Precision Thermocouple Measurement and Logging for Lab and Science Experiments

Introduction

Accurate temperature monitoring is essential for any laboratory—whether in a home workshop, a school science class, or a research facility. Thermocouple sensors are the standard for measuring wide temperature ranges, but interfacing them with low‑cost single‑board computers can be challenging. This guide demonstrates how to connect a high‑resolution ADS1118 thermocouple amplifier to a Raspberry Pi, enabling precise, fast, and reliable temperature logging at a fraction of the cost of commercial systems.

Design Overview

Many hobbyists default to the MAX31855, but the Texas Instruments ADS1118 offers 16‑bit resolution, dual analog inputs, and built‑in cold‑junction compensation. A ready‑made ADS1118 development board—including an LCD display and thermocouple connector—is available for under $20 and can be mounted directly on a Raspberry Pi via SPI. If you prefer a custom build, a small adapter board or perforated PCB can be fabricated using standard 10‑way SIL headers; the board layout files are provided in the project archive.

Hardware Setup

1. Connect the ADS1118 board to the Pi’s 40‑pin header:

• VCC (ADS1118) → 3.3 V (Pin 1)
• GND → GND (Pin 9)
• CLK → SPI SCLK (Pin 23)
• ADS_CS → SPI CE1 (Pin 26)
• LCD_CS → SPI CE0 (Pin 24)
• LCD_RS → GPIO 17 (Pin 11)
• SIMO → MOSI (Pin 19)
• SOMI → MISO (Pin 21)

2. Enable the Pi’s SPI, I²C, and SSH interfaces via raspi-config or the Raspberry Pi Imager.

3. Optionally, secure the board with a lightweight epoxy such as Araldite, leaving enough room for future soldering if needed.

Software Overview

The firmware is split into three main files: therm.c (core driver), lcd.c (LCD handling), and main.c (application logic). The driver uses TI’s ADS1118 library, adapted for the Pi’s SPI implementation. Cold‑junction compensation is handled automatically by reading the internal temperature sensor; the raw ADC value is then converted to degrees Celsius using the thermocouple lookup tables supplied by the manufacturer.

Compiling and Running

mkdir -p ~/development/therm
cd ~/development/therm
cp ~/Downloads/therm.zip .
unzip therm.zip
gcc -o therm therm.c -lrt
sudo ./therm

Run the program with sudo or as the root user. The application samples the internal sensor once per second and the thermocouple ten times in rapid succession, averages the readings, and outputs a 0.1 °C resolution measurement. The LCD displays the current temperature on the bottom line and a user‑defined experiment label on the top line.

Web Interface

The project also includes a lightweight HTTP server that serves real‑time temperature data and allows remote monitoring via any web browser. Visit https://pi‑address:8000 to view live readings, historical logs, and configuration options.

Best Practices and Troubleshooting

• Use a dedicated 3.3 V regulator if you anticipate high current draw to keep the sensor isolated from the Pi’s noisy power rail.
• Keep the thermocouple leads short and shielded; longer runs introduce noise and thermal lag.
• Verify that the Pi’s SPI clock speed is set to 1 MHz or lower for optimal ADS1118 performance.
• When assembling on a perf‑board, place the ADS1118 and the Pi adapter side‑by‑side rather than stacked to reduce thermal cross‑talk.

Conclusion

This system delivers high‑accuracy, low‑cost temperature logging suitable for educational labs, hobbyist projects, and research experiments. Its modular design, open‑source code, and straightforward hardware make it an ideal starting point for anyone looking to integrate precise thermocouple measurements into a Raspberry Pi platform.