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 1967 Raytheon documentation, this circuit uses a company coupler consisting of a lamp and...
In a modern version this circuit can use an LED in series with a resistor in place of the lamp. The...
This circuit, when triggered, locks, and remains so until the source is switched off for a moment or...