This interesting sensor for RF signals was found in an article in the June 1989 Radio Electronics magazine. It is a field strength meter with an LED bargraph indicator.
This circuit was found in a 1962 publication. At the time there were high voltage batteries like the...
Suggested by Sprague in the 1980s documentation, this circuit can control three-phase AC motors....
This circuit divides the frequency of a 2 kHz signal by 2. The circuit was found in Philips...