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Strain Gauge Circuit Diagram: Wheatstone Bridge Wiring

#electronics #diy #tutorial

A strain gauge is a resistive sensor whose resistance changes in proportion to mechanical deformation. The change is small -- a typical metal foil gauge has a gauge factor of about 2, meaning a 0.1% strain produces a 0.2% change in resistance. For a 120Ω gauge, 0.2% is 0.24Ω. To measure a 0.24Ω change on a 120Ω baseline you cannot just connect it to a voltage divider and call it done. You need a Wheatstone bridge.

Why a Wheatstone Bridge?

A Wheatstone bridge has four resistive legs. At balance, the differential voltage across the bridge midpoints is zero. When one (or more) legs change value -- like a strain gauge under load -- the bridge goes out of balance and a small differential voltage appears. This differential voltage is proportional to the resistance change.

The advantage over a simple voltage divider: the bridge rejects the large common-mode component (the DC excitation voltage) and amplifies only the difference. A precision instrumentation amplifier amplifies this differential signal to a readable level.

Quarter Bridge, Half Bridge, and Full Bridge

The Wheatstone bridge has four arms. Replacing one, two, or four of those arms with active gauges determines the bridge type.

Quarter Bridge

One active gauge (R_sg), three fixed precision resistors (all equal to gauge nominal resistance):

         +Vex
          |
    R3 ──┤── R_sg
          |        |
         Vout+    Vout-
          |        |
    R4 ──┤── R2
          |
         GND
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The output voltage at small strains:
Vout ≈ (Vex / 4) × (ΔR / R)

For Vex = 5V, ΔR/R = 0.002 (0.2% change): Vout ≈ 5V/4 × 0.002 = 2.5mV

This is the signal level you are dealing with -- millivolts. A gain of 100--500 is needed before feeding an ADC.

Quarter bridge is the simplest to wire and is fine for measuring bending or axial strain where temperature compensation is not critical. The three fixed resistors should be matched metal-film types (0.1% tolerance or better) to keep the bridge balanced at zero strain.

Half Bridge

Two active gauges in adjacent arms of the bridge. The typical arrangement puts one gauge in tension and one in compression (as in a bending beam), which doubles the output compared to a quarter bridge and provides inherent temperature compensation -- both gauges see the same temperature change, which cancels out.

Vout ≈ (Vex / 2) × (ΔR / R)

For the same 0.2% change: Vout ≈ 5V/2 × 0.002 = 5.0mV

This arrangement is common in commercial load cells and torque sensors.

Full Bridge (Wheatstone Bridge with Four Active Gauges)

All four arms are active gauges. Two in tension, two in compression, arranged so opposing gauges are in opposite arms.

Vout ≈ Vex × (ΔR / R)

For 0.2% change: Vout ≈ 5V × 0.002 = 10mV

Full bridge configurations are used in precision force and weight sensors. Virtually all commercial load cells -- the kind you find in kitchen scales and industrial weigh platforms -- use a full bridge inside a machined aluminum or steel body.

Load Cell Color Codes

Commercial load cells typically use a 4-wire system:

Wire Color Connection
Red +Vex (excitation positive)
Black -Vex / GND (excitation negative)
White Signal + (Vout+)
Green Signal - (Vout-)

Some manufacturers use Red/Black/White/Blue. A few use a 6-wire configuration with additional Sense+ and Sense- wires that allow the amplifier to compensate for excitation voltage drops in long cable runs (Kelvin sensing). When a 6-wire load cell feeds a 4-wire input, tie Sense+ to Exc+ and Sense- to Exc-.

Excitation Voltage

The bridge needs a stable, clean excitation voltage. Noise on the excitation supply directly appears as noise on the output signal.

Typical excitation voltages:

  • 5V: Common when the system runs from USB or a 5V regulated supply. Works well with the HX711.
  • 3.3V: Lower output signal but fine for short-range analog or when the ADC reference is also 3.3V.
  • 10V: Higher signal-to-noise ratio; used in precision industrial systems.

