Pencil Graphite Strain Gauges on Paper

ISEF Category: Materials Science

Subcategory: Electronic, Optical, and Magnetic Materials  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

A pencil line can act like a tiny sensor. When the paper bends, the graphite network shifts and the resistance changes. That means a doodle can become a strain gauge. Your job is to find out which pencil hardness makes the best one.

What Is It?

A strain gauge is a sensor that changes its electrical resistance when it gets stretched or compressed. In this project, you make that sensor from pencil graphite on paper. Graphite conducts electricity because its carbon particles form a connected path. When the paper bends, that path changes, and the resistance changes too.

Think of the graphite layer like a sidewalk made of tiny stepping stones. A soft pencil, like 9B, leaves more graphite on the paper than a hard pencil like HB. More graphite can mean a thicker, messier path for current, but it can also make the sensor less stable. You can test which hardness gives the biggest and most repeatable signal.

An Arduino Wheatstone bridge helps you measure those small resistance changes. A Wheatstone bridge is a circuit that turns tiny resistance shifts into a voltage signal you can record. That lets you compare pencil grades with real numbers instead of guessing by eye.

Why This Is a Good Topic

This is a strong science fair topic because you can test a clear variable, pencil hardness, and measure a real output, gauge factor. You also connect to a real problem, low-cost sensing for wearables, robotics, and structural monitoring. You can learn about circuits, calibration, repeatability, and data analysis without needing a university lab.

Research Questions

• How does pencil hardness from HB to 9B affect the gauge factor of a graphite-on-paper strain sensor?
• What is the effect of paper type on the sensitivity and noise of a pencil-graphite strain gauge?
• Does the number of graphite layers change the repeatability of resistance change under bending?
• To what extent does drawing direction relative to the paper fiber direction affect sensor response?
• Which pencil hardness gives the best balance of sensitivity and signal stability?
• How does sensor length affect the linearity of the resistance change during bending?

Basic Materials

• Assorted pencils from HB through 9B.
• Smooth printer paper, cardstock, and notebook paper.
• Ruler or straightedge.
• Digital multimeter.
• Arduino board.
• Breadboard.
• Fixed resistors for a Wheatstone bridge.
• Jumper wires.
• Alligator clips.
• Tape.
• Binder clips or a simple bending fixture.
• Laptop for data logging.
• Smartphone camera for documenting samples.

Advanced Materials

• Assorted pencils from HB through 9B.
• Paper substrates with known fiber direction.
• Precision scale for graphite mass comparison.
• Digital multimeter with higher resolution.
• Arduino or microcontroller with an external analog-to-digital converter.
• Instrumentation amplifier.
• High-stability reference resistors.
• Mechanical bending stage or cantilever fixture.
• Force gauge or load cell.
• Oscilloscope or data acquisition system.
• Environmental sensor for humidity and temperature.
• Conductive adhesive or silver paint for stable contacts.

Software & Tools

• Arduino IDE: Programs the microcontroller and reads bridge voltage during bending tests.
• Excel or Google Sheets: Organizes measurements, builds graphs, and compares gauge factors.
• ImageJ: Measures line width, coverage, and drawing consistency from sample photos.
• Python: Fits calibration curves, calculates repeatability, and tests statistical differences between pencil grades.
• GeoGebra: Helps you visualize linear and nonlinear trends in your sensor data.

Experiment Steps

1. Define the one sensor variable you will change first, such as pencil hardness, while holding the paper, geometry, and bending setup constant.
2. Design a repeatable graphite pattern and decide how you will measure its resistance before and during strain.
3. Build a calibration plan that turns bridge voltage into relative resistance change and then into gauge factor.
4. Set up controls that separate real strain response from contact noise, humidity drift, and drawing inconsistency.
5. Plan how you will compare multiple samples per pencil grade so you can test repeatability, not just one lucky sensor.
6. Choose the graphs and statistics you need before you collect data, so your final analysis matches your research question.

Common Pitfalls

• Drawing graphite lines with uneven pressure, which changes resistance more than the strain itself.
• Letting clip contacts slide on the paper, which creates fake signal spikes during bending.
• Testing different pencil grades with different line widths, which makes hardness comparisons invalid.
• Ignoring humidity, which can change paper resistance and hide the graphite effect.
• Using only one sample per pencil grade, which makes random noise look like a real trend.

What Makes This Competitive

A class-level version just compares pencil grades and graphs resistance change. A stronger project goes farther. You can quantify repeatability across many samples, correct for humidity, and report gauge factor with uncertainty. You can also compare paper type, drawing direction, or layer thickness, then use statistics to show which factor matters most.

Project Variations

• Test how cardstock, printer paper, and recycled paper change the same graphite sensor response.
• Compare straight graphite lines with crosshatched patterns to see whether texture improves stability.
• Swap the bending test for a touch or pressure test, then see whether the same pencil grades act differently under compression.

Learn More

• MIT OpenCourseWare: Search for introductory circuits and instrumentation lectures to learn Wheatstone bridges and sensor readout basics.
• NASA NTRS: Search for papers on flexible sensors, strain gauges, and piezoresistive materials in aerospace monitoring.
• PubMed: Search for review articles on paper-based sensors and carbon-based piezoresistive materials.
• Google Scholar: Search for studies on pencil graphite strain sensors, paper electronics, and gauge factor comparisons.
• Sensors journal: Search the journal site for peer-reviewed papers on flexible and low-cost strain sensors.