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 for very long-time intervals was found in the March 1993 Radio Electronics magazine. Times...
Suggested by National Semiconductor in documentation from the 90s, this circuit uses a switched...
Found in a 1960s RCA manual, this circuit uses components from the era. For the indicated circuit R1 is...