The sensor

The flow sensor connected to the rain water pump.

I bought from a "YF-B7 Water Flow Sensor with Temperature Sensor - Brass - G1/2" from tinytronics.nl. This device uses a magnetic propeller sitting in the water feed to trigger pulses in a hall effect sensor.

A summary of its specifications:

Water flow sensor:
    Supply voltage: 5-18V DC
    Signal voltage: 4.7V (with 5V DC power supply)
    Connector: JST-SM 3p female
    Pulse frequency per L/min: 11Hz*
    Duty cycle pulse: 50% ±10%
    Measuring range: 1-25L/min
    Accuracy: ±3%*
Temperature sensor:
    Type: NTC
    Resistance: 50KΩ
    Beta value: 3950
    Connector: JST-SM 2p female
    Thread: M8
Water connection: G1/2"
Maximum water pressure: 17.5bar
Working temperature: 1-100°C
Suitable liquids: Water**
Material: Brass
Dimensions: See images

Pinout (water flow sensor):

Red: Vcc/plus (supply voltage)
Black: GND/min
Yellow: Signal

Pulse counter

I have several sensors in my garden shed, and they are all read via a 2 wire RS485 serial bus.

I figured I could use an off the shelve pulse counter to keep track of the water use. This way the more complex real-time tracking part can be kept close to the sensor since modbus is probably too slow and it keeps things simple.

The €34.89 "WJ150-485 1 Way Encoder Pulse Signal Counter or 2 Way DI High-speed Counter Modbus RTU Module" from Aliexpress was a good fit, it had good documentation and I got it to work in an hour.

Mechanical sensor parts assembled with a very short sensor tube to fit the picture.

In set-up mode, which is enabled by connecting a wire to one of the module's screw terminals, the module can receive ASCII commands, so a modbus RTU client is not even necessary.

I sent this command to it using a serial terminal to set module address from '01' to '05', module type '00', baudrate at 9600 ('6'):

%01050006

After a powercycle I checked if I could read the pulse count, and after blowing on the water flow meter, the count register increased!

thomas@roper ~ % modbus  -b 9600 -s 1 /dev/ttyUSB0  h@32/\<I
Parsed 0 registers definitions from 1 files
32: 2819 0xb03

Home assistant setup

I added this modbus.yaml entry:

- name: water_counter
  unique_id: water_counter
  slave: 5
  address: 32
  swap: word
  data_type: uint32

And a template sensor to convert the pulse count to liters (template.yaml):

- trigger:
    - platform: time_pattern
      minutes: "/1"
    - platform: state
      entity_id:
        - sensor.water_counter
      not_from:
        - "unknown"
        - "unavailable"
      not_to:
        - "unknown"
        - "unavailable"
  sensor:
    - name: regenwater
      unique_id: regenwater
      unit_of_measurement: L
      device_class: water
      state_class: total_increasing
      state: >
        {% set count = states('sensor.water_counter') | float %}
        {{ '%0.3f' | format(count*6.0/(4*660)) }}
      attributes:
        dummy: "{{ now().minute }}"

Now I can view pretty bar charts in the Home Assistant Energy overview.

The accuracy might be rough, but I did some napkin math and it seems to be ballpark-accurate.