DIY Bluetooth Coffee/Espresso Scale

prusaprinters

<p>&nbsp;</p><figure class="media"><oembed url="https://www.youtube.com/watch?v=_Y4ka5QkGaM"></oembed></figure><p>&nbsp;</p><p>Seeing how smart espresso or coffee scales can cost hundreds of dollars, I decided to build my own. I designed all of the 3D files and created a companion web-app that runs in any browser that supports the Bluetooth API (Chrome would be my choice).</p><p>The open-source code for the app and firmware can be found at <a href="https://github.com/beeb/coffee-scale-app">https://github.com/beeb/coffee-scale-app</a></p><p>I sourced the springs and contacts for the 18650 Li-ion cell from some AA batteries holder I had laying around, see picture attached (you lift the small rivet prongs with a blade to remove it from the plastic housing, then transfer to the printed base and bend them back outwards to secure them.</p><p>Bill of Materials:</p><ul><li>Sparkfun Mini Load Cell - 500g, Straight Bar (TAL221)</li><li>HX711 Load cell amplifier (any manufacturer)</li><li>Sparkfun ESP32 Thing</li><li>128x32 OLED module with I2C capability (40x12mm - SSD1306)</li><li>2 x 47kOhm or 100kOhm or 220kOhm resistors (for battery voltage divider - they need to have both the same value)</li><li>SPST Rocker switch</li><li>M3 screws</li><li>Spring and contact from battery case</li><li>18650 Li-ion battery</li></ul><p>The OLED screen, HX711 breakout board, and ESP32 are held in place with thin double-sided sticky tape (a small surface on either extremity is enough). Depending on the thickness of the tape, you might need to sand down the button cap a little bit so that it doesn't depress the button in its resting position.</p><p><strong>NOTE: </strong>If your HX711 breakout board has separate VCC and VDD pins, one user mentioned they had to short/bridge the two pins together in order to have correct readings.</p><p>Since my power switch was so big, I couldn't fit it nicely on the sides of the scale, so I decided to make it accessible from the bottom.</p><p>The ESP32 Thing has a built-in charging circuit which means that USB can charge the Li-ion cell when the switch is in the ON position.</p><p>The battery voltage is in the range of 2.5V to 4.2V which exceeds the maximum 3.6V of the ESP32 pins. As such, I made a very basic voltage divider with 2 resistors having the same value. I directly soldered the resistors on the GND + VBAT pins and ran a couple of wires to the sensing pin. The resulting sensed voltage between the two resistors will be half of the real voltage. The code has a lookup table (with interpolation) to determine the remaining capacity percentage.</p><h3>Print instructions</h3><p>Printed in PolyMax PLA but pretty much any material should work.</p><p>No supports needed, overhangs print as bridges.</p>