This 4-tube counter was found in Radio Electronics magazine from December 1959. The circuit uses a coil as a sensor to detect the passage of metallic parts or magnets.
This circuit is from a 2005 Linear Technology manual. The circuit is powered by 3.3 V but can...
This circuit uses two operational amplifiers, which can be of the type 741, in a full wave...
This simple circuit for an audio link uses a lamp, which has a very poor frequency response, but...