LEDs on pin 0/1 not as bright as on other pins?

I'm using the 3.3V version of the Arduino Pro and a bunch of low-current-LEDs (they need about 2mA each). Since I need every port the Arduino offers, I've also connected LEDs to the Rx and Tx pins, which doesn't affect the upload of new sketches (they're just flickering simular to the orignal Rx/Tx LEDs on the Board).
(I don't know if this is important, but I connected the ground-pin of every LED to the Arduino, so setting the pin as HIGH the LED is off and setting it as LOW the LED is on.)

So here comes the Problem:
When the Arduino is running the sketch (which does not use any serial communication by the way) and turning the LEDs on, all the other LEDs are glowing about twice as bright
as the LEDs connected to pin 0 and 1.
Has anyone got an idea how this comes and what I can do about it?

You could post your sketch.

Are you using external series resistors for all the leds?


Each LED has it's own resistor (550 Ohm).

The sketch is a little long to post it here (it's a binary clock), but here are the extracts, I'd consider as important:

void setup()   {     

  pinMode(dcfpin, INPUT);
  pinMode(buttonpin, INPUT);
  for(int i=0; i<11; i++)
  { pinMode(i, OUTPUT); }
  pinMode(dcfpowerpin, OUTPUT);
  for(int i=14; i<20; i++)
  { pinMode(i, OUTPUT); }
  //This is a function which simply sets all the output pins to HIGH
 //This function encodes the DCF- signal
//This function sets the status of the LEDs due to the current time   

void loop()                     
//sekunden, minuten, stunden and tag are the variables which hold the current time
   if(sekunden==60){minuten++; sekunden=0;}
   if(minuten==60){stunden++; minuten=0; }
   if(stunden==24){tag++; stunden=0;}


According to the schematic, both rx and tx have 1k ohm series resistors fitted.

Have you removed them?

I did't know they exist...thanks a lot.
According to the schematic the pins of the programming header aren't affected by the resistors, so I'll try to connect the LEDs to them.
Do you know a reason why these resistors are there?

Do you know a reason why these resistors are there?

If these pins are the dedicated tx/rx pins, the resistors probably ensure the pins don't float for clean serial communication.

I'd use 1k resistors on all the other pins with 0 & 1 without. They'd all be the same brightness then. :slight_smile:

But then they would't be bright enough...
The connection trough the programming-header workst fine, thanks for the help all.

Connecting anything to the hardware serial is generally a bad idea.

I suggest you take a look a I/O port expanders (you can buy i2c ones) and look at shift registers/multiplexers etc.