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 is from a technical documentation from the 70s, and equivalents can be used for...
This circuit was obtained in an English publication from 1980 but can still be assembled with...
This circuit was found in the July 1992 Radio Electronics magazine. Sensitivity adjustment is made...