Then one choice is to add a HDD ( hard drive ) Tachometer.
This tach will show the HDD activity on a desktop computer. Not for automotive use.
Built with the 555 and 4017 combination. But the " Interface " is what makes this work.
Tricky thing is the HDD LED has full V+ all the time and it is the ground that is varied to make the LED flash.
That means the HDD LED ground becomes the input signal for the tachometer, or pin2 of the 555.
There is more descriptions in the pictures.
I did not go in-depth of the chip operations and theory, just a down and dirty hack.
Actually i do not know exactly why / how the interface works or why it works.
Somebody with a physics or electronics background could probably understand and explain how this works.
For me, IT WORKS.
My regular camera is not here so the pictures are, well not good.