Cats love digging in indoor pots. The standard chemical deterrents (citrus sprays, spike mats) work for a week then the cat habituates. Ultrasonic deterrents work indoors but bother dogs and some humans. The water-burst approach trades a tiny inconvenience (pot needs water topped up weekly) for a cat-proof solution that the cat actually respects long-term — they hate the surprise spray and they don't habituate to a sensation that makes them physically wet.
The minimum-viable version is a PIR motion sensor pointed at the plants, a 12V water pump in a small reservoir, and a servo aiming the nozzle. Total cost is about $35. The non-obvious detail is the height filter — you don't want to spray your kid or your dog. A simple ToF (time-of-flight) sensor mounted parallel to the floor returns the height of whatever triggered the PIR. If it's above 50cm, ignore it. Below 50cm, fire the spray.
The pump runs for 200ms — long enough for one good burst, short enough that you don't refill the reservoir for two weeks of typical use. The nozzle is aimed via a single SG90 servo with about 60° of pan range, which covers a 1-meter-wide planter from 50cm away. For larger areas, mount two units in opposite corners.