Aren't there little flow sensors for small pumps that have a rotating vane with magnets on the inside and a hall effect sensor on the outside, or something like that? IIRC they are pretty low cost.

ak

 

RPM can be extracted from current with careful BPF and cycle interval detector 1-shot but the current approach will work better. I imagine no-load current may have spike noise so I added a 100 nF LPF cap to the original https://tinyurl.com/25pr34l7

I used 50 mOhm current sensor ( or 2x 0.1 Ohm) with a 100x gain amplifier to reduce power dissipation. I expect 5mVpk during operation. You test this on a protoboard if wires are kept short. Add decoupling caps.


BTW the FET driver senses the current peak detector cap thus preventing a flyback voltage so the coil diode is redundant. https://tinyurl.com/23pp2hz2




I simulated motor load with 150 ohms and no-load with 1k5.
The AC motor is sensed relative to neutral, so the DC supply is not isolated.


If you need an explanation why I designed it this way, just ask

 

What will be a lot simpler is to have a flow sensing switch in line with the pump, either inlet or discharge, with a bypass switch to allow starting the pump and getting the water flowing. When the tank is empty and the flow stops, the pump will switch off. No electronics and no adjustment and it will also avoid seal damage from running the pump dry.
My method is simpler, probably cheaper also.