Push-pull XOR gate

"No, no, you're not thinking; you're just being logical." - Niels Bohr

This page has details about the XOR gate. An XOR gate differs from a normal OR gate in that the 1,1 input gives a zero out. The XOR gate is of great importance as it is a fundamental component of the adder.

The XOR gate

By far the most difficult gate to build has been the XOR gate. This is mainly because it is essentially an composite gate made from two ANDs, two NOTs and an OR gate. Sure I could have made it that way, but this isn't satisfactory and given the inertia and gear slippage, it meant that a half adder was probably never going to happen. But around Jan 05 I finally cracked it.
The gate makes use of a sort of lever system. These are pushed from their top or bottom and will rotate about their centre if pushed only from either. Here's the trick, if the lever is pushed from the top and bottom at the same time it will move laterally. OK that's not the clearest of explanations, but hopefully the photos below, which show the gate in its four possible states along with a cut away photo showing the internal workings will make things more lucid. Failing that have a look at the XOR gate video that is available to download.
Here is the video of the XOR gate being operated.