Dip-a-Drip Dunk Tank

By Marc Stevens

7-Eleven approached my company, Real Art, to help design an online dunk tank shaped like a giant coffee cup to celebrate National Coffee Day. The dunk tank, named "Dip-a-Drip ," was set up in Union Station near the capitol in Washington, D.C. where in person players and 7-Eleven's Facebook friends could use an online interface to submerge a "drip," from anywhere in the world.

waiting to be dipped A "drip" sits waiting to be dipped into a giant coffee cup.

The key component was a customized baseball pitching machine outfitted with digital controls and a webcam that let players control the ball launcher's direction and aim at the target. From the machine's software to the circuitry and weighted stand, Real Art created an awesome beast of a pitching cannon.

The basis of our contraption was the Batting Tutor Pitching Machine, which is capable of throwing baseballs from 25 to 60 mph. A top priority was establishing safety measures. We literally installed a "safety net" along the entire path from pitching machine to dunk tank, as well as did extensive software testing and exploring worst-case scenarios for the electrical hardware.

It was clear that the device's stock stand would not be strong or stable enough to handle the forces induced by our quick motion, so we created a custom stand to prevent tipping and also facilitate easily mounting our hardware.

After balancing it front to back using weights, we investigated turntables, motors and actuators according to the amount of weight and resultant torques that would be necessary. We found that 4" stroke length actuators would suffice for our desired range of motion, a 35 lb. rated load actuator for the left/right rotation and a 150 lb. one for the elevation.

The actuators were driven by a motor controller powered by a bulletproof Mean Well 12V, 150W power supply. Although other distributors offer a few types of Mean Well supplies, we chose Jameco because they offer the entire line of Mean Well products through a handy parametric filtering tool.

Jameco had nearly all of the parts we required for the project: Arduino Uno, switches, fuses, relays, diodes, terminal blocks, transistors, and more. We also purchased XLR jacks for all of our signal connections between the pitching machine, power supply box and computer. These cables are commonly used for audio signals, but we've found that XLR is very easy to connect with a simple color code system on cables and jacks. Because these cables are readily available at any music store, it is easy to replace a broken cable or procure a longer cable if necessary.

We hacked into the control circuit board to let online players adjust the pitch speed and also trigger the pitch. The machine's existing buttons were pulling open collector inputs to ground, so we installed our transistors in parallel with the original buttons, which allowed us to keep manual control even after adding the circuitry for software control. We also mounted a large red warning light on top of the machine to alert everyone when a pitch was coming and mounted a kill switch for emergency shutoff.

On the software side, the pitching machine was controlled by an Arduino Uno connected to a Windows computer on-site. The Arduino simply turns ASCII commands into digital outputs, sending aiming instructions to the motor controllers and the pitch speed/throw instructions to our hacked control circuit board. The Windows machine also contains a C# console application that is connected to a Java socket server along with all clients connected over the Internet. This program handles the queuing of users, game time and relays all events from the end user to the Arduino in the least latent way possible. The video camera is attached to the machine itself to deliver 1:1 motion as the user controls the direction of the machine and streamed the video over RTMP to give the end user the most lag free experience possible.
Dip-a-Drip Pitching Cannon Control Panel SketchDip-a-Drip Pitching Cannon Control Panel Sketch
(Click to enlarge)

Bottom viewBottom view of actuator for rotation (top right) and the
black coil cord that connects the power supply box to the stock control panel for software control.

XLR Connections to Pitching MachineXLR Connections to Pitching Machine,
Power Supply Box and Computer

Testing the Dip-a-Drip

After a bit of testing, carting the gear to Union Station and setting it all up, Dip-a-Drip went off without a hitch. In seven hours of game time, more than 23,000 people registered for the live event and over 700 online players took aim at the drips. The project garnered 1,326 publicity placements and more than 367 million brand impressions for 7-Eleven. The project was quite a success!
Marc Stevens lives in Dayton, Ohio and works as an electrical engineer for Real Art, an innovative digital and creative agency. His interests include digital communications, satellite design, bicycling, and studying Japanese language and culture. Marc has been an electronics enthusiast since the day he purchased Getting Started in Electronics by Forrest M. Mims III.