Here’s a cool source I designed at work. It’s for the medical industry. They use it to treat tubular thingies that are implanted into the body. The idea is that you feed a tube (or other cylindrical object) through this enclosure. As the tube passes through the enclosure the plasma works it’s magic. This particular customer didn’t want to feed through the device but feed in and stop at a certain point. So, to accomplish this I built a photo gate. The circuit is pretty basic… there is a source on one side and a sensor on the other. When the tube passes between the two the output at the collector of the photo trasistor changes. Using an op amp as a comparator the analog output is converted to a nice on/off signal. But that’s not what’s really neat… what’s really neat about this is the fact that this board is embedded/concealed into the side of this 1/4″ cover plate AND there are components on both sides of the board.