2 LEDs and one pin left

[Darrel Taylor]

Actually, I'd rather know your conditions and do the math.
What is it you need to do with your LED's?
VDD, LED Forward Voltage / Forward Current?
See if the theory works?

... As for the actual circuit, I will have to mull over why the 2 resistors in series with the led's doesn't create a voltage divider.

Because the voltages across the resistors are changed by the voltage drops that are controlled by the LED's.

I would just LOVE to haave proteus. I tried looking it up 1 time, seems it was in the triple digits, just a little much for me

That's what I thought too at first.
But they have a $250 "starter version" that gives you a 12F, 16F and an 18F.
Of course it comes with all the other logic chips, analog stuff and test instruments.

Even if it's not the exact chip you target, the "Family" is usually close enough to know your program has a chance.

Beware:
Once you get used to it ...
You probably will spend $1000 for the full PIC version.

[cncmachineguy]

The LED's will be board level indicators, showing drive enables, disabled, or in a fault
red is 2Vf @ 10-20ma green is 2.2Vf @ 10-20ma

Vcc 5V

Now I could use 3.3Vsupply for the LED's But was trying not to add that.

$250 I could deal with, its the beware I am listing too!

[rsocor01]

Yes, after I posted that attachment I realized it was wrong. The correct value for the resistors should be between 100 and 330 ohms depending on the brightness that you want. I'm sorry for the confusion.

Robert

[sayzer]

I am using this technique with 6K8 resistors on 3.1V/1500mcd LEDs.
Vdd is 5.0V.

I am using one pin to simulate TX and RX transmission with two LEDs inside Hserin and Hserout routines.
The light level is more than good enough for the purpose.
I am %100 satisfied with the result.

__________________________

[cncmachineguy]

here was the setup:

5Vdc->R1->2.2vF LED->1.9vF LED->R2->GND
R1=R2
first tried 240 ohm, Great brightness for each, pretty bright with no input
next up 1K, simular results just dimmer
last 4.7K, No real output for off, bright enough to see with input, but not really enough for an indicator in this app.

So my opinion is everybody in this thread is correct in some respect.

I feel like the best solution is to use a 3.3V supply. that way there is NO way to light both, and each can be nice and bright. Now, question for the masses: can we use a voltage divider to get this 3.3 from 5? This seems to me to be the least costly answer. Just 2 resistors and off we go. Hang a resistor inline with the signal pin and life is good. My perticular application will have 6 sets of LED's so if 1 from each set were on at once, could be 120mA.droping 1.7 volts, that would be 1.7*.12=.2W. That part doesn't seem too bad. Of course they could be strobed so there would never be more than 1 on at a time, then it would be 1.7*.02=.034W

[mackrackit]

can we use a voltage divider to get this 3.3 from 5?

And run the whole thing? Nope.

Using 5 volts with 470 resistors the thing I posted works fine.

[cncmachineguy]

Dave thank you! I actually intended to hook it up this way, but somewhere between yesterday and today I got myself turned around. I even drew it up this way in my schmatic. double DOH! So 470 both sides, makes sense to me!

Prolly the only size sips I didn't order.