2 LEDs and one pin left

**rsocor01** · - 9th November 2010, 06:08

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** · - 9th November 2010, 09:13

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** · - 10th November 2010, 00:15

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** · - 10th November 2010, 00:35

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** · - 10th November 2010, 00:49

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.

**Darrel Taylor** · - 11th November 2010, 17:21

I want to get a better idea of how these LED circuits will work.

So let's take a look at how the DOH! circuit will perform ...

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It's more significant since you will be using 3 sets of 2-LEDs.

**Dave** · - 11th November 2010, 17:58

cncmachineguy, The only problem with this circuit arrangement is there will always be current flowing in the circuit even if the LED's are supposedly off...

Dave Purola,
N8NTA