I built your circuit as per diagram:I'm still assuming that the output is open-collector. I found a photo of the internal workings, and you can see the pull-up resistor, which goes to the positive supply.
So with a 5V supply, it will be TTL compatible.
With a 12V supply, it will have a 12V output, so my circuit will work.
By just running the sensor under the tap, at what i guess will be the water flow in reality(when the engine in running) the alarm switches on /off very fast.
If I speed up the sensor, by blowing into it, the alarm stops ( a much faster rotation)
On the rotor slowing down( run out of breath) The alarm starts to switch on/off rapidly until a contentious sound is heard.
So the circuit does work.
What do you suggest I change , to set the threshold lower? thanks