On Mon, 12 May 2008 21:59:16 +0200, Arlet Ottens <usenet+5...@c-scape.nl>
wrote:

>Tomás Ó hÉilidhe wrote:
>> On May 12, 10:43 am, Arlet Ottens <usene...@c-scape.nl> wrote:
>> 
>>> If you turn off both LEDs, there will be a small current through the
>>> pull-up resistor, the 200 Ohm resistor, and the red LED.
>>>
>>> For example, if your pull-up resistor is 100k, the current will be about
>>> 3/100k = 30uA. The voltage drop across your 200 Ohm resistor is then
>>> reduced to only 200R*30uA = 6mV, and your gate voltage approximately
>>> equal to Vf = 2V. This may be sufficient to turn it on enough to make
>>> the green LED light up.
>> 
>> 
>> Oh I see now, thanks for that.
>> 
>> I think I've got two options in that scenario:
>> 1) Decrease the value of the pull-up resistor so that less voltage is
>> dropped across it. The only problem with this though is that the Red
>> LED might light dimly at all times because of the increased current.
>
>Yes, any pull-up low enough to turn off the green LED will turn on the 
>red one.
>
>> 2) Choose a PMOS transistor that needs a very low gate voltage to turn
>> on.
>
>This might work, but it still needs to turn on reliably, for a range of 
>devices and temperatures.
>
>> How would you go about it?
>
>My 2 favourite solutions are a) pick a controller with more pins, or b) 
>use one or more 74HC595 devices to create additional outputs.

This solution works for LEDs that do not have a common: 

http://electronicdesign.com/Articles/Print.cfm?ArticleID=1683

This solution works for bipolar LEDs (two leads): 

http://www.edn.com/contents/images/112201di.pdf

For your 3-lead LED with common cathode, there's a simple circuit
along the lines of the above that uses four parts (a dual optoisolator
and three resistors). 

Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
s...@interlog.com             Info for manufacturers: http://www.trexon.com
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Tomás Ó hÉilidhe wrote:

> 2) Choose a PMOS transistor that needs a very low gate voltage to turn
> on.

Do you mean higher gate threshold voltage ?

A high threshold PMOS part, would be turned on by the Port LOW, but
not (fully) turned on by a Red Led voltage above that.

Green LEDS commonly have higher Vf, so you could swap R/G here to
get more margin. (then, the Green Led Vf is trying to turn on the PMOS 
driving the Red Led )

-jg