Patent US 4,236,181

A visible image of an apparatus including a movable element to be watched is formed on the CRT of a television by means of an industrial television camera. One or more photosensors are provided on the face plate of the CRT to receive light from the image of the apparatus. When the movable element of the apparatus is moved from an original position, the output of the photosensor is changed. This, it is electrically detected whether or not the movable element exists in its given position by watching the output level of the photosensor.