Found in a PE Hobbyst Handbook of 1991, this circuit was originally proposed to time photographic baths in developing rooms but can be used for other purposes. The circuit beeps at 30-second intervals.
This circuit was found in a 90s Radio Electronics. The FM encoder or multiplexer is an uncommon...
This touch-controlled circuit was found in English documentation from the 1970s. The components...
Found in a book on transistors from 1954, this circuit can modernly be used as a relay shield. A 1k...