LoadCell Scale

A standalone digital scale built on a Raspberry Pi Pico running MicroPython. Reads a load cell via an HX711 amplifier, displays weight on a 2×16 I2C LCD, and supports multiple units. Calibration is handled via the REPL when a PC is connected.

Features

• Live weight display in g, kg, oz, or lbs
• Adaptive lowpass filter — snappy response to load changes, stable at rest
• Short press tare button: tare and zero the scale
• Long press tare button (≥1s): cycle through units
• Peak hold mode — captures the stable peak weight, ignoring loading impulses
• Item count mode — for coin and pill counting; shows count on line 1, weight on line 2
• Battery monitor — custom LCD icon with fill level, low and critical warnings
• Calibration and tare saved to flash (cal.json) — survives power cycles
• Timer-driven measurement loop — REPL remains available while the scale runs

Hardware

Component	Details
Microcontroller	Raspberry Pi Pico (RP2040)
Load cell amplifier	HX711
Display	HD44780 2×16 LCD via PCF8574 I2C backpack
Power	LiPo via VSYS

Pin Assignments

Function	GPIO
HX711 SCK	GP6
HX711 DATA	GP7
LCD SDA (I2C0)	GP4
LCD SCL (I2C0)	GP5
Tare button	GP2 (active-low)
Battery ADC	GP26 (100kΩ/100kΩ divider from LiPo+)
I2C pull-ups	GP4/GP5 internal pull-ups enabled; external 4.7kΩ recommended if backpack has none

Files

File	Description
scale.py	Main application — copy to main.py on the Pico for auto-run
lcd_i2c.py	Minimal HD44780 driver over PCF8574 I2C backpack
hx711/hx711_gpio.py	HX711 driver with adaptive lowpass filter

Getting Started

1. Copy hx711/hx711_gpio.py, lcd_i2c.py, and scale.py to the Pico's filesystem
2. Update the pin assignments and LCD I2C address (0x27 or 0x3F) in scale.py if needed
3. Rename scale.py to main.py on the Pico to auto-run on boot
4. Connect via REPL and run the calibration workflow below

Calibration

Calibration is done once via the REPL (e.g. with mpremote or Thonny). The values are saved to cal.json and loaded automatically on boot.

# 1. Empty platform — zero the scale
tare()

# 2. Place a known weight on the platform, then:
calibrate(500, "2025-05-05")   # known weight in grams, date as string

To reload calibration from flash without rebooting:

load_cal()

Unit Switching

From the REPL:

set_unit('oz')   # g, kg, oz, lbs

Or hold the tare button for ≥1 second to cycle through units on the device.

Peak Hold

Captures the stable peak weight — loading impulses are ignored. The peak updates only once the reading has been steady for ~500ms.

peak_hold()       # enable
peak_hold(False)  # disable
peak_reset()      # clear stored peak without disabling

Line 1 shows live weight; line 2 shows PK: 1234.5 g when active.

Item Count Mode

Useful for counting coins, pills, or other uniform items.

tare()        # empty platform
peak_hold()   # enable peak hold
# place one item on the platform and wait ~1s for a stable reading
count_mode()  # switch to count mode using the captured unit weight

Or with a known unit weight:

count_mode(5.23)   # 5.23g per item — skips the peak hold step
count_mode(False)  # return to normal mode

Line 1 shows the item count; line 2 shows the total weight.

Battery Monitor

A 100kΩ/100kΩ resistor divider on GP26 measures the single-cell LiPo voltage (3.0–4.2V). The Pico's SMPS is put into power-save mode on boot for better efficiency at light loads.

A custom LCD character (5×8 pixel battery icon) shows the charge level with fill rising from the bottom. Line 2 displays the icon and voltage at all times when not occupied by other content.

Voltage	Icon fill	Status
≥ 4.0V	Full	—
≥ 3.8V	3/4	—
≥ 3.6V	1/2	—
≥ 3.5V	1/4	Low warning
< 3.2V	Empty	CRIT! (always visible)

Check battery voltage from the REPL:

get_battery_v()

LCD I2C Address

The address is set to 0x3F in scale.py. Change to 0x27 if your backpack uses that address. Scan from the REPL to check:

from machine import I2C, Pin
I2C(0, sda=Pin(4, pull=Pin.PULL_UP), scl=Pin(5, pull=Pin.PULL_UP)).scan()

RP2040 Notes

machine.Timer callbacks run at interrupt level on RP2040 and cannot perform I2C or HX711 GPIO bit-banging. The measurement loop runs on core 1 via _thread instead. GPIO IRQ callbacks also only run on core 0, so the HX711 driver uses polling rather than IRQ to detect conversion-ready.