Found in a 90’s American documentation, this circuit indicates the frequency of input pulses up to 1 kHz. The circuit has a very explored configuration on this site.
Texas Instruments ESD122 Electrostatic Discharge Diodes (ESD) can be ordered from Mouser...
This circuit was suggested in a 1976 documentation. We can use the pair BD135 and BD136 on the...
This interesting circuit found in an Elektor from the 90s controls loads in four quadrants of the driving...