This simple circuit to monitor the frequency of the power grid uses an analog indicator. The adjustment trimpot can be 4k7 and the indicator is 100 mA full scale.
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...