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 circuit was found in a publication in 2002 and can be powered by voltages from 5 to 12 V. The...
Found in an American 80's documentation, this circuit can use common FETs like the BF245 and cover...
Found in a publication for radio amateurs from the 1980s, this circuit amplifies signals in the 60...