[nullc]

You can just buy off the shelf a ordinary lawn sprinkler controlled by a motion sensor. Though maybe the total amount of water might be undesirable on a balcony.

I found them quite effective at supplementing a fence to keep deer out of areas they shouldn't be in.

[everybodyknows]

Any particular motion sensor? Low voltage?

[nullc]

The ones I use and have seen use PIR, and are battery powered. The batteries last a long time esp. if it isn't triggered often.

The biggest limitation I've found is that it takes a bit of soldering iron adventure if you want to have a remote disable switch. :)