Circuit Project Electronic

This is an old discrete component design, now superceded by a PIC.
This thing is a very nifty capacitive sensor. For you europeans, this little gem is used in north america mostly for detecting wooden beams behind drywall or plaster. I’ll take one of these over the new design any day, because these ones don’t error out every time you try to use them in a non-standard way.

Read more source:
http://www.members.shaw.ca/novotill/StudSensorAutomatic/index.htm