⚖️ Precision Digital Weighing Scale

Build a 0.1 g accuracy scale using a load cell and HX711 24-bit ADC.

📋 Overview

A load cell is a transducer that converts mechanical force into an electrical signal via strain gauges in a Wheatstone bridge configuration. The HX711 is a purpose-built 24-bit ADC amplifier that amplifies the tiny microvolt signals from the load cell up to a measurable level, achieving 0.1 gram resolution with a 5 kg cell.

What you'll learn: Load cell bridge theory, HX711 2-wire serial protocol, multi-point calibration with known weights, tare (zero) functionality, and running-average noise filtering.

Estimated time: 1.5–2 hours. Difficulty: ⭐⭐⭐ Intermediate.

🧩 Components Needed

Component	Specification	Qty	Notes
Arduino Nano	ATmega328P, 5V	1	Compact form factor
5 kg Load Cell	Straight bar type	1	Wheatstone bridge, 4-wire
HX711 Amplifier Module	24-bit ADC	1	Gain 128 by default
16x2 I2C LCD	PCF8574	1	Displays weight
Tare Button	Momentary	1	Zero reset
Acrylic Platform	100mm × 100mm	1	Weighing surface

📖 Step-by-Step Tutorial

Mount the Load Cell

Fix one end of the load cell rigidly to a base plate using M4 bolts. Attach the weighing platform to the free end. The load cell must flex only under the applied load.

Wire Load Cell to HX711

Load cell has 4 wires: Red (+Excitation) → E+, Black (-Excitation) → E-, White (+Signal) → A+, Green (-Signal) → A-. Connect HX711 VCC → 5V, GND → GND, DT → Pin 3, SCK → Pin 2.

Install HX711 Library

Install HX711 Arduino Library by Bogdan Necula from Library Manager.

Calibrate the Scale

Upload the calibration sketch. Note the raw value with no weight, then place a known weight (e.g., 100g) and note that value. The calibration factor = raw_diff / known_grams.

Add Tare & Display

Connect a button to Pin 7 (INPUT_PULLUP). When pressed, call scale.tare() to zero the reading. Display grams on the LCD with scale.get_units(10) (average of 10 readings).

💡

To find your exact calibration factor, place a known certified weight (postal scale weight or calibration mass) on the platform. Divide the raw HX711 reading by the actual weight in grams. This number varies between cells, so always calibrate individually.

💻 Code / Configuration

digital_scale.ino

INO

// Digital Weighing Scale - Volt X
#include <HX711.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

HX711 scale;
LiquidCrystal_I2C lcd(0x27, 16, 2);

const int DT_PIN      = 3;
const int SCK_PIN     = 2;
const int TARE_BUTTON = 7;

// Calibration factor — run calibration sketch first to find your value
float calibrationFactor = -7050.0; // Adjust for your load cell

void setup() {
  Serial.begin(9600);
  pinMode(TARE_BUTTON, INPUT_PULLUP);

  scale.begin(DT_PIN, SCK_PIN);
  scale.set_scale(calibrationFactor);
  scale.tare(); // Zero at startup

  lcd.init(); lcd.backlight();
  lcd.setCursor(0, 0);

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