
> 
> *Peter*
>