This RF circuit measures transmission power from 0.25 W to 10 W. The maximum frequency depends on the diode and the gauge is an analog instrument. A multimeter can be used instead.
This fast-acting circuit uses any phototransistor as a sensor. The circuit drives a 30 V relay, but...
This circuit is from 1995 documentation. Integrated circuits are used which can still be found...
This circuit was found in a Popular Electronics from the 90s. The circuit uses common components...