Keeping Time with Rolling Balls - An Arduino-Controlled Gravitram Clock

An Arduino Zero turns out to be the perfect controller when Kagan sets out to keep time with stacked columns of rolling balls in a novel desktop clock

- By Aubrey Kagan

At the onset of my retirement in 2021, a friend challenged me to design a control board to make an antique clock keep time accurately, without modifying mechanical pieces of the clock. I described the project in my article, “Antique Clock Accuracy Project: Reloaded” (Circuit Cellar 381, April 2022) [1].

After I completed the project, I cast around for a new one, and my thoughts continued in a timekeeping vein.

Although the word “gravitram” may not be familiar, you are certainly familiar with the object. Also known as a “rolling ball sculpture” or “marble run,” it consists of a ball running through a series of twists and turns under the influence of gravity, often with a Heath Robinson ethos. Many years ago, a cousin had a clock of this nature—except it only had a single electric motor and mechanical interlocks. I thought I would use the concept for my own clock.

The first hurdle was how to raise the ball to the start of the run. I tried a primitive rail gun to launch the metal ball without success. I tried using a large solenoid to launch the ball, but I couldn’t get enough height. Then I investigated a robotic arm, and I found one with sufficient height (460mm). The “wow” factor for my grandchildren led me to opt for this approach.

In order to present the time, I wanted four vertical columns (marked B in Figure 1), where the number of balls in each tube would indicate a digit of the time. Rather than create open channels for the balls, I decided on PETG transparent tubing. It is easy enough to shape the tube with a heat gun—together with a silicone bending insert and a bending kit—although I did occasionally shape the tube freehand [2].