Proximity Sensor

After deciding on proximity sensor i decided to research different possibilities and alternatives

This one way of doing it with light sensors but i think it would be too much work and not practical for the project
http://www.youtube.com/watch?v=nEfdMw3gY0M&feature=player_embedded

This was another alternative using a infrared sensor
http://www.instructables.com/id/Arduino-Powered-Pumpkin/

The most probable and practical way is to use a ping Ultrasonic Sensor
http://arduino.cc/en/Tutorial/Ping?from=Tutorial.UltrasoundSensor

Here is an example of it in work
http://www.youtube.com/watch?v=bZIKbuZaJSQ

There about 30 euros
http://www.sparkfun.com/commerce/product_info.php?products_id=639

Alternatively you could make one
http://blog.makezine.com/archive/2009/02/how_to_diy_ultrasonic_rangefinder.html