This circuit for the 40 MHz band was published in Radio Electronics magazine in December 1957. Its purpose was to monitor the signals of the Russian satellite launched at the time.
This circuit was found in a 1990 Electronics Handbook. The transistors can also be the BC548 and...
This circuit is from a 1987 American publication called Electronics Handbook. The circuit can be...
This 1995 documentation circuit detects when an extension on a phone line is active. Adjustment of the...