In this episode, Shahriar explores a large Stirling cryocooler and its custom controller from a medical-grade ultra-low-temperature freezer:

https://www.stirlingultracold.com/refrigerators-freezers/

The internal components were kindly provided by a viewer. The original LCD interface is designed for freezer operation and offers limited control over the engine. The goal is to build a new controller and explore temperatures far below the freezer’s intended operating range.

Reverse engineering begins with the Debian-based onboard computer and its executable software. Analysis of these programs, combined with captured serial communications, reveals the protocol between the main controller and the engine control board.

New interface hardware is designed to read the temperature sensors and collect telemetry from the engine controller. Custom firmware provides control over the engine’s operating parameters, with real-time safeguards to enforce operating limits. It also incorporates a PID temperature-control loop and a web server for visualizing data and adjusting settings.

With the new controller, the cold head reaches approximately −160°C which is far below its temperature during normal freezer operation.

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