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.
Found in a Maxim documentation from the 1990s, this current-sensing amplifier uses components from the...
We found this circuit in a 1982 documentation. An intercom to be good must be sensitive and have...
This circuit is from a 1980s documentation, but the configuration can be applied to most modern...