Hall effect sensor, water flow indicator alarm

I also got individual Hall effect sensors, 44E's and 3144's

There is no good link for the China crap sensor that I've bought, which i did for a long time ago,

But the idea is to read the pwm input, and monitor it to a certain pulse width?

Let's say that we have a normal pulse of about 10hz? When it's at its top of flow.

And if it then drops below ~5hz then it will trigger a pulsed output to a piezo? And maybe trigger a ssr relay with some lights? or a mosfet?

I have no idea how fast it will spin,

found a schematic for the Hall sensor module.