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.
The frequency of this Pierce oscillator, found in 1969 documentation, depends on the crystal. The...
This simple timer with unijunction transistor was found in an old American technical...
Found in documentation from the 1960s, this power supply employs germanium diodes from the time....