With this circuit it is possible to use a frequency meter as a tachometer. The circuit was found in American documentation from the 90s. The phototransistor can be of any type.
This configuration is from an old Motorola documentation from the 70s. The circuit uses operations that...
This National Semiconductor circuit appears elsewhere on this site. The circuit must be powered by...
This is the diagram of a Japanese commercial radio from the 1960s. The circuit uses germanium...