The Physics: How the Strain Gauge and HX711 Sensor Working Principle Align
At the physical level, a load cell like the common TAL220 (a 5kg single-point aluminum beam) relies on Wheatstone bridge circuitry. Four strain gauges are bonded to the metal beam in areas of maximum mechanical stress. When you apply force, the metal deforms microscopically, stretching two gauges (increasing resistance) and compressing the other two (decreasing resistance). This unbalances the bridge, generating a differential voltage in the microvolt range proportional to the applied weight. Because this signal is far too small for any microcontroller's internal ADC to read accurately, it requires dedicated amplification.
The HX711 IC acts as the critical bridge between this analog microvolt signal and your digital microcontroller. It contains a 24-bit analog-to-digital converter (ADC) paired with a Programmable Gain Amplifier (PGA) that boosts the load cell's differential output by a factor of 128. Instead of outputting a varying voltage or a standard I2C/SPI data stream, the HX711 outputs a precise 24-bit signed integer via a custom two-wire serial protocol, giving you the raw digital representation of the physical force applied to the beam.
Hardware Interfacing: Pinout, Supply Ranges, and Signal Types
A common mistake when wiring the HX711 is conflating its custom serial output with standard protocols like I2C or SPI. The HX711 uses a strict clock-and-data handshake. Furthermore, while the HX711 can be powered by 5V, its digital logic outputs will match its supply voltage. If you are using a 3.3V microcontroller like the ESP32, you must power the HX711's VCC with 3.3V to avoid frying your GPIO pins.
Do not wire the HX711 DT (Data) pin to an analog input (ADC) on your Arduino or ESP32. The output is a timed digital bitstream, not a variable voltage. Wire it to a standard digital GPIO.
| HX711 Pin | Function | ESP32 DevKit V1 Pin | Arduino Uno Pin | Notes & Supply Range |
|---|---|---|---|---|
| VCC | Logic & Analog Supply | 3V3 | 5V | Accepts 2.6V to 5.5V. Match to MCU logic level. |
| GND | Common Ground | GND | GND | Must share ground with MCU and load cell. |
| DT (DOUT) | Digital Data Out | GPIO 4 | D2 | Serial data line. Do not use internal pull-ups. |
| SCK (PD_SCK) | Serial Clock In | GPIO 5 | D3 | Clock line. Timing must be precise (no interrupts). |
| E+ / E- | Excitation Voltage | N/A | N/A | Connects to Load Cell Red/Black wires. |
| A+ / A- | Signal Input | N/A | N/A | Connects to Load Cell White/Green wires. |
The Math: Converting Raw 24-Bit Hex to Physical Weight
The HX711 outputs a 24-bit two's complement signed integer. This means your raw reading will range from -8,388,608 to +8,388,607. To turn this raw number into a usable physical unit like kilograms, you must perform a two-step linear calibration: establishing a Tare Offset and calculating a Calibration Factor.
Step 1: Establish the Tare Offset (Zero Point)
With nothing on the scale, read the raw value from the HX711 multiple times and average it. Let's assume your empty scale averages a raw reading of 8,000,000. This is your Tare Offset. All future readings will subtract this number to establish a baseline of zero.
Step 2: Calculate the Calibration Factor (Scaling)
Place an object of known, precise weight on the scale. For this example, we use a calibrated 1.000 kg mass. The HX711 outputs a new raw reading of 8,421,500.
- Delta Raw = Raw Read - Tare Offset (8,421,500 - 8,000,000 = 421,500)
- Calibration Factor = Delta Raw / Known Weight (421,500 / 1.000 = 421,500)
Step 3: The Final Conversion Formula
For any subsequent measurement, apply this exact formula in your C++ code:
Weight_kg = (Current_Raw_Read - Tare_Offset) / Calibration_Factor
Worked Example: You place an unknown object on the scale. The HX711 outputs 8,210,750.
Weight = (8,210,750 - 8,000,000) / 421,500
Weight = 210,750 / 421,500 = 0.500 kg.
If you mount the load cell upside down or apply upward force, the raw reading will drop below the Tare Offset, resulting in a negative Delta. The math remains identical, but your code should use signed 32-bit integers (
int32_t) rather than unsigned integers to prevent overflow errors when crossing the zero threshold.
Real-World Interference and Signal Degradation
The HX711 breakout boards are highly sensitive, which is a double-edged sword. On the bench, you will encounter three primary interference sources that corrupt the raw 24-bit data:
- Electromagnetic Interference (EMI): The wires connecting the load cell to the HX711 (A+, A-, E+, E-) act as antennas. If routed near stepper motors, AC relays, or even the ESP32's WiFi antenna, they will inject 50Hz/60Hz mains noise and high-frequency RF spikes into the microvolt signal. Fix: Use shielded 4-core cable for the load cell and keep it under 30cm.
- Thermal Drift: Strain gauges are temperature-sensitive. If your workspace temperature shifts by 5°C, the metal beam expands, altering the baseline resistance and shifting your Tare Offset. Fix: Implement a software auto-tare routine that triggers when the scale reads stable but non-zero for an extended period, or use a temperature-compensated load cell.
- Creep and Hysteresis: If you leave a 4kg weight on a 5kg load cell for an hour, the raw reading will slowly drift upward due to mechanical creep in the aluminum. When removed, it won't immediately return to zero. Fix: Design your mechanical enclosure to fully unload the beam between measurements if high precision is required over long durations.
Frequently Asked Questions: HX711 Sensor Working Nuances
Why is my load cell sensor working erratically with an ESP32?
Erratic readings on an ESP32 usually stem from two specific issues. First, the ESP32 runs a real-time operating system (FreeRTOS) with background tasks like WiFi and Bluetooth. The HX711 requires strict microsecond timing on the SCK (clock) pin to clock out the 24 bits. If a WiFi interrupt pauses the clock signal for too long, the HX711 resets or outputs garbage data. Second, if you powered the HX711 with 5V but connected the DT pin to a 3.3V ESP32 GPIO, you are backfeeding 5V into a 3.3V pin, causing logic threshold confusion and potential silicon damage. Power the HX711 from the ESP32's 3V3 pin to solve both logic level and timing issues (using hx711.read() inside a critical section or disabling interrupts during the read).
How does the HX711 sensor working cycle handle 50Hz/60Hz noise?
The HX711 IC features a hardware notch filter designed to reject mains frequency noise, controlled by the RATE pin on the physical chip (often a jumper pad on cheap breakout boards). If the RATE pin is pulled LOW (default on most modules), the ADC samples at 10 SPS (Samples Per Second) and applies a 50Hz notch filter. If pulled HIGH, it samples at 80 SPS with a 60Hz filter. For most bench and DIY scale applications in North America, pulling the RATE pin HIGH to enable the 60Hz rejection and 80 SPS rate yields faster, cleaner data, which you can then average in software.
Is the HX711 sensor working principle compatible with multiple load cells?
You cannot simply wire multiple load cells in parallel to a single HX711 A+/A- input; doing so creates a short between their internal Wheatstone bridges, destroying the differential signal and yielding a static, unchanging raw output. To use four load cells (like in a bathroom scale), you must either use an analog summing board that physically combines the bridge signals before feeding a single HX711, or use four separate HX711 modules, each with its own dedicated SCK and DT GPIO pins on your microcontroller, summing the final kilogram values in software.