Use a precision voltage reference (LM4040, REF3025) or the regulated supply from a low-noise LDO rather than a raw microcontroller VCC pin, which carries switching noise from the digital circuitry.

Amplifying the Signal: HX711 vs. INA125

HX711

The HX711 is the most popular load-cell amplifier for maker/hobbyist use. It combines a precision instrumentation amplifier with a 24-bit delta-sigma ADC, communicates over a simple 2-wire serial interface (CLK/DOUT), and includes an internal voltage reference.

Wiring a load cell to an HX711:

  • Red (Exc+) → E+ pin
  • Black (Exc-) → E- pin
  • White (Sig+) → A+ pin
  • Green (Sig-) → A- pin
  • VCC: 2.7--5.5V (from 3.3V or 5V microcontroller supply)
  • GND: Common ground
  • CLK, DOUT → Arduino digital pins

Gain is set by channel selection: channel A at gain 128 (default), channel A at gain 64, or channel B at gain 32. At gain 128 with 5V excitation, the full-scale input range is ±20mV -- fine for most load cells.

Arduino library: SparkFun's HX711 library or bogde's HX711 library both work well.

INA125 / INA128

For analog output (to feed a microcontroller ADC directly), an instrumentation amplifier like the INA125 (includes internal voltage reference and bridge excitation) or INA128 is a cleaner approach.

Gain is set by a single external resistor:
G = 1 + (49.4kΩ / Rg)

For G = 500: Rg = 49.4kΩ / (500-1) ≈ 99Ω → use 100Ω.

Output voltage at full load: Vout = G × Vbridge = 500 × 10mV = 5V (matches 5V ADC reference).

Noise and Layout Considerations

At gain 128, the HX711 amplifies everything 128 times -- including 50/60Hz interference, power supply noise, and radio-frequency pickup.

Key practices:

  • Run the four bridge wires together (twisted pair or flat cable with shield)
  • Keep the amplifier board physically close to the load cell
  • Use short wires from the load cell to the amplifier; long runs pick up noise
  • Decouple the HX711 VCC pin with a 100nF capacitor as close to the pin as possible
  • Average multiple readings in firmware -- 10 reads averaged out reduces random noise effectively

Simulating a Strain Gauge Circuit

Sketch the Wheatstone bridge in CircuitDiagramMaker with the four resistors and an instrumentation amplifier symbol. Set one resistor to R+ΔR to simulate the loaded condition and run DC analysis. The simulator will show the exact differential voltage across the bridge midpoints, which lets you confirm the amplifier gain setting before ordering parts.

Create Your Own Strain Gauge Circuit Diagram

  • Place the four Wheatstone bridge arms, labeling the active gauge(s)
  • Add the excitation voltage source and instrumentation amplifier
  • Show the HX711 or INA125 connections with pin labels
  • Document wire colors from the load cell connector
  • Export as a reference for assembly and troubleshooting

Create your own strain gauge circuit diagram -- free

Key Takeaways

  • Strain gauges produce tiny resistance changes (fractions of an ohm); a Wheatstone bridge converts this to a differential millivolt signal.
  • Quarter bridge uses one active gauge; half bridge uses two; full bridge uses four. More active gauges = higher output and better temperature compensation.
  • Commercial load cells are full bridges with standard color coding: Red = Exc+, Black = Exc-, White = Sig+, Green = Sig-.
  • The HX711 is the standard low-cost solution -- 24-bit ADC plus instrumentation amp in one chip, communicates over a simple 2-wire serial protocol.
  • Use a clean, stable excitation voltage; noise on the excitation supply appears amplified in the output.
  • Layout matters: keep wiring short, keep bridge wires together, and add firmware averaging to reduce noise.

Originally published at https://circuitdiagrammaker.app/blog/strain-gauge-circuit-diagram.

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