This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This circuit has an LED indication of the timing progress, having been found in an old...
This circuit is from a Radio Electronics magazine from August 1974. It used one of the first...
This charger is powered by the car's 12 V socket, the number of diodes being calculated according to...