Before home computers existed, E.S.R. launched the Digi-Comp I in 1963 as an educational toy that simulated binary computation using purely mechanical parts. The company had originally set out to build real computers but pivoted to educational toys and became a leading maker of them throughout the decade.

The device works by having users slide soda-straw-like tubes onto rails to set up a program. These tubes interact with spring-loaded levers that shift position each time an operator flips a manual clock switch, mimicking the clock cycle of an electronic computer and producing binary outputs.

Restorer Chris Staecker got hold of a remarkably intact original unit and returned it to working order using rubber bands and a few 3D-printed replacement parts. Despite the manual tinkering required to operate it, the machine is not Turing-complete; it functions as a state machine capable of 16,777,216 possible states, or 8 to the 8th power.

The Digi-Comp I later got a rather different successor called the Digi-Comp II, and it joins other mid-century educational near-computers such as the Geniac as a reminder of how teaching devices hinted at where computing was headed.