How to light LEDs with Arduino without having any resistors
#include&lt;avr/io.h&gt;
#include&lt;util/delay.h&gt;

int main(void)
{
    SFIOR &amp;=~ (0&lt;&lt;PUD);
    PORTC |= 0x01;

    while(1)
    {
    DDRC ^= (1&lt;&lt;0);
    _delay_ms(10);
    }
}  

AVR microcontroller has a built-in 10K resistor with every output pins. Arduino should have libraries related to it. They are called pull up resistors. I checked it using the code I provided.

This screenshot is taken from the AVR datasheet. You can see that there is a Resistor inside that can be operated by switching the internal MOSFET.
So, ask in the Arduino forum about the library associated to it.