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 well-designed bistable configuration is for educational purposes. We found it in the...
This low power bass booster is from the 1977 National Semiconductor Audio Handbook. The circuit is...
This circuit, well known to our readers, only has a slightly different schematic representation, as it was...