AVR-Chat

Russell Shaw wrote:
> 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.

                                       +5VCC
                                  --+----------+--
                                    |          |
                                    |          >
                                    |          < 10k
AVR                                |c   NPN   >
---+                                \|b_______|
    |                                /|        |
    |                               |e         |
    |                              _|_         |
    |                              \ / LED     |
    |                              ---         |
    |pin                100ohms     |          >
    +-------------------\/\/\-------+          < 39k
    |                              _|_         >
    |                              \ / LED     |
    |                              ---         |
    |                               |e         |
    |                                \|b_______|
---+                                /|        |
                                    |c   PNP   >
                                    |          < 10k
                                    |          >
                                ----+----------+---- GND