[Glen Akins] had a WW2-era aircraft engine cowl flap indicator lying around (as you do) and thought it would make a jolly fine USB-attached indicator. The model in question is a General Electric model 8DJ4PBV DC Selsyn, which was intended for four-engined aircraft. For those not familiar with the purpose [Glen] explains in his detailed writeup, that piston-engine aircraft of that era were air-cooled, and during conditions of maximum engine power — such as during take-off — flaps on the side of the engine cowling could be opened to admit additional cooling airflow. These indicator dials were connected to a sender unit on each of the flap actuators, providing the pilots an indication of the flaps’ positions.
The mode of operation in the DC power environment of WW2-era aircraft utilised the concept of variable magnetic field orientation. The sender is a potentiometer, sending a voltage down the wire between 24V and ground. The indicator unit has a pair of coils set at 120 degrees around a ring, with the coils wired in series, and
To interface this thing to modern technology, a custom PCB was constructed leveraging the USB functionality of the PIC16F1459 microcontroller, that [Glen] was already familiar with. Four Microchip MCP31HV41-502 digital potentiometers were pressed into service directly driving the coils of the indicator units. That might seem like an odd if not viable way to drive such a thing, but [Glen] goes into some extensive theory and some modeling to determine which devices would have sufficient margin, which is worth a read for the unfamiliar. After bit-banging the SPI connection to the digipots (even though the PIC has hardware SPI) [Glen] goes on to describe how the USB endpoints work, finishing off with a .NET application to drive it all.
We’ve seen plenty of hacks bringing retro hardware back to life, connecting to modern computing. Here’s a project that goes the other way, building custom aircraft instrumentation from modern parts.
