The folks at Evil Mad Scientist Labs just put up a post on the giant mechanical binary computer they brought to last month’s Maker Faire.

As a faithful reproduction of the Digi-Comp II from the 1960s, every operation is powered by balls falling onto levers. Unlike the original, the larger version is powered by billiard balls instead of half-inch marbles. The Digi-Comp II is able to count, add, subtract, multiply, divide, get the 1s or 2s complement and zero all of it’s bits. With a 7-bit accumulator, the Digi-Comp II is able to calculate anything where the result is less than 127, so we wouldn’t recommend doing your taxes on it. In the demo video, it took the Digi-Comp II about two minutes and twenty seconds to multiply 3 by 13. We’re not going to venture a guess on the equivalent seconds per cycle for an electronic calculator, but it’s an impressive build

The Digi-Comp II is a great way to show the process of binary arithmetic in a computer and we were wondering why there aren’t any educational toys like the Digi-Comp II out today. A site linked from the build page tells us there will be kits available this summer, we’re hoping the kit doesn’t fill the bed of a pickup truck.

Check out the video after the break for the multiplication demo.

This analog computer can multiply, divide, square numbers, and find square roots. It has a maximum result of ten billion with an average precision of 2-3%. [Miroslav's] build recreates something he saw in a Popular Electronics magazine. It uses a resistor network made up of three potentiometers with a digital multimeter is an integral part of the machine. To multiply a number you set the needles on the first two knobs to the numbers on which you are operating. To find the result turn the third knob until the multimeter has been zeroed out and read the value that knob is pointing to. It seems much more simple than some of the discrete logic computers we’ve seen, yet it’s just as interesting.

[Eric Archer] constructed an analog computer to model the physics of a bouncing ball. The core is a TL074 opamp that does all the integral math. He had no trouble finding descriptions of analog computers, but how to set the initial conditions was rarely covered. The controls include potentiometers to set the initial velocity, force of gravity, and coefficient of restitution (how much energy is lost in the bounce). The output is displayed on an oscilloscope. He mentions that this output could be used in electronic music, citing Aphex Twin’s Bucephalus Bouncing Ball. Watch the video below for a demo of all the features.