Ever wanted an LED tree? Well now you can have your LED tree and eat it too (no, wait, you probably shouldn't)...

Step 1: Gather the Parts

You will need:

1x ATtiny85
10x male header pins
Solid core wire, 2 colors
A bunch of LEDs

1x Arduino Uno to program the ATtiny

Step 2: Program the ATtiny

To prep the Arduino for programming the ATtiny, follow the guide on this page.

After you have done that, upload this sketch to the ATtiny.

//CODE STARTS HERE

//ATtiny85 RGB color fading tree

const int 2Pin = 2;
const int 1Pin = 1;
const int 0Pin = 0;

void setup()
{
pinMode(2Pin, OUTPUT);
pinMode(1Pin, OUTPUT);
pinMode(0Pin, OUTPUT);

}

void loop() {
    2to21();
    21to1();
    1to10();
    10to0();
    0to02();
    02to2();
}

void 2to21()
{
digitalWrite(redPin, HIGH);
digitalWrite(bluPin, LOW);

// fade up green
for(byte i=1; i<100; i++) {
    byte on = i;
    byte off = 100-on;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(grnPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(grnPin, LOW);
      delayMicroseconds(off);
    }
}
}

void 21to1()
{
digitalWrite(grnPin, HIGH);
digitalWrite(bluPin, LOW);

// fade down red
for(byte i=1; i<100; i++) {
    byte on = 100-i;
    byte off = i;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(redPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(redPin, LOW);
      delayMicroseconds(off);
    }
}
}

void 1to10()
{
digitalWrite(grnPin, HIGH);
digitalWrite(redPin, LOW);

// fade up blue
for(byte i=1; i<100; i++) {
    byte on = i;
    byte off = 100-on;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(bluPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(bluPin, LOW);
      delayMicroseconds(off);
    }
}
}

void 10to0()
{
digitalWrite(bluPin, HIGH);
digitalWrite(redPin, LOW);

// fade down green
for(byte i=1; i<100; i++) {
    byte on = 100-i;
    byte off = i;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(grnPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(grnPin, LOW);
      delayMicroseconds(off);
    }
}
}

void 0to02()
{
digitalWrite(bluPin, HIGH);
digitalWrite(grnPin, LOW);

// fade up red
for(byte i=1; i<100; i++) {
    byte on = i;
    byte off = 100-on;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(redPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(redPin, LOW);
      delayMicroseconds(off);
    }
}
}

void 02to2()
{
digitalWrite(redPin, HIGH);
digitalWrite(grnPin, LOW);

// fade down blue
for(byte i=1; i<100; i++) {
    byte on = 100-i;
    byte off = i;
    for( byte a=0; a<100; a++ ) {
      digitalWrite(bluPin, HIGH);
      delayMicroseconds(on);
      digitalWrite(bluPin, LOW);
      delayMicroseconds(off);
    }
}
}

//CODE ENDS HERE

Step 3: Construct the Tree

First, start by taking two different colors of wire and cutting them into pieces (I cut them about 6 inches long). Then strip the ends off all the wires.

Solder an LED to the end of two different colors of wire (for example, I used black wire for the cathode and green wire for the anode). Then, separate the LEDs with wires attached into 3 groups.

Solder all the anodes of one group together, and all the cathodes of all the groups together, so you have 3 different groups all with a common cathode. I soldered header pins to the end of these groups so that I could easily plug it into a breadboard.

Step 4: Wire It All Up

Connect the 3 anode groups of the tree to ATtiny digital pins 0, 1 and 2 (actual pins 5, 6 and 7 on the ATtiny). Connect the single common cathode to ground.

Connect ATtiny pin 4 (ground) to the negative wire of a 3 volt 2xAA battery pack, and pin 8 (Vcc) to +3 volts.

Don't use any resistors, because if you do, the ATtiny will not have enough power to light up all the LEDs. I tried with some 1/4w 220 ohm resistors and the LEDs don't light up at all.