HX711 load cell with NodeMCU ESP8266 — wiring and scale code

The HX711 turns a load cell’s tiny bridge signal into a 24-bit number. Paired with a NodeMCU it becomes a kitchen scale, a milk-collection weigher, or an LPG-cylinder level monitor that reports over Wi-Fi.

Wire DT to D5 and SCK to D6, power it from 3V3, and keep D3, D4 and D8 out of it — those pins decide how the ESP8266 boots.

Wiring

Board quirks

• The HX711 is not I2C, even though its pins say DT and SCK — it is a simple clocked protocol, so any two free GPIOs work. Using D5/D6 leaves D1/D2 free for an OLED or LCD on I2C.
• Avoid D8 for SCK: holding SCK high for over 60 µs powers the HX711 down, and D8 (GPIO15) must be LOW at boot. Avoid D3/D4 for DT: the HX711 drives DT, and a LOW there during reset stops the ESP8266 booting.
• Reading a sample bit-bangs 25 clock pulses with interrupts briefly off. That is short, but don’t wait in a bare while (!scale.is_ready()) loop — on the ESP8266 a wait without yield() trips the watchdog. wait_ready_timeout() yields for you.
• Wire colours vary between load cells. If the reading falls when you add weight, swap A+ and A−.
• Calibrate once: tare with nothing on, place a known weight (a 1 kg atta packet works), and set CALIBRATION = raw reading ÷ grams. Write the number down — it changes if you rewire.

Code

// HX711 load-cell amplifier on a NodeMCU (ESP8266).
// VCC → 3V3, GND → GND, DT → D5 (GPIO14), SCK → D6 (GPIO12). Load cell on E+/E-/A-/A+.
// Send t to tare. Put a known weight on and adjust CALIBRATION until the reading matches.
#include <HX711.h>

const uint8_t HX_DT = D5;
const uint8_t HX_SCK = D6;
const float CALIBRATION = 420.0;  // raw counts per gram for your load cell — calibrate this

HX711 scale;

void setup() {
  Serial.begin(115200);
  Serial.println();
  scale.begin(HX_DT, HX_SCK);
  if (!scale.wait_ready_timeout(1000)) {
    Serial.println("HX711 not responding - check DT/SCK and power");
  }
  scale.set_scale(CALIBRATION);
  scale.tare(10);  // the empty platform reads 0
  Serial.println("Scale tared. Send t to tare again.");
}

void loop() {
  if (Serial.available() && Serial.read() == 't') {
    scale.tare(10);
    Serial.println("Tared");
  }
  if (scale.wait_ready_timeout(200)) {  // waits with delay(), so the watchdog stays fed
    float grams = scale.get_units(5);
    Serial.printf("Weight %.1f g\n", grams);
  } else {
    Serial.println("HX711 not ready");
  }
  delay(500);
}

// HX711 load-cell amplifier on a NodeMCU (ESP8266).
// VCC → 3V3, GND → GND, DT → D5 (GPIO14), SCK → D6 (GPIO12). Load cell on E+/E-/A-/A+.
// Send t to tare. Put a known weight on and adjust CALIBRATION until the reading matches.
#include <HX711.h>

const uint8_t HX_DT = D5;
const uint8_t HX_SCK = D6;
const float CALIBRATION = 420.0;  // raw counts per gram for your load cell — calibrate this

HX711 scale;

void setup() {
  Serial.begin(115200);
  Serial.println();
  scale.begin(HX_DT, HX_SCK);
  if (!scale.wait_ready_timeout(1000)) {
    Serial.println("HX711 not responding - check DT/SCK and power");
  }
  scale.set_scale(CALIBRATION);
  scale.tare(10);  // the empty platform reads 0
  Serial.println("Scale tared. Send t to tare again.");
}

void loop() {
  if (Serial.available() && Serial.read() == 't') {
    scale.tare(10);
    Serial.println("Tared");
  }
  if (scale.wait_ready_timeout(200)) {  // waits with delay(), so the watchdog stays fed
    float grams = scale.get_units(5);
    Serial.printf("Weight %.1f g\n", grams);
  } else {
    Serial.println("HX711 not ready");
  }
  delay(500);
}

Library: HX711 Arduino Library (Bogdan Necula) — compiled with HX711 Arduino Library 0.7.5.

Expected serial output

Scale tared. Send t to tare again.
Weight 0.2 g
Weight -0.1 g
Weight 999.6 g
Weight 1000.3 g

Scale tared. Send t to tare again.
Weight 0.2 g
Weight -0.1 g
Weight 999.6 g
Weight 1000.3 g

Common errors