Post-in-Ring Touch Sensor:
Often you don't really care where a robot is being touched since your avoidance algorithm probably consists of simply backing up and turning. It would be nice to have an all-around touch sensor, but it is pretty hard to achieve this with Mindstorms Touch sensors even with the most elaborate bumper system. Here are the plans for the simple Post-in-Ring touch sensor shown below.
The Ring is made from heavy wire formed to the shape shown below. I made mine from heavy copper house wire, but I would think a paperclip wire could be used also. The ring has an 8mm inside diameter. The wire is held in place using two Motor Mount 1x2s stacked up like the photo above. The connector to the RCX is a spare 9V connector wire (available from S@H) cut in half. One of the two leads is soldered to the Ring wire as shown.
The Post is made from a piece of the thin hollow yellow tubing typically used for "feelers". The other 9V connector wire lead is soldered to a piece of copper foil. The foil is then wrapped around the tubing at the Ring location and mounted as shown below.
Finally a shell or body is made and supported by the Post as shown in the example below. No matter where the shell is bumped, the Post will be pushed to the side slightly and come in contact with the Ring.