The planter primarily senses smell, entry/exit and water level. There is also a mechanical system (construction is going on) that mixes an "appropriate" amount of water with pee, so that it is usable by the plant.

So in all we can sense:

- number of people that went into the planter (approx)

- amount of pee was collected

- smell levels based on usage/pee levels

there are also some more things related to plant growth.

Probably an "electronic nose" that detects a particular chemical or mixture of chemicals.

http://en.wikipedia.org/wiki/Electronic_nose