We found this circuit in a Popular Electronics from the 90s. Its purpose is to monitor a power line. The circuit uses optical couplers of the time.
This circuit was found in a documentation of the 80s, having didactic purpose. For R1 values...
This 1965 General Electric circuit uses a "Thyrector" that is a suppressor of the time. The...
This old diode test, from a documentation from the 1970s, uses two small 6 V lamps up to 50 mA to...