AVR-Chat

Peter Gargano wrote:
> Roy wrote:
> 
>>See attached WORD file of the Switcher CAD III plot of the circuit below.
> 
> Thanks Roy for the analysis info.
> 
>>*From:*  *Russell Shaw
> 
> Thanks Russell for the original circuit.
> 
> One problem of course is getting enough current to drive the LEDs to a 
> reasonable brightness. This is mostly due to having them run from what 
> is effectively a 2.5 Volt rail. Differences in brightness between red 
> and green LEDs because of different operating voltages is also 
> exacerbated.
> 
> I'm leaning towards using a shift register like the 74HC595 and 
> sending it data via the SPI - in my current design this allows me to 
> control up to 8 LEDs with just the original extra pin!  And it takes 
> about the same PCB real estate too.

I would not have posted that circuit as shown if i realized the stuff-up.
I calculated that 220ohm resistor for 5V - diode-drops, but it should have
been 2.5V - diode-drops, which is inadequate because 2.1V(red) + 0.7V is > 2.5V.

The circuit can be simpler if separate LEDs are used:

                                       +5VCC
                       --+----------+---------+-----------+--
                         |          |         |           |
                         >          |         |           |
                         < 10k      |         |           |
                         >          |         |          e|
                         |          |         |        b|/  PNP
                         +------------------------------|
                         |          |         |         |\
AVR                     |c   NPN   >         +-----+    c|
----+                    \|b__     < 10k    _|_LEDs|_    |
     |                    /|   |    >        \ /   \ /    >
     |pin     10k        |e    |    |        ---   ---    < 10k
     +-------\/\/\-------+     +----+         |     |     >
     |                   |e    |    |         <     >     |
     |                    \|b__|    >         >330R <330R |
----+                    /|        < 10k     <     >     |
                         |c   PNP   >        c|     |c    |
                         |          |      b|/ NPN   \|b  |
                         +------------------|         |---+
                         |          |       |\   NPN /|   |
                         >          |        e|     |e    <
                         <          |         |     |     > 10k
                         >          |         |     |     >
                         |          |         |     |     |
                     ----+----------+---------+-----+-----+---- GND