Arduino Light Pen!

  A simple Light Pen, created using the TVout library with the Arduino and a simple phototransistor sensor. 

#include <TVout.h>
#include <fontALL.h>
#include <video_gen.h>

long s1,s2;
TVout TV;
int sz=8;

void setup() {
TV.begin(0,128,96);
TV.select_font(font6x8);
}

void loop() { 
  for (int x=3; x<112; x=x+16) {
    for (int y=32; y<40; y=y+16) {
      s1=analogRead(0);
      TV.draw_rect(x,y,sz,sz,1,1);
      TV.delay_frame(2);
      s2=analogRead(0);
      TV.draw_rect(x-2,y-2,sz+5,sz+5,0,0);
      if (abs(s2-s1)>10) {
      TV.draw_rect(x-2,y-2,sz+5,sz+5,1,0);
      TV.delay_frame(5);
      };
    };
  };
 }

Virtually any light sensor will work, as long as it can provide an output that can be handled by the ATMEGA328's Analog to Digital Converter.  For the simplest form, a CDS photocell simply connected to +5v on one side and the Analog 0 input pin on the Arduino on the other may suffice.

To increase sensitivity or adjust response, a simple voltage divider can be created by adding a variable resistance between Analog 0 and Ground with the above sensor.