I’d use a small processor module like a Seeeduino XIAO to produce the PWM pulses to turn the drive transistor on. A BJT should be fine.

Because the on time is brief you can overdrive the LED(s) above their specified current without shortening their lifetime significantly.

Use a 4 digit 7 segment display module to show RPM, DIP switch to select frequency ranges, pot to adjust frequency.

Use the micros() function to check frequency, a variable loop size and/or delaymicroseconds() function to control frequency.

The XIAO is pretty fast and you only need to update the display maybe three times a second so if the update takes too long you can skip a cycle or more and you won’t notice it so long as it uses micros() to synchronize with where it left off

 

Just some thoughts (from an old article from the time when white LEDs were getting bright enough for some serious uses).

A little off topic, @DickCappels, but I really wanted to tell you how much I enjoy your project articles. Very readable, complete, and competent. Really excellent work. I ran across some of them before joining here, and it was a pleasure to discover that it was that Dick Cappels (OK, to be completely honest, I hadn’t remembered your name and it took you posting one, also about LEDs, to get me to twig to your secret identity).

Anyway, always a pleasure to read one, even if I don’t have any intention to build something from it. I always learn, or am reminded about, something important. Thanks for some great reading.