If I had to get a LED to blink at exactly 40 Hz, I would use an Arduino Nano clone to control it.
That may seem like overkill, but the Chinese Nano clones only cost $3 shipped w/USB cable if you are in the USA.
The Nano would be accurate to within (roughly) 4/1,000,000ths of a second; no analog circuit you could build for the same price would do nearly as well.
Plus you can alter the blink frequency just by changing a numeric value in a line of computer code.
If you want to construct a circuit on your own just because you want to do it, then just put "circuit to blink LED" into Google and stand back.
Many use a 555 chip, but many don't--and you can build a circuit to mimic the 555 chip if you want.
But none of these is going to be cheaper, faster or more accurate than just using the Arduino Nano.