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.
With this name, we find a photorelay in a publication from 1966. The transistor is made of...
This circuit was found in an old documentation for radio amateurs. The circuit makes use of an...
Shown in Radio Electronics June 1953, this circuit needs a small adjustable source of 0 to 3 V and...