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