Electrical Design

As already described in the overview, I took a used yoke and disassembled it to finally only have the front and back-frame of the plastic horn including the switches, push buttons and associated cabling, as shown in the right picture. Before continuing with the electrical setup, both frames were sanded and then sprayed with beige paint.

I took a cable harness from my grab box having 32 isolated cables and long enough for building two yokes.This harness was fed through the aluminium pipe and soldered to the original cables of the yoke - the ones going directly to the switches and the push buttons. To isolate the soldered cables I used heat shrink tubes.

For the new LCD display with an internal controller of type HD 44780 a dedicated board was developed, to plug the display directly into the integrated left and right 9-pin connectors. This allows to disassemble the LCD from the board in case of problems, because it is impossible to de-solder an LCD display, which is soldered on a board.

This LCD board also includes a Schmitt-Trigger IC of type 74LS14, because without this signal shaping element the LCD controller does not work.

If you ever read the HD 44780 datasheet of the LCD internal controller, you will notice that somewhere in the “signal timing” table is written: “Enable Signal Fall Time max. 25ns” at pin 6. That means the voltage on the HD 44780 pin called “Enable” has to fall from 5 Volt to GND within 25 nanoseconds and the other way round. They should have better printed that in BIG fat red letters, because these HD 44780 LCD controllers are really picky about this Enable signal fall time.

And that is the problem: If you count together the bad driving characteristics of the relative long (ca. 50 cm) flat ribbon cables from the ATmega8 output port in the Flight Control Unit over the Yoke Switch Distributor board to the LCD controller input port, including three connectors, the total capacitance may easily get an order of magnitude, resulting in a fall time, significant slower than the allowed one. It is therefore necessary to 'shape' the Enable signal by an additional Schmitt-Trigger as close as possible to the LCD controller.

Because the cockpit consists of two yokes, a sophisticated control architecture was necessary to allow the control of the aircraft either from the pilot or from the co-pilot yoke, as shown in the right picture. Therefore an intermediary Yoke Switch Distributor was designed. The distributor acts as an electronic switch, which routes either the pilot yoke commands or the co-pilot yoke commands to the Yoke Switch Sub-Controller. This electronic switch is realised by a quad AND/OR select gate CD4019 IC.

The switching of the analogue signals from the elevator and the aileron potentiometer position is implemented in the Flight-Control Switch board (see below schematics) by an electronic switch, which consists of an analogue quad bilateral CD4066 IC. This board also switches the analogue signals from the two pedals - rudder and brake potentiometer positions.

The yoke switch sub-controller is a dedicated board equipped with an ATmega32 micro-controller. The schematics of this board is shown below. This board also serves the LCD Display Slave Controller board, which consists of an ATmega8 controller. The communication between both controllers is realised by a handshake protocol, which runs via the 6-pin Bank-2a connector. The interface to the PC and hence to FSX is realised by a commonly used Main-Controller via an RS-232 link.

Based on this setup it is very simple to switch very quickly from one yoke to the other. This is accomplished by a single mechanical switch at the lower right switch panel, which is connected to the Yoke-Switch-Controller at pin 5 of Bank-3. In addition, there is a small green LED above the LCD display in the yoke, indicating that this yoke is the active one.

It shall be noted that the readout of the analogue signals of the elevator and the aileron potentiometers are fed to the Flight-Control Sub-Controller, which is described elsewhere.

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