Seth Robinson writes:

I enjoy throwing the ball for my Rat Terrier, Maple, every afternoon, but I injured my shoulder a while back and some days it can be of painful. Naturally, as a maker I wondered if there was a “better” way to throw a tennis ball. I’d seen similar projects before, notably the popular YouTube video with the too-cute daschund, and reckoned I should give it a try.

This catapult-style tennis ball launcher is my first attempt. I’m sure there’s an engineering principle that covers this kind of machine, but I am uninformed, so bear with me. The shredder motor winds the elastic band until it gains enough potential energy to start the arm moving. Once the arm starts, it accelerates quickly and slams into the forward stop, throwing the ball. I have fitted the arm with a second, weaker elastic band which tensions as the arm reaches the forward stop. When the direction of the motor is reversed, the tension is released from the main elastic band, allowing the second band to return the arm to its resting position.

Except for the wood frame, the launcher is made of scavenged parts from:

  • dead paper shredder
  • old SCSI scanner
  • Theraband that my Physical Therapist gave me (yep, shoulder)
  • slingshot-style tennis ball launcher
  • Chuckit tennis ball thrower
  • 55 gallon plastic barrel
  • cardboard box

I threw the whole thing together in about 2 hours and was surprised when it worked on the first try. The ball currently travels about 30 feet, which is far from ideal as my goal is 200 feet. Relays are on the way for the motor control and I have a Make Controller sitting here waiting for firmware.