A small 5 cm hollow sphere which has a base and a hole for a 5mm LED.

I used mine with an RGB led and Arduino, and made it into a Pomodoro timer. It starts green, fades to red over 25 minutes, then for 4 minutes glows blue, signalling a stretch break.

Detailed instructions for creating the Pomodoro timer are below.

Print out the Sphere in a neutral or white colour. Insert 5mm LED of your choice into the bottom and power it however you like!

If you want to build the Pomodoro timer which I did, follow this instructable for building the RGB LED circuit for the Arduino, which is what I used: http://www.instructables.com/id/Fading-RGB-LED-Arduino/. It give clear steps and materials required.

The code I wrote for the Pomodoro specific logic is here:

#define RED 9  
#define GREEN 10  
#define BLUE 11  

void setup() {  

  pinMode(GREEN, OUTPUT);  
  pinMode(BLUE, OUTPUT);  
  pinMode(RED, OUTPUT);  
  digitalWrite(GREEN, HIGH);  
  digitalWrite(BLUE, LOW);  
  digitalWrite(RED, LOW);  
}  

int redVal;  
int blueVal;  
int greenVal;  
int val;  
 
void loop() {  
 
  // 25 minute green to red   
  
  int redVal = 0;  
  int blueVal = 0;  
  int greenVal = 255;  
  for( int i = 0 ; i < 255 ; i += 1 ){  
    greenVal -= 1;  
    redVal += 1;  
    analogWrite( GREEN, greenVal );  
    analogWrite( RED, redVal );  

    delay( 5882 );  
  }  
  
  
  // blink red   
  
  for ( int i = 0; i < 60; i++ ) {  
    redVal = ( redVal == 0 ) ? 255 : 0;  
    analogWrite( RED, redVal );  

    delay ( 500 );  
  }  
  
  
  // 4 minute solid blue  
  
  analogWrite( RED, 0 );  
  analogWrite( BLUE, 255 );  
  delay( 240000 );  
  
  
  // blink blue   
  
  for ( int i = 0; i < 60; i++ ) {  
    val = ( val == 0 ) ? 255 : 0;  
    analogWrite( BLUE, val );  

    delay ( 500 );  
  }  
  
  analogWrite ( BLUE, 0 );  
  
}