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.
We found the circuit of this Japanese transistorized commercial receiver in a documentation from...
This circuit was found in a 90's Electronic Design. It takes advantage of the nonlinearity...
This three-transistor amplifier uses a JFET at the input to increase the input impedance. The...