This circuit is from a Hands-On Electronics from the 90s. The integrated amplifiers used are well known. The range is a few tens of meters depending on the resistance of the cables.
This circuit generates a rectangular signal whose frequency depends on the electrolytic...
This circuit was found in an old publicatio n from the year 1958. The circuit controls an external load by...
This circuit that converts very low currents into voltage was obtained in the 80's used component...