modelKitchenScale

Information

A complete kitchen scale, from the mass placed on the pan to the display:

load + pan (masses) → g, weight (weight: a force) → loadCell (load cell body, which deforms) → bridge (gauges: the strain unbalances the bridge, 1 mV/V at full load) → hx (HX711: amplification × 128 and 24-bit conversion) → mcu (program kitchen_scale.py) → lcd (Grove LCD RGB screen, I2C bus).

The program uses two off-the-shelf MicroPython drivers, unmodified: hx711_gpio.py (robert-hh/hx711) and driver_grove_lcd_rgb.py. It tares at start-up (the 200 g pan becomes the zero), converts the HX711 points into grams with a calibration constant (429.497 points per gram) and shows the result to the nearest gram. Only the characters that change are sent to the screen: each one costs an I2C transaction. Pressing the TARE button (GP0, interrupt on falling edge) resets the display to zero, bowl included.

Scenario:

• t = 0 to 1.1 s: start-up, tare of the empty pan ("Tare...");
• t = 2 s: a 350 g bowl is placed → "350 g";
• t = 3.5 s: TARE pressed → "Tare...", then "0 g" around 4.2 s;
• t = 4.5 to 5.5 s: 250 g of flour are poured → the display rises up to "250 g".

To watch: the icon of lcd during the animated replay; hx.code (the raw points, noise included); mcu.GP6.v and mcu.GP7.v (the HX711 frames); mcu.GP4.v/mcu.GP5.v (the I2C bus, active only when the display changes).

Lab ideas: calibrate the scale (find 429.497 points per gram again from a known mass); tune the filter of the driver (set_time_constant) and observe the trade-off between stability and speed; increase hx.noiseLsb; set hx.rate to 80 samples per second.

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