Re: bi-color LEDs with mixed common lead


"Lostgallifreyan" <no-one@xxxxxxxxxxx> wrote in message
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"James Thompson" <Jamesthompson2002@xxxxxxxxxxx> wrote in
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"Carrie" <cnuncio@xxxxxxxxxxxxx> wrote in message
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Starting on a new PIC project using bi-color LEDs. First thought was
to use
a hex inverter to change colors between RED and GREEN. However it
occured to me that if there were a bi-color LED (RED/GREEN) that had
a mixed
common lead it would save me 4 74ALS04 hex inverters, the board
real estate and the board size. I can find common anode and common
cathode
LEDs but, so far, no mixed common. By mixed common I mean that the
anode of one LED is internally connected to the cathode of the other
LED.

Anyone heard of such an animal??

Thanks !



I'm not sure if have it figured out by now, but here is a simple way
to figure it.

Consider the led's red or green has a forward drop of about 2 volt,
and its equivalent resistance at 20 ma will be 100 ohms. Now take 2)
100 ohm resistors to make a voltage divider ( + 5 volt to resistor a
with resistor b in series then connecting to ground). now with the
back to back led connected to the center of your divider being 2.5
volt and the other end of the led to the Pic output, A high or a low
will give roughly 2.5 volt difference on the led. You may or may not
even need a resistor to drop the extra .5 volt but if you do, you end
up with 3 resistors and the led instead of the extra opamp's. I would
still consider at least a transistor buffer on the Pic outputs to
drive the leds. JTT




Do you really think that 24 transistors are easier to handle than an
op-amp
IC to make the 2.5V rail?
If the transistors were sot-23 then they could be incorporated to the led
board itself.
But if the pic can handle the led directly, its just 3 simple resistors.
That would make 48 resistor for the divider plus 24 led's and maybe an
equalizing resistor in series with the led.

Question? does this Pic have 24 I/O lines for the led output. Also if space
is a premium , then surface mount should not take too much room for the
whole thing.
JTT


.

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