Note: Try to complete tasks 4.1.2, 4.3.1, 4.4.1, 4.4.2, 4.5, and 4.6.1 before coming to the WorkINGLab so that you have more time for the experimental part.

If you want to print this description, the pdf export might not work properly, so you can download one here: (version Nov 28, 2025 )

1. Context of the Experiment

This experiment concerns resistive sensors and especially their usage in Wheatstone-bridges. Resistive sensors transmit information by a change in resistance due to external influences such as temperature, bending, light, etc. In the course of this experiment, you will learn, how to measure this change, even if it is very small, and how to compensate for some common errors.

2. Learning Goals of this Experiment

• Know: Wheatstone-bridge and why it is useful

• Abilities: choosing the correct setup for resistive sensors, handling Wheatstone-bridge/multimeter/(oscilloscope)/strain gauges/amplifiers

• Understand: how to compensate for certain errors

3. Literature

[1] Lectures: resistive sensors, electrical measurement technology

[2] Keil, Dehnungsmessstreifen. Wiesbaden: Springer Fachmedien Wiesbaden, 2017.

[3] P. L. Regtien, Hg., Sensors for Mechatronics // Sensors for mechatronics. Amsterdam: Elsevier, 2012.

[4] HBM-homepage: Wheatstone-bridge circuit (German webpage with correct table: https://www.hbm.com/de/7163/die-wheatstonesche-brueckenschaltung-kurz-erklaert/)

Necessary further reading/datasheets:

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4. Basics/Fundamentals

Wheatstone-Bridge

To measure small resistances or measure their value very exact, using a measurement bridge is reasonable. One common measurement bridge is the Wheatstone-bridge (fig. 1) that consists of two balanced voltage dividers. One to four unknown resistances are compared to (known) resistances, which leads to a much more exact measurement of the resistances than by purely measuring voltage and current.

Figure 1: Wheatstone-Bridge

Strain-Gauges

In this experiment, you will be using strain gauges in different measurement setups and bridges. A strain gauge basically consists of a metal measurement grid on top of a backing film that is covered by a protective sheet. The metal measurement grid consists of many turns, therefore, it changes its resistance once it is being stretched.

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Figure 2: strain gauge structure

Strain gauges change their resistance as a result of applied strain. The change in resistance in comparison to the nominal resistance equals the k-factor of the strain gauge multiplied by strain:

For calculating the mechanical stress applied, the law of hook with the mechanical stress σ, the elasticity module E and the strain ε can be used:

The Wheatstone bridge can be used in a full-, half- or quarter bridge-setup, using zero, two or three known fixed resistors respectively that have the same resistance as the strain gauges nominal resistance. Using the voltage divider, the following equation can be used for the Wheatstone-bridge

As strain gauges only have small changes in resistance, assume all strain gauges and all other resistors have the exact same nominal value R₁=R₂=R₃=R₄.

Due to the difference voltage being in the range of mV, amplification is necessary. The amplification factor

From these equations, you can obtain the necessary formula for the experiments tasks for all different bridge setups.

ADCs

In this experiment a HX711-sparkfun 24-bit Analog-to-digital-Converter (ADC) will be used. This device also amplifies the measured signal before converting to digital values. Please read the electrical measurement technology lecture for further information on ADCs.

5. Technical Basics & preparations

• All strain gauges have a nominal resistance of 350 Ohms and a k-factor of 2.

• Build your own Wheatstone bridge using the supplied resistors

  • amplification is necessary, use the sparkfun HX711

General instructions

• Always balance the Wheatstone-bridge anew after you changed the setup!

• The supply voltage is set by the HX711.

• For different bridge setups, it might be necessary to consider the strain you want to measure and its direction/the direction in which the strain gauges are placed in your formula! Superposition of factors might happen.

• Always make sure, the Arduino is set to the correct baud-rate and the port is correctly chosen.

Preparations:

• Gather all the necessary measurement objects and instruments:

  • Sparkfun HX711

  • 350 Ohm resistors

  • Breadboard

  • Cables to connect

  • Arduino Uno & Cable

• Install the Arduino IDE and the packages and download this program:
  
  

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Troubleshooting:

In case you find your sensor reading always a raw value of ~16 mio or 0, check if one of the following errors might have occurred:

• Are all the resistors and cables well-connected? There might be a loose contact.

• Is your USB-cable plugged in? Does your Arduino have power?

• Did you connect the correct Arduino-pins?

• Check with the multimeter, if the Arduino actually outputs 5V.

• Check if your cables are connected to the luster clamp and the strain gauge.

• Do the Breadboard-interconnections work?