My Story: Digi-Comp Revival

By Lenore Edman

"Can you dig it?" and "Keep on Truckin" aren't all the cool that came out of the 1960s. Digi-Comp was a fully functional binary digital mechanical computer, which sparked many future engineers' imagination. Capable of conducting basic operations like adding, multiplying, subtracting, dividing and counting, the mechanical toy of old has become a bit of a phenomenon of late. Today, numerous websites, blogs and networking groups sell trade or just relive the glory of the Digi-Comp series.

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Original Digi-Comp 1963
Original Digi-Comp 1963
Digi-Comp II, in larger-than-life scale at Maker Faire 2011
Digi-Comp II, in larger-than-life scale at Maker Faire 2011

Our friends at Evil Mad Science recently revived the game with a larger than life version displayed at several Maker Faires.

Digi-Comp, the first real binary computer, worked mechanically the same way as the giant computers of the 1960s, which work electrically. In fact, with the addition of many more parts, Digi-Comp could solve very large problems just as an electronic digital computer does. Coolest of all, these operations are all conducted by the action of marbles rolling down a slope, directed by mechanical switches and flip flops, all powered by gravity.

Most calculations are semi-automatic. For example, once you enter two numbers that you wish to multiply together (and set the appropriate configuration switches); you pull the start lever to release the first marble. Running the full calculation can take quite a few marbles. But, once the first ball makes its way through to the bottom, it releases the next ball from the top, and so on, until the calculation is complete at which point it stops.

SchematicClick for larger schematic
Billiard balls are used in place of marblesBilliard balls are used in place of marbles

For our "larger than life" model at Maker Faire, we've scaled it up to use billiard balls (specifically, 2 1/4" diameter 8-balls). The overall size of the top deck of the machine is just under 4 x 8 feet. It's made of CNC-routed plywood and is sturdy enough that it might make a good museum exhibit someday. We've posted a video of our giant Digi-Comp II operating (see below).

Evil Mad Science is about to launch the newest generation of Digi-Comp. Check out this link for updated information.

The next generation Digi-Comp II is modeled after the original 1967 DIGI-COMP II. These educational computers work similarly to an electronic, digital computer.

With appropriate settings DIGI-COMP II can:

1. Count out a specified number of balls
2. Count in binary arithmetic
3. Add
4. Multiply
5. Obtain either the "1's" or "2's" complement
6. Subtract
7. Divide
8. Zero or clear
9. Overflow
10. Halt

The rolling ball acts as the electrical impulse. The gravity powered balls are directed by levers, which equate to the wires in electrical computers, and through a succession of binary logic. In an electrical computer, these logic calculations are performed in the ALU, or arithmetic logic unit.

Some additional links that may be of interest:
  • The Friends of DigiComp group on Yahoo, whose members provided crucial photos and scans of the original Digi-Comp II, along with its documentation. This project would not have been possible without their help.
  • "How can they learn?", an article by Jack Crenshaw in EEtimes, about the educational value of mechanical computers including the Digi-Comp II
  • "My first computer - the Digi-Comp II", an article by Joshua S. Levine
  • The Digi-Comp I, predecessor to the Digi-Comp II, available as a reproduction kit from Minds-On Toys.
  • Updates for the digi-Comp II table-top version at
Lenore M. Edman runs Evil Mad Science LLC, a small hobby electronics business, along with Windell H. Oskay. They also blog about projects involving art, electronics, food, design, and whatever else piques their interest at Evil Mad Scientist Laboratories.

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