This project contains all the necessary software to program the ESP32 microcontroller and interface with it through a Bluetooth API-enabled web browser (Chrome, Opera).

The firmware folder contains the .py files that need to be uploaded into the ESP32's memory running the MicroPython interpreter.

The firmware code has been tested on esp32-idf4-20200930-unstable-v1.13-77-g0fd0eb00a.bin (later version should work too) and can be flashed with esptool according to the instructions on the micropython site.

To upload the source files to the microcontroller, either use WebREPL or ampy.

The src folder contains the source for the Progressive Web App that allows to retrieve the scale's reading in real time (10Hz refresh rate) as well as the battery level.

The application is hosted at beeb.li/coffee for convenience of users desiring building the scale.

The app works as a timer to achieve the desired extraction in the desired time. A reference curve is displayed, taking into account the user parameters:

• Coffee Weight: how much coffee grounds were put in the basket (the "Read" button puts the current scale value into this field)
• Target Ratio: the ratio of "coffee out" to "grounds in". The expected output weight is calculated next to the field.
• Pre-infusion Time: this is an amount of time that is added to the timer as soon as >0.5g are detected in the cup. This allows to indicate the desired total brew time including pre-infusion. It's the user responsibility to perform the pre-infusion with this amount of time.
• Total Time: the total brew time, including any pre-infusion.

• The basket or portafilter is put on the scale and tared with the scale's button.
• The desired amount of coffee grounds is put inside the basket
• the basket or portafilter is put back on the scales, thereby measuring the weight of the coffee grounds
• The "Read" button next to the "Coffee Weight" input is clicked to read the current value into it
• The cup is placed on the scale and it is tared with the button
• The "Start recording" button is pressed (only available when the scale reads 0g). The app now waits for an increase in weight.
• As soon as the weight exceeds 0.5g, the timer starts counting from the pre-infusion time (default 5s).
• The extraction can be followed in real time and should match the grey reference curve (in a direct drive lever machine, the user would adjust the pressure to match the curve)
• When the extraction is finished, the cup is lifted from the scale, thus stopping the recording.

• Dual-core microcontroller allows for fast sampling rate of the load cell and fast refresh rate of the 128x32 OLED display
• Load cell input is filtered with basic Kalman filter for fast response and good smoothing
• The weight is communicated through Bluetooth Low Energy every 100ms
• The microcontroller can charge a LiPo or Li-ion battery and report its charge level
• The web-app persists user settings in the browser's local storage
• Can be added to the home screen of smartphones (e.g. with Chrome on Android, look almost like a native app)

The 3D printable parts are available from the prusaprinters page of this project.

This project was made possible by the following projects and people (amongst others). Thanks to them!

• hx711-lopy by robert-hh
• SimpleKalmanFilter by denyssene
• vue-svg-gauge by hellocomet
• StayAwake.js by jamesbachini
• Vue.js
• BootstrapVue
• MicroPython

Distributed under the MIT License