Functional Diagram. Haley and Constance

Input: Money into the piggy bank 

Output: Visual display of progress to goal (lights)

Processing: How much money is being put in and how much the light increases 

Sensors: 

Linear CCD Sensor 

http://www.element14.com/community/thread/29637/l/arduino-coin-counting-machine-question

or 

LDR Sensor 

http://forum.arduino.cc/index.php?topic=131901.0