Interfacing a Course Deviation Indicator

I try to catch up with all my projects ... eventually. Good news for everyone trying to get their dust-collecting old instruments to life. After using the ESP32 for some work-related projects I was confident that it is the perfect platform for this - cheap, performant, and not affected too badly by the ongoing shortages.

So there we are, I got the VOR signal generation running, even wrote a small plugin to integrate with X-Plane. You can find the alpha code for the ESP32 on GitHub. More details and the plugin following shortly. Also, small extension of the project scope, I will also implement localizer and glideslope signal generation, so that you can really get the full functionality of the CDI.

The first step of this project is to figure out how the instrument works in his native environment. Luckily, avionics tend to be documented really well. So I started out searching for an installation manual that tells how its usually wired up in the aircraft. This is the pinout diagram I found there:

The Instrument is supplied by a voltage of 11 to 32 VDC between Pin b and v. This is to accommodate for both 14 V and 28 V power buses used in different aircraft.

The instrument lighting consists of two lamps in series between Pin B and E with a center-tap on Pin D. So depending on the system voltage, you can put +14 V on Pin D and ground the Pins B and E, or connect +28 V between Pin B and E.

Pin K is called ILS energize. Pulling it to ground will switch the instrument from accepting a VOR input signal to accepting a ILS input signal. 

The actual control signal comes in from Pin Y - Composite. This signal comes out of the navigation receiver of the radio and is just a raw AM demodulated base band signal. 

In the next post, I'll describe how this composite signal is formed by the real system, how it can be described mathematically, and how I first simulated it using MATLAB and my PC sound card.