OHP - Left

OHP Overview | Pneumatics | OHP-Top

Located on the left side of the cockpit, just below the throttle quadrant and the warning buttons, this panel contains the following elements of the original B737 OHP:

  • Fuel management panel incl. Fuel Temp gauge
  • Engine Start switches
  • Electric Bus and Generators panel
  • APU panel with EGT gauge
  • Probe Heat panel
  • Wing anti ice panel
  • A and B Hydraulic System panels
Every item in my cockpit first saw the light of day on my computer screen - as a Corel Draw file. Since the panel will be backlit, I decided to build it from two layers of acrylic glass. The picture on the left shows the base plate in grey and the transparent parts in blue.

The base plate is laser cut from black 3mm acrylic glass, painted grey on one side. The individual sub-panels will later be glued on this plate. The irregular openings allow the backlight to pass where necessary.

The small holes will hold 2mm threaded rods. These act as spacers to provide equal distance between backplate and the backlighting plate that holds the LED's.

A close up of the Bus Transfer panel as an example for the 2nd layer sub-panels. They are made from transparent 3mm acrylic glass to allow for backlighting. The edges are painted grey to block the light.

Information on how to make a backlit panel and how to build an annunciator light can be found in the "Building Tips" section.

Reverse side of the Bus Transfer sub-panel. Note the black acrylic frames around the anunciators. They isolate each anunciator and avoid light spilling. When an LED goes on, only that anunciator will be lit.

More info on the making of the anunciator lights can be found here.

The (almost) finished OHP_Left panel. The APU EGT gauge and Fuel Temp gauges are still missing, and so are some annunciator lights for the pitots.

The openings in the base plate allow backlight to illuminate the labels, lines and symbols on the sub-panels. Note the mirror letters on the back of the transparent panels. They are part of my backlighting design.

The black bolts around the subpanels are only for show. The panels are simply glued to the base plate.

Another close-up...

The two engine start switches in the B737 are solenoid held in the GND position. I used electo-magnets to mimmick this behavior. The magnets and switches are installed in the gray frame in the center of this picture.

Detailed info on how the solenoid switches are built can be found here.

All backlighting plates are installed. They hold arrays of white spider LEDs that produce plenty of light. Since they are mounted at a distance of 40mm from the panel surface, they provide very uniform lighting. If LEDs are mounted to close, the backlighting will not be uniform and individual LEDs might produce "hot spots" with too much light on the surface of the panel.

 

Serious cabel madness!

Since the panel is so densly packed with LEDs and switches, cable chaos during the soldering phase can hardly be avoided.

Admittedly, this panel will not win a beauty contest, even after the wiring and harnessing is done...

All LED are wired to a Phidgets LED64 card, mounted on top of the Engine Start Rotaries. I made sure all cables are labeled to make documentation and programing easier. I emphazise again how important it is do write down as much detail about wiring as possible. It is not the most glorious work, but clear references about what goes where will help greatly with all subseqent tasks.

A close-up of the Phidgets card. You may note that not all connectors are used. The reason is simple. There are 96 LED on my three OHP subpanels, so in any case two Phidgets 64 cards were needed. In order to keep the wires short, each board is used only for the LED that are close to it.

Note the two relais marked "R" and "L". They are connected to the EPIC 32-point output module and - when fired - activate the 15v DC solenoids switches.

Another look at the finished panel. Come to think of it, this is my first 5-level panel...

The connectors to the EPIC Expansion Card and my distribution board.

While all LED in this panel connect to the Phidgets 64LED card, the switches are wired to the EPIC. I was slowly running out of free mod rows on the EPIC expansion modules 1 and 2, so I had to start using rows on module 0. That module is usually reserved for throttle, stick and rudder. Since I upgraded to the USB based Cougar system some time ago, I could use all 53 buttons on module 0.

The silver DB-25 connector marked with "7" goes to distribution board 2.

The business side of the panel...

Both OHP-Left and warning panel fit into a plywood support frame. That frame is attached to the cockpit structure with just two screws.

If the cockpit needs to be transported, this unit can be detached within minutes. It is small enough to allow for easy packaging without the need for further disassembly.

At long last, the panel is installed in the cockpit.

Finally there is direct tactile control over the fuel system, pitot heat, anti icing, engine start switches and the like! How could I ever fly the virtual skies without them? Just think of the risks I took...