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 transmitter operates with a voltage of only 1.4 V, and can then be powered by a single...
Found in an Electronic Design of 1964 this circuit has a control for the frequency and another for the...
This old circuit, from the 80s, was found in American documentation. The circuit uses dual gate...