The elevator shaft was also constructed from cardboard box, and 680 ohm resistors were placed in pairs to form a resistor ladder. You can see in the pic below how the legs protruded to make contact with the aluminium foil backing when the lift body moves to a level. The resistors were soldered into a series connection, making two rows of resistors, one down each side. In theory, the 5V will be put over the the complete resistor ladder as well as one extra resistor, effectively creating a voltage divider between the ladder and the referance resistor. All resistors being 680 ohm. Why 680? Well, we had lots of them, so they won the opportunity :)

As the lift body moves past a pair of resistors, they form part of the voltage divider, and thus the Arduino's ADC can determine the current level. The contact points were made on the bottom of each level. In retrospect, halfway might have been better, but I will leave this for DigiGram to fix with code.