A load cell and HX711 pairing combines a force-sensitive strain gauge transducer with a dedicated 24-bit analog-to-digital converter to translate physical weight into precise digital data for a microcontroller.
In a real circuit or installation, this combination changes a high-impedance, microvolt-level analog differential signal into a robust, noise-immune digital serial stream. Instead of forcing a microcontroller's weak 10-bit internal ADC to read a noisy 2mV signal, the HX711 amplifies and digitizes the data at the source, outputting it via a simple two-wire clock/data interface (DT and SCK) that any GPIO can read.
The Anatomy of the Signal Chain
To understand why this specific pairing dominates the DIY and light-industrial weighing space, you have to look at the raw electrical characteristics of the sensors involved. A standard strain gauge load cell relies on a Wheatstone bridge circuit. When physical force bends the metal element, the resistance of the bonded strain gauges changes by fractions of an ohm, unbalancing the bridge and producing a tiny differential voltage.
The HX711, designed by Avia Semiconductor, is built specifically to read this exact topology. It features an integrated programmable gain amplifier (PGA) and a precision 24-bit ADC. Below is the critical specification data you need when matching common hobbyist and light-industrial load cells to the HX711's input channels.
| Component / Channel | Rated Capacity / Gain | Sensitivity / Input Range | Excitation / VREF | Typical Use Case |
|---|---|---|---|---|
| CZL601 (5kg Single Point) | 5 kg (49 N) | 1.0 mV/V ± 0.15 | 5V - 12V DC | Small hoppers, espresso dosing |
| CZL601 (20kg Single Point) | 20 kg (196 N) | 2.0 mV/V ± 0.15 | 5V - 12V DC | Kitchen scales, brewing kettles |
| CZL601 (50kg Single Point) | 50 kg (490 N) | 2.0 mV/V ± 0.15 | 5V - 12V DC | Luggage scales, bench inventory |
| HX711 Channel A | Gain = 128 | ±20 mV (Differential) | Internal ~4.2V | Primary high-sensitivity load cell |
| HX711 Channel B | Gain = 32 | ±80 mV (Differential) | Internal ~4.2V | Secondary sensor or high-output bridge |
Notice that Channel A and Channel B are not interchangeable backups. Channel A is hardwired to a gain of 128, making it perfect for the microvolt outputs of standard strain gauges. Channel B is fixed at a gain of 32, intended for a second sensor with a much higher output voltage, or for applications where the signal is already pre-amplified. According to the HX711 datasheet, attempting to read a standard 2mV/V load cell on Channel B will result in severe quantization noise because the signal won't span enough of the ADC's input range.
The Math: From Physical Force to Digital Counts
Abstract definitions don't help when you are staring at a serial monitor full of raw, fluctuating integers. Let's walk through a concrete numeric example to see exactly how physical kilograms translate into digital counts.
- Calculate Full-Scale Analog Output: The load cell datasheet specifies a rated output of 2.0 mV/V. If you power the HX711 with a clean 5V supply, the excitation voltage (E+ to E-) across the Wheatstone bridge is 5V. At maximum capacity (20kg), the differential output across the signal wires (A+ and A-) is: 2.0 mV/V × 5V = 10 mV.
- Apply the HX711 PGA Gain: Channel A amplifies this 10 mV signal by a factor of 128 before it hits the ADC. 10 mV × 128 = 1.28 V.
- Determine ADC Counts: The HX711 is a 24-bit ADC. Theoretically, 2^24 equals 16,777,216 total counts. Because it uses two's complement for bipolar measurements, the positive range is 8,388,608 counts. The HX711's internal reference voltage (AVDD) is typically around 4.2V. Our 1.28V amplified signal represents roughly 30% of the full-scale range.
- Calculate the Calibration Factor: If 20kg (20,000 grams) yields roughly 2,500,000 usable counts on your serial monitor, your calibration factor is 2,500,000 / 20,000 = 125 counts per gram. In your Arduino code, you divide the raw reading by 125 to get grams.
Where You Meet This in Practice (and What Goes Wrong)
You will encounter the load cell and HX711 combination anywhere physical mass needs to be logged or controlled by a microcontroller. Common applications include automated homebrewing systems (weighing mash tuns to calculate liquid volume), 3D printer filament spool monitors, and robotics gripper force-feedback loops. For a deeper dive into the underlying bridge theory, All About Circuits provides an excellent primer on Wheatstone bridges and how temperature affects strain gauge resistance.
Despite its popularity, builders frequently fall into three specific traps when deploying this hardware:
1. Confusing Resolution with Accuracy
People see '24-bit' and assume they can measure a 50kg load down to the milligram. Resolution is the size of the smallest step the ADC can detect; accuracy is how close that step is to reality. Thermal drift, load cell 'creep' (the gradual deformation of the metal under constant load), and EMI from nearby stepper motors will destroy your accuracy long before you hit the ADC's resolution limit.
2. The USB Power Trap
Powering the HX711 directly from an Arduino's USB 5V pin is a recipe for drifting readings. USB power from a PC is notoriously noisy, often carrying 50mV of high-frequency ripple. Because the HX711 uses the excitation voltage as its reference baseline, any noise on the 5V line is directly injected into your weight calculation. Always power the HX711 from a dedicated linear voltage regulator (like an LM7805 or AMS1117) or use a high-quality isolated DC-DC converter.
3. Wiring Length and Shielding
The signal between the load cell and the HX711 is an unamplified, high-impedance differential signal in the millivolt range. If you run 22 AWG unshielded wire for three feet alongside a 120V AC mains line or a PWM-driven motor cable, the wires will act as an antenna. Keep load cell wires under 12 inches, use shielded twisted-pair cable, and tie the shield to the HX711 ground at one end only to prevent ground loops.
Frequently Asked Questions
Why does my HX711 output negative numbers when I add weight?
This simply means your A+ and A- signal wires are swapped. The Wheatstone bridge outputs a differential voltage; reversing the sense wires inverts the polarity. You can either physically swap the two signal wires on the HX711 terminal block or multiply the final reading by -1 in your firmware.
Do I need to use the tare() function every time the microcontroller boots?
Yes, unless you have implemented non-volatile storage (like EEPROM) to save a zero-offset value. The HX711 does not remember its zero point across power cycles, and strain gauges are subject to thermal expansion. Taring at startup ensures your baseline accounts for the current ambient temperature and the empty weight of your platform.
Can I connect two load cells to a single HX711?
Yes, but not by using Channel A and Channel B simultaneously for the same weight platform. If you want to sum two load cells (e.g., a platform with one cell in each corner), you wire them in parallel (E+ to E+, E- to E-, A+ to A+, A- to A-) into Channel A. Ensure the combined resistance doesn't drop below the HX711's minimum drive capability (usually safe for two 1kΩ bridges in parallel yielding 500Ω).
My readings are jumping by 500+ grams. How do I fix it?
First, verify your mechanical mounting. The load cell must be clamped rigidly at one end, and the load must be applied to the free end. If the metal beam is touching the enclosure or resting on a soft foam pad, the mechanical binding will cause erratic, non-linear jumps. Second, check your sample rate. The HX711 can run at 10 SPS (samples per second) or 80 SPS. Pin 15 (RATE) dictates this; tie it to GND for 10 SPS, which provides better noise rejection for slow-moving weight measurements.
