In about 2004 my alcoholic mechanical-genius of a roommate looks at me and says: "I think this computer thing is staying; here's a cool fact: a car's differenti

In about 2004 my alcoholic mechanical-genius of a roommate looks at me and says: "I think this computer thing is staying; here's a cool fact: a car's differential is an adding machine, but it's analog and not digital." Making an analog device into a digital device requires finding a way to add saturation in finite time. For differentials, this can be done by letting rotation represent the value of the device. Here's the plan we sketched out in ~2005... let `D(x,y) = (x+y)/2` represent a differential (we used spur gear differentials because they're more robust & easier to manufacture). Then, if 1=clockwise, and -1=counterclockwise, then we get the following truth-table for `I0 := D(L,R)`: I0 := D(L,R) == 1 1 = 1 1 -1 = 0 -1 1 = 0 -1 -1 = -1 We then can use another spur-gear differential with a floating input and two palls on the floating (F) side and the output to "clamp" the result (which has to be scaled by 2x): `I1 := D(I0,_)*2`: I1 := D(I0,_)*2 == 1 F = 1 0 F = 0 -1 F = 0 Finally, we scale the output and add -1: `I2 := D(2*I1,-1)*2`, converting back to 1,-1. This is an "AND" gate. You can build "OR", "XOR", "NOT", etc., fairly easily. EDIT: A similar device can be made from those Japanese bamboo water gates. If you've got a bucket with a hole in it, then the bucket will drain if the amount of water coming in is less than going out. So, if the input is A, and the output is 3/2A, then if we've got two inputs we've got (A+A) > 3/2A. So, the result is that with 0 or 1 inputs, the bucket drains, with two inputs it overflows. The bucket is attached to an arm, and it is the far end of the arm that has a channel with water coming in to the channel; notice, that the water in the bucket only controls the flow of the water in the channel; it is not the water in the bucket providing water into the channel. If the bucket is full, then the channel is up and no water flows; otherwise the bucket allows water through. This forms a NAND gate. I've never found a way to make this reliable, although I didn't really try.