The Pax Instruments T400 datalogger is an open source four-channel thermocouple temperature datalogger based on the Arduino™ Leonardo platform. It is ready to use out of the box with the features you want most. Measurements can be logged to MicoSD card, printed to serial port, and graphed. The T400 is a great tool for anything from live thermal process monitoring in the lab to long-term environmental data collection in the field.

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Professional design

The Pax Instruments T400 datalogger is designed to be out of the box ready for professionals and hobbyists alike. If you need a temperature datalogger that works every time, this is the device for you.

Open source spirit

The hardware and software design files are available to you at no cost to use, modify, or redistribute. This allows you and others to extend the devices capabilities or tailor it to your specific application.

Arduino™ Compatible

Arduino™-compatible hardware means while hacking on the platform you will be able leverage the work of others while sharing your own work with large community of hackers and makers. Sharing is caring.


MicroSD slot

Readings can be saved to a microSD card in standard CSV format for processing in Microsoft Excel, LibreOffice, or your favorite data analysis tool.

USB serial port

Readings can captured live via the USB serial port. This is perfect for live process monitoring in lab experiments or connecting to an internet-enabled device.

Mini-TC connectors

Thermocouples connect via standard mini thermocouple connectors. The T400 is compatible with a wide variety of K-type thermocouple sensor types from stainless steel probes to rolling surface-contact sensors.

Ambient temperature

The MCP9800 temperature sensor is used for cold junction compensation.


The MCP3424 analog-to-digital converter measures the voltage produced by each thermocouple.

Real-time clock

The DS3231 real time clock is used to trigger readings. Between readings the device is put into low power sleep mode. The RTC wakes up the unit to take a reading. This gives a longer battery life.

Li-Po battery

The T400 uses a standard BL-5C battery. This is great for battery replacement in the field.


The generous 132×64 LCD is capable of displaying the current temperature for each thermocouple as well as a graph of the most recent readings.

AVR processor

The T400 runs on the ATmega32u4 AVR processor.


NOTE: Source files are being migrated from the original private repository to the repositories below.
T400-v0.7 schematic
Electronics on Github
Firmware on Github
Enclosure on Github

Additional resources
T400 on Hackaday.io
T400 on Thingiverse

ATmega32U4 – Microcontroller
DS3231 – Real-time-clock
MCP3424 – Analog-to-digital-converter
MCP9800 – Ambient temperature sensor
MCP73831 – LiPo battery charge controller
MIC5219 – 3.3 V regulator
PGB1010603 – ESD suppressor


Q: Why does the board operate at 3.3 V?
A: So the board can be run from the 3.7 V LiPo battery without a step-up converter. The LCD operates on 3.3V. Lots of nice sensors operate at 3.3V.

Q: Why does this board operate at 8 MHz rather than 16 MHz like other Arduinos?
A: 8 MHz is the highest clock frequency available when operating the ATmega32U4 at 3.3 V. The Arduino LilyPad and several other boards also operate at 8 MHz.

Q: This is an opensource design; why not use KiCAD?
A: KiCAD would be awesome, but there were already lots of good Eagle libraries for the components.