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 triac control circuit with transformer and unijunction transistor was found in the June 1970...
This circuit was found in an old GE manual. It uses a thermocouple as a sensor. The SCR must be in...
Found in a publication by Gernsback from 1960, this circuit makes use of germanium transistors of the time....