Step 12.4, Home built avionics

Just when it looks like I might be making some actual progress on this project, I find a new way to slow it down.  One might reasonably assume that I've missed my calling;  That with a skill such as this, I could have been a mid-level manager.  Perhaps.  But I digress...

The most recent impediment to my project's completion is the idea that I should make some of my own avionics.  I'm not talking about anything big, like a radio or auto pilot.  Something really small, I tell myself, should be no trouble at all.  Like an annunciator panel or relay deck.  How hard could it really be?  They're so small.

With this truly terrible idea rattling about in the back of my consciousness, I then compounded the injury to my project by inventing several new solutions that have yet to be matched with actual problems. In all, I've identified 5 circuit boards to make.

The screen on the right shows the circuit layout underway for a few of them.  Clockwise from the top:  annunciator panel, relay deck, and two copies of the control column serializer.  The screen on the left is my instrument panel which was also being designed at the same time.

Now without further adieu.  The five boards my project can not live with out are:

Annunciator Panel:  This is a set of lights that signal changes in various conditions within the aircraft.  There are six conditions monitored by my annunciator: Master warning,  Master caution, Low oil pressure, Low voltage, Canopy not locked, Auxiliary fuel pump on.  The six lights are also push-buttons that are monitored by a microprocessor whose principal responsibility is watching the input signals and updating the status of the lights -- on, off, or flash.  The big advantage of having a microprocessor do all of this is that it is more complicated.

There are some people who think that 8 indicators are the absolute minimum for an annunciator panel, but I've settled for only six because somehow six seems less ostentatious.

WIG/WAG Controller:  A potential problem at an uncontrolled airport is an aircraft or other vehicle pulling out onto the runway when you are about to land.  One way to reduce the likelihood of this occurring is to increase your visibility from the ground.  The wig/wag controller does this by alternately flashing the taxi and landing lights.  A microprocessor interprets the state of three switches from the instrument panel, Landing light, Taxi light and Wig/Wag enable and then sets its outputs appropriately to control the landing and taxi lights.  

Relay Deck: A relay deck is just a collection of relays.  My implementation of a relay deck contains 10 relays and, --- wait for it --- a microprocessor.  Relays such as these are typically used to isolate low current instrument panel switches from higher powered devices like motors. In my case, I need the relays for the trim motors, flap motor, and the push to talk switch.  In addition, I wanted to reduce the size of the wire bundle going down through the control columns by serializing the data emanating from the stick mounted switches.  The serial data input function (on a relay deck) is not available commercially as far as I am aware.  My relay deck has three data channels for input.  One serial channel each for the pilot and passenger, and one parallel channel that permits panel mounted switches to get in on the act as well.  All of this allows me more flexibility on how and when the relays should be activated and by whom.

Control Column Serializer:  This board reads the control column switches that are mounted within the grips and converts this data to a serial data stream going to the relay deck.  The data are packetized and a CRC is computed to validate the data's integrity at the receiving end.

Fuel Capacity Transducer: This board is required to translate the fuel tank's quantitative transducer, which is of the capacitive type, to an analog voltage that the Garmin EFIS (Electronic Flight Information System) can then use to then display the fuel quantity on the PFD (Primary Flight Display).

Pictured below are the 5 bare boards listed above, plus a sixth (bonus) board at right that doesn't do anything -- well, it doesn't seem to do a lot because it simply translates some signals from the Honeywell AML34 instrument panel switches. (how useful could it really be without a microprocessor?)

I've always felt that any problem worth fixing can be made more complex by adding a computer to it's solution.  It's a kind of occupational hazard for me.  I just can't help myself.

At the core of each of these boards are 8 bit micro controllers of various types.  Having a computer right there surely invites feature creep which is the enemy of CPS (Completed Plane Syndrome), so I'll have to watch that.  I've already added the ability for a couple of these boards to listen to the Garmin EFIS RS-232 serial data stream.  Whether or not I'll do anything with that data source remains to be seen. At the very least, it will be something I can play with after I finish the plane.

At this point I've built the 5 boards and have tested 4 of them.  A lot more software will have to be written before they are finished, but at this point I'm just trying to verify the correctness of the hardware before I move on.  I'll have more to say about them in upcoming posts.  By then I will have had a chance to learn some new acronyms.  TTFN.