Using a bunch of 3d printed parts, I made a robot to chase squirrels.
First, the nerf barrel (with a squirrel on top) rotates every 30 seconds from left, to center, then to right. If the robot detects a squirrel, stage one is initiated. The robot charges forward then retreats. If the squirrel is still detected, the robot fires a nerf dart toward the offending creature. If the pest persists, the robot blows up a balloon and (using a heated resistor) pops the balloon.
First, there are a lot of parts. I used an Arduino Uno, a motor shield, a robot base (from MakerShed or Amazon), nine AA batteries, two servo motors, a pump motor (vacuum pump, Karlssonrobotics.com), a motion detector (parallax), a ball bearing (vxb bearings) and many 3d printed parts.
The design and print files for 3d parts can be found at here
You can find someone nearby to print the files using 3dhubs
Attach the servo mount to the barrel using a soldering iron to weld the parts together. Add the servo motor. It is the release mechanism for the rubber band that fires the nerf dart.
Add a servo horn to the connector. Affix the connector/horn to the servo motor on the top plate.
Add the pump holder and balloon adapter.
Attach the bearing holder and bearing so that they line up with the servo adapter.
Drill a hole through the adapter and nerf barrel. Fasten with a small bolt.
Attach the pir sensor to the nerf barrel. Route the wires so that they will not bind when the assembly rotates
Add the front warning holder. Print and insert the "No Squirrel" warning.
The "Balloon Popper" is a resistor on a stick. The little pump will not reliably pop balloons (at least I can't find cheap crummy balloons when I want them). After the balloon is inflated, the resistor gets hot and assures a pop.
Put it all together and you will have the most complex, delicate, high tech squirrel repeller available.