This 50 Hz rejector or notch filter was found in a 1975 documentation. Other frequencies can be obtained by changing components. The filter uses a Wien network and must use a symmetrical power supply.
We found this 3-digit digital odometer for bicycles (or another application) in a 1995...
This circuit was published in a 1977 English magazine, but remains current, since we can assemble it...
In the figure we have the configuration of series regulator protector suggested by Motorola in 1968...