This circuit was suggested by National Semiconductor and is present elsewhere on this site. The LM2575 is not very common today and should be mounted on a heatsink.
This circuit was found in a 1980s American documentation. The circuit is based on integrated CMOS 4001...
This 1970 Philips circuit uses a mechanical counter. The circuit is triggered by a reed switch...
This traditional photodiode amplifier configuration uses an operational one that is no longer...