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 traditional configuration of astable was found in a 1965 documentation. Transistors can be...
This variable frequency oscillator generates signals that have a recurrence frequency determined by...
This is an old circuit, using operating equipment of the time, modern operational amplifiers can...