Monitor remaining water bottles with ESPHome and Home Assistant

After we ran out of sparkling water twice in a row, I decided to do something against that. The plan is to receive a notification when there are only three bottles left:

Ideas

I need a way to sense how many bottles are left. Here are some ideas I had:

• put a light sensor under each bottle in the box
• put a scale under the crate
• use a camera and a neural network to do some object detection

I don't know enough about neural networks and object detection, but I think this would be rather complicated and over-engineered for what I want to achieve. It may be really hard to distinguish full bottles of water from empty ones, even if I lift them slightly to check if they are full or empty. It could work for colored beverages though.

Using light sensors for each bottle is still an idea I want to check out in the future, as this solves the weight problem I'll go into in the next section.

Why weighing the bottles may not work for you

Before we begin weighing bottles, this approach probably only works for a few plastic bottles. I put a scale under the bottles and weight them (I normalized the result by subtracting the weight of the crate). With the result, let's call it W, we can calculate the number of bottles like this:

\[n_{full bottles} = \lfloor\frac{W}{1070g}\rfloor\]

Those brackets mean “round the result down”. I measured that one full bottle weighs 1070g, and one crate holds 12 bottles, so the maximum weight is 12840g.

For my example, we will get the right result every time, because if all bottles are empty, the whole crate weighs 70g × 12 = 840g which is equal to zero bottles. Here is the problem: If you have glass bottles, and probably a crate of 24 bottles, this approach won't work for you. Let's assume an empty glass bottle weighs *200g. *The drink itself weighs 330g for a 0,33l bottle. Then, our formula would be:

\[n_{full bottles} = \lfloor\frac{W}{530g}\rfloor\]

For W = 1795g, this can either be 3 full and 1 empty bottle (1790g) or 9 empty bottles (1800g). As the scale doesn't return exact values and not every bottle weighs the same, it is impossible to tell these cases apart. If you have an idea of how to solve this, please leave a comment below.

Now we learned that weighting the bottles will only work if:

\[W_{empty bottle} \times n_{bottles} \lt W_{full bottle}\]

Put a scale under the crate

I decided to make the scale myself and use a cheap HX711 module from a Chinese supplier. Check out this instructables to see how to wire them up.

I'll use ESPHome to program a WeMOS D1 Mini which is connected to the HX711.

esphome:
  name: water_bottles
  platform: ESP8266
  board: d1_mini

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

api:
ota:
logger:

sensor:
  - platform: hx711
    name: "water_bottles_weight"
    id: water_bottles_weight
    dout_pin: D2
    clk_pin: D1
    gain: 128
    update_interval: 60s
    filters:
      - lambda: |-
          auto first_mass = 0.0; // first known mass was 0kg
          auto first_value = 25136; // value for the first known mass was 
          auto second_mass = 12840; // second mass was 
          auto second_value = 24568; // second value was
          auto r = map(x/1000, first_value, second_value, first_mass, second_mass);
          if (r > 0) return r;
          return 0;
    unit_of_measurement: g

  - platform: template
    name: "water_bottles_count"
    lambda: |-
      return floor(id(water_bottles_weight).state / 1000);
    update_interval: 60s

Above, you see the code on how I programmed the scale. It sends the weight and the count to home assistant, where you only need to go to Config > Integrations > ESPHome (Add) and enter water_bottles.local. To get started with ESPHome, check out the documentation.

Now, you can add automations as desired. My automation reminds me to buy new water if there are fewer than 4 bottles left.

Do you have an alternative idea to achieve this? Toot me on Mastodon and tell me about it.