# double led flasher problem

hello everybody, i am new to Arduino , very new... i make a simple LED flasher project with a 100ohm resistor and a LED connected to pin 10 all ok. when i tried to connect a second led to pin 9 the compiler write "a*vrdude: ser_open(): can't set com-state for "\.\COM7"" when i connected the same led to **pin 11* all works fine. can anybody help me and tell me why this happened? my arduino is in com 7 and working fine/.

A 100ohm resistor is too low. It will allow 50 mA to be drawn from the I/O pin when 40mA is the absolute max and 20mA would be sensible. You may damage your Arduino. Use a 250 ohm resistor - or higher.

Edit to say these calculations are incorrect - see Reply #2

If that does not solve the problem …

What Arduino board are you using?

You need to post your code so we can see what you are talking about.
And please use the code button </>

``````so your code looks like this
``````

and is easy to copy to a text editor.

…R

Robin2:
A 100ohm resistor is too low. It will allow 50 mA to be drawn from the I/O pin

Huh? If the led forward voltage is 1.8V (e.g. Red), it would draw around 32mA. If the led forward voltage is 3.2V (e.g. Blue, White) it would draw 18mA. How did you get 50mA, Robin?

PaulRB: Huh? If the led forward voltage is 1.8V (e.g. Red), it would draw around 32mA. If the led forward voltage is 3.2V (e.g. Blue, White) it would draw 18mA. How did you get 50mA, Robin?

You are quite right. I forgot about the diode voltage drop.

I was seduced by the idea that the problem was due to overloading the I/O pins.

...R

You are quite right. I forgot about the diode voltage drop.

?

This really sounds like a hardware problem. I've had Uno's or variants in the past where the chip mostly works, but some pins are kaput (this was always due to my own follishness).

raschemmel: ?

Are you saying I was silly to forget the diode voltage drop - if so I agree.

If you have something else in mind I don't understand.

...R

You're talking about a led, so yes, it was silly. That's LED101 , is it not? ( I 'm NOT saying I'm perfect and don't make mistakes, just not that one.) I recently stated optos had 6 pins , not 4 and next thing you know there's a datsheet for a 4-pin Darlington opto in my face. Maybe we're just getting old.

It's almost like we're expected to be perfect. Are we ?

What are they going to do , ask for a refund ? You could offer the OP a Forum Gift Card (good for 2 FREE consultations)

raschemmel: You're talking about a led, so yes, it was silly.

That's fine. I was just concerned that I had missed something else as well.

I usually put a 1k resistor on my LEDs - plenty bright enough.

...R

raschemmel: It's almost like we're expected to be perfect. Are we ?

If you have anything to do with the public, or the media, you are!

Yes, expecting optos to have 6 pins is a bit dated. Since two are never used, it is considered wasteful to provide them. And I do like the AC ones (with back-to-back LEDs).