DIY Automatic Dog Ball Thrower

Sophie, our golden doodle has been even more hyper than usual since our young son has been taking more of our attention. Perhaps there is a way to burn off some of her energy when we are away at work. When we saw a recent issue of Make magazine, (awesome magazine by the way) we knew we had the answer. They had a detailed build of a DIY automatic ball thrower- the "Fetch-O-Matic." I had much of the materials lying around the house so we built an adapted version of the automatic dog toy.

The Make design is a very elegant design. It uses a freely rotating "striker" bar, which is spring loaded. The drive mechanism turns the striker about 180 degrees, loading up the spring. Once the striker rotates past center, the spring rapidly accelerates the striker and brings it back to home position. There is no electronics required, just a simple switch to energize the mechanism when the ball is in the proper location.

The Make version of the ball thrower used a windshield wiper motor. These seem to be rather expensive on E-bay, let alone the local auto parts thrower. We had an old Black & Decker battery powered screwdriver sitting in the junk pile that I thought would work great to drive the device. Tons of torque, works on about 5 volts, and most importantly, free.

I used 3/4" square basswood for both the drive bar and striker. After ten or so ball strikes, the drive bar failed- split along grain lines. I made a new drive bar out of 3/4" plywood and that seems to be quite durable. The drive bar is attached to the motor shaft using a screw threaded into a tapped hole I drilled into the lower shaft collar. I ordered the 7/16" ID shaft collars from Amazon.com, utilizing Amazon Prime free shipping. Without the free shipping I probably would have ordered these from mcmastercarr.com or grainger.com. I used another shaft collar to hold the striker in place.

For power, I used a UBEC from Hobbyking to convert 11.1V from a 3 cell LIPO battery to 5V and to provide low voltage protection for the battery. The lighted on/off switch and ball switch is from Radioshack.

Other than the drive mechanism, the build is very similar to the Make Magazine build. 

Drive mechanism (lower 3/4" square) and striker (upper 3/4" square) attached to motor shaft

Top view of mechanism, upper bar rotates freely about motor shaft, lower is affixed to motor shaft.

Motor Mounted on ball thrower floor

3/4 view of mechanism

I used three small shelf brackets and two hose clamps to mount the motor

Dry Fitting the sides and lower ball guide

Extension Spring- purchased from Lowe's

Spring is mounted- note ball guide and cutout for switch on right side of photo

New Plywood Drive and Switch mounted

Ball wasn't heavy enough to activate the switch, so I added an extension lever cut from 1/8" aluminum

Switch Detail

Switch Detail

Testing the switch- ball held in place with guides. Note there must be clearance for the striker!

Switch Visible

Final, cleaned up wiring on bottom of thrower

"Business End" of the thrower- note the extra ball guides to keep the ball in place

Lighted On/Off switch

Electronics compartment- On/Off switch, UBEC, and 2200 mAh 3s LIPO battery

Completed DIY Automatic Dog Ball Thrower!