[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 the center tap connected to the sender signal. The other ends of each coil connect to the DC power bus so that as the signal voltage varies, the coils produce a varying magnetic field. Lower voltages bias the field towards the coil connected to 24V, and higher voltages the other way. A permanent magnet in the center is attached to the indicator dial, with a small spring to bias it to the center. A very simple but effective arrangement, giving analog feedback of the actual flap position.
To interface this thing to modern technology, a custom PCB was constructed leveraging the USB functionality of the PIC16F1459 m ..
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