Load Cell / Strain Gauge
Sensor
A machined metal beam with strain gauges glued to it: load bends the beam microscopically, unbalancing a Wheatstone bridge by microvolts. An HX711 amplifier turns that into weight readings.
What it looks like
Load Cell / Strain Gauge
A typical sensor — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Rated capacity
kg1/5/10/20 kg bars are common hobby sizes.
Sensitivity
mV/VFull-scale bridge output per volt of excitation, typically 1–2 mV/V.
Accuracy class
%Combined error as % of full scale.
Bridge resistance
Ω350 Ω or 1 kΩ typical — used to sanity-check wiring.
Markings
Capacity and wiring key on a sticker: red/black = excitation (E+/E−), green/white = signal (S+/S−).
Standard values
5 kg and 10 kg bar cells with HX711 breakouts dominate hobby scales.
How to choose
Capacity ~1.5–2× max expected load (headroom for shocks), bar type for platform scales, S-type for hanging loads. Mount exactly as the arrow indicates — load direction matters.
Pinout & package
4 wires to the bridge (some add shield). Bar cells mount cantilevered between two plates with spacers.
Example circuits
- Kitchen scale: bar cell + HX711 + display
- Beehive weight logger
- Force gauge for a test rig
Common failures
Overload permanently bends the beam (zero shifts, never reads right again), moisture ingress into the gauge epoxy causes drift, broken flexing wires.
How to test
Resistance: E+ to E− and S+ to S− ≈ bridge resistance (350 Ω/1 kΩ); each input-to-output pin ≈ 75% of it. With HX711 running, readings should track known weights linearly.
Substitutes
Another cell of the same capacity and mV/V recalibrates in software; FSR pads for crude force sensing where accuracy doesn't matter.