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 is'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 make use of a sort of lever system. These are pushed from their top or bottom and rotate about their centre. And 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 (659KB) that is available to download.