I’m a sucker for mechanical computers.
Students Matt Donahoe, Jeff DeCew, and Olek Lorenc are building a K’NEX Computer for their Computer Architecture class at Olin College:
The K’NEX calculator stands over 10 feet tall, and can perform 4 bit addition and subtraction operations in about 30 seconds. The slowest part of the operation is the user entering the balls. From there the balls trickle down, computing the result of the operation, and then sending that through a 4 bit decoder, which flips a flag that tells the user the answer. Since it is 4 bit, we can add and subtract numbers from 0 to 15.
They explain the details of how thinking with balls is done. I didn’t immediately recognize this complication in using balls rather than electrical states:
Now things start to get tricky. A normal AND gate has two inputs, and a single output that is active only when both inputs are active. Since we are using balls, we need to have the same number of inputs as outputs: if two balls go in, two balls better come out.