The avioncs of the beech baron 58 aircraft is a set of several units and is based on a production series of the US company Bendix / King, widely used in numerous aircrafts. To build this unit, the original design had to be modified in some areas because of the following reasons:
- The 7-segment LED display elements shall have a height of 10 mm.
- The dimensions of the complete avionics box shall not exceed a height of 250 mm and a width of 150 mm to fit into the middle part of the main panel.
- It was therefore decided to combine the ADF, DME and the transponder elements in one physical unit.
- The push buttons, LEDs and the turn knobs shall be positioned close together to allow for a single electronics board for each unit.
The result of these considerations is shown in the below right picture, which was used as a design template for the manufacturing phase.
The ready to use avionics design is shown in the right picture and shows the following features:
- The mechanical parts of the panels are based on white rigid FOREX foam. This includes the panel frames and the push buttons. The panels are sprayed with black dull paint and the rims of the panels were finally sanded to be 're-coloured' from black to white. This manufacturing idea was taken from www.simvim.com/b58.
- To set the numerical content of the 7-segment LED displays (frequencies, altitude or vertical speed), rotation encoders with a push button feature are used. The yellow LEDs indicate, whether we are setting the whole or the fractional part of the frequency values. Switching between both is accomplished by pushing the rotation encoder knob.
- For the lettering I used permanent adhesive paper and printed it with white text on black background.
- The electrical design features 5 micro-controllers of type ATmega32, one for each unit, because only these have enough input/output ports to control all the buttons, the yellow LEDs, the rotation encoders and the 7-segment LED digits.
- The electrical board design and layout for these controllers were made by the EAGLE PCB tool.
- These five controllers (also called sub-controllers) are driven by one main-controller of type ATmega8. The interface between the main-controller and any sub-controller is based on a Two-Wire-Interface (TWI) protocol. See right bottom picture. The controllers are running with a clock frequency of 10 MHz, which is sufficient to allow an uninterrupted processing of all commands from the pilot and from FSX.
- To drive the 7-segment LED displays, I used the traditional MAX7219 IC. Although this IC can drive up to 8 digits, only 5 of them are used. The drawback of these ICs is that they are relatively expensive and I needed 14 of them. The brightness of the LED displays will automatically be set by FSX, depending on a day, dawn-dusk or night flight. In addition, it can be set individually by the pilot to three different levels.
- The displays of ADF, DME, tranponder and autopilot can be individually switched on or off, as needed.
- Pressing the TEST button of a unit will light-up all segments of the LED digits. This allows the pilot to verify, that the display is correctly working and the correct values are shown.
- Pressing the TEST button and then turning either the COMM or NAV knob will increase or decrease the illumination strenght of all 7-segment LED displays.
- The plexiglas covers of the display cut-outs are backed by a dark printed foil. This makes the 7-segment LED elements almost invisible when they are not turned on.
- The entire unit is driven by a single 5V stabilised power supply, serving the micro-controller and the 7-segment LED displays. Although the MAX7219 is working in the interleave mode when driving the displays, there are no significant voltage peaks on the power supply line.
- Only the main-controller is connected to the PC via a serial RS-232 interface line.
- The software for the main and sub-controllers was written in C using the Atmel Studio 6.
The picture below shows as an example the COMM / NAV electronics schematics. It is built around an ATmega32 micro-processor. The connector JP2 serves the four 7-segment LED displays, while the connectors JP3 and JP4 serve the COMM and NAV panel part controls, consisting of push buttons, LEDs and a rotation encoder. The connector SV1 is the interface to the main-controller. There is an additional board, where the four 7-segment MAX7219 LED drivers are located. This board is connected via connector JP2. The electronics schematics of the other units are nearly identical and are not repeated here.
Although it took about 800 hours to build this unit and to develop the software, it was well worth it because this avionics unit offers more features than the virtual FSX unit and the resulting appearance of the entire panel is very impressive. The complete documentation of the avionics is described here.