Glen Akins' Raspberry Pi Pico-Powered Selsyn Adapter Brings a Vintage Aircraft Display Back to Life

Maker Glen Akins has been working on another upcycled aircraft display, this time converting a World War II-era landing gear and flaps indicator to USB for use with a modern computer — thanks to a Raspberry Pi Pico adapter.

"This specific meter is used to indicate the position of the landing gear on an aircraft with nose and wing gear, whether the landing gear are locked in place, and the position of the flaps that can be extended from the trailing edges of the wings," Akins explains of the object of his attention. "The flaps are used during take offs and landing to produce more lift at slower speeds. The flaps are then raised during normal flight to reduce drag and increase efficiency at higher speeds."

The meter, however, isn't the easiest thing to integrate into modern devices. Built by General Electric for use in World War II-era aircraft, the display uses Selsyn technology — a form of synchro, technology first developed in the early 1900s and deployed in the control desk of the Panama Canal. A synchro looks like a motor, but is typically deployed as a single transmitter typically controlling multiple receivers connected to dials. As an analog technology, though, it needs a special interface to bridge it to a digital device — which is where Akins' project comes in.

"The dials on the indicator could be driven using PWM [Pulse-Width Modulation] or programmable current sinks. In both cases, the current through each of the indicator’s dials' coils needs to be limited to a maximum of about 50mA," Akins explains. "If PWM is used, current limiting resistors are required to limit the current. This is similar to the 290Ω current limiting resistor in the transmitter. If programmable current sinks are used, the current is limited by the design of the current sink. I used programmable current sinks for this project."

To start, Akins designed a custom current sink board with four channels — enough to control two dials on the indicator at the same time, meaning a full control implementation would need two of the boards. For the USB interface, Akins turned to the popular and low-cost Raspberry Pi Pico development board and its dual-core RP2040 microcontroller. The Raspberry Pi Pico is hosted on a custom carrier board, with all power coming from the board's micro-USB port.

The resulting three-board stack runs a custom firmware written in the RP2040 C software development kit (SDK), which communicates with a C# .NET application running on a Windows host — providing an easy way to switch the meter on and off, raise or lower the landing gear indicator, provides analog control of the flaps position indicator, and offers the ability to deliberately trigger fault displays on the nose, left wing, and right wing.

To streamline the project, Akins designed a final board that puts all eight current sink channels plus the Raspberry Pi Pico onto a single PCB — but warns that the design is untested. "I'm highly unlikely to build a board, buy parts, and assemble them myself," Akins notes.

This isn't the first piece of aircraft instrumentation Akins has turned into a USB peripheral: late last month Akins showed off a custom-built adapter board, based around a Microchip PIC16F1459 and four MCP31HV41-502 digital potentiometers, designed to interface with a General Electric model 8DJ4PBV DC Selsyn indicator originally used to monitor cowl flap positions on a World War II-era aircraft. Prior to that, Akins had been working on a Python-based decoder for a more modern synchro-based altitude warning monitor.

Akins has pledged to make the design files and source code for the project available on his GitHub repository in the near future, though at the time of writing the files had not yet been made public; more information is available on Akins' website.