Backlighting and Dimming with OEM and Reproduction Panels

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FDS-IBL-DIST-DIM.  A card that makes diming backlighting very easy.  Potentiometer is not shown

Many enthusiasts are now using Original Aircraft Equipment (OEM) panels in their simulators.  These panels are connected to Flight Simulator using a variety of interface cards.  Unless the flight deck uses all OEM panels, or all reproduction panels, there will be a difference in backlighting when the light plates are illuminated.

Reproduction panels, with the exception of expensive very high end types, will have exceptionally bright backlighting.  Manufacturers of reproduction panels want their panel to look good and appeal to a prospective buyer – this is why they have bright backlighting.  In contrast, OEM panels do not have  bright backlighting, and in some cases, depending upon the manufacturer of the panel, the backlighting will appear rather dim.  

Therefore, the brightness of the backlighting when using ‘run of the mill’ reproduction panels is not realistic in comparison to that observed in a real aircraft.

So how does a cockpit builder solve this conundrum of brightness if he or she has a mix of reproduction and OEM panels.  The solution is very simple – install a dimmer switch into your flight deck.

Dimmer Control

There are a number of 5 volt dimmer switches on the market and some are better than others.  For those with electrical knowledge it’s relatively straightforward to make your own dimmer switch, but what about the rest of us?  An excellent solution is the distribution board with built in dimmer control manufactured by Flight Deck Solutions (FDS).  The board keeps with the principle of KIS (keep it simple).  

FDS-IBL-DIST-DIM

The distribution board is well made, small, is fuse protected, and have the capability to connect up to 14 accessory LEDS or bulbs via propriety board connectors.  The board also can be used as a slave, meaning it can be daisy-chained to another board to increase the number items attached.

The distribution board includes a pre-wired metal potentiometer which allows all the LEDS/bulbs attached to the board to be dimmed from on to off or anywhere in-between.  The potentiometer is a standard size and fits the hole located in the panel lights panel on either a reproduction panel or an OEM panel.

One limiting feature that should be noted is that each distribution board will only support 10 amps - the rating of the fuse.  Therefore, depending upon the number of panels that you wish to connect to the board, it may be necessary to use two boards in parallel rather one board or an extension to the board.

Of more importance, the board operates flawlessly and is a very easy solution to maintaining an even brightness across reproduction and OEM panels; adjust the brightness of the reproduction panels to the same level as the OEM panels.

Connection

Connection is straightforward and requires +- 5 volts to be connected to the board.  Each LED (or bulb) that requires dim control is then connected to the board connectors.  If using an FDS panel this is very easy as the FDS panels already use the correct female attachment plugs (FDS also use bulbs and not LEDS).  Failing this, a little extra work is required to source the correct plugs and wire them to the +- wires that connect to the light plate.

Bulbs and LEDS

On another note, with the exception of late model airframes, the Next Generation B737 use 5 volt incandescent bulbs in their panels for backlighting.  This is in contrast to reproduction panels that, for the most part, use LEDS.  

The difference between bulbs and LEDS, other than construction, is the temperature they generate when turned on.  A bulb will generate considerable heat and the colour of the light will appear as a warmer hue.  A LED does not generate heat when turned on.  Therefore, an LED will have a cooler temperature and the colour of the light will be colder and more stark in its appearance.

However, before changing out all your bulbs or LEDS to maintain colour consistency, study the flight deck of a real aircraft.  Panels on all aircraft fail or need upgrading from time to time.  Therefore, it is not unrealistic to have a flight deck consisting of both LEDS and bulbs.  Airlines are in the business of making money, and pilots fly.  Neither are particularly interested in whether the ADF radio has a bulb or LED.

Additional Information

Soar-By-Wire has also discussed this subject.  Although his information relates to the Airbus, the same procedure can be done for Boeing OEM panels.

Disclaimer

I do not represent Flight Deck Solutions or any other manufacturer and have no received any fee or reward for discussing one of their interface components.

Further information pertaining to the distribution board can be found on the Flight Deck Solutions website.

A fellow enthusiast has written more information on his website about the distribution board as it relates to Airbus - Soarbywire.  What he has written is well worth the time reading.

BRT / DIM Functionality - Lights Test Switch

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Lights Test switch.  The three way switch located on the Main Instrument Panel (MIP) Captain-side is used to toggle the intensity of connected annunciators.  The panel label reads TEST, BRT and DIM.  The switch in the photograph is an OEM switch which has been retrofitted to a Flight Deck Solutions (FDS) MIP

The annunciators in the Boeing 737 are very bright when illuminated, and the reason for the high intensity is justified - the designers want to ensure that any system warnings or cautions are quickly noted by a flight crew.

However, when flying at night for extended periods of time the bright lights can be tiring on your eyes.  Also, during critical flight phases such as during a night-time approach, the bright lights can become distracting.  At this time, the flight deck is usually dimmed in an attempt to conserve night vision. 

For example, the three green landing annunciators (Christmas tree lights), speed brake and flaps extension annunciators are all illuminated during the final segment of the approach.  At full intensity these annunciators can, at the very least, be distracting.

To help minimize eye strain and to enable night vision to be maintained as much as possible, pilots can select from two light intensity levels to control the brightness output of the annunciators. 

Anatomy of the Lights Test Switch

The switch (a three-way toggle) which controls the light intensity (brightness level) is called the Lights Test switch.  The switch is located on the Main Instrument Panel (MIP).  The switch is not a momentary switch and whatever position the switch is left at it will stay at until toggled to another position.  The switch has three labelled positions: Lights Test, BRT and DIM. 

(i)           UP controls the lights test (labeled Lights Test);

(ii)          CENTER is the normal position which enables the annunciators to illuminate at full intensity (labeled BRT); and,

(iii)         DOWN lowers the brightness level of the annunciators (labeled DIM).

OEM annunciators have a built-in Push-To-Test function, and each annunciator will illuminate when pushed.  The brightness level is pursuant to the position the Lights Test switch (DIM or BRT). 

The Lights Test will always illuminate all the annunciators at their full intensity (maximum brightness). An earlier article explains the Lights Test switch in more detail.

Special Conditions

When the Light Test switch is set to DIM, all the annunciators will be display at their minimum brightness.  The exception is the annunciators belonging to the Master Caution System (MCS), which are the master warning, fire bell and six packs, and the Autopilot Flight Director System (AFDS).  These annunciators will always illuminate at their full intensity because they are construed as primary caution and warning lights.

Variable Voltage

There is nothing magical about the design Boeing has used to allow DIM functionality; it is very simplistic.

Annunciators for the most part are powered by 28 volts; therefore, when the Lights Test switch is in the neutral position (center position labeled BRT) the bulbs are receiving 28 volts and will illuminate at full intensity.  Moving the switch to the DIM position reduces the voltage from 28 volts to 16.5 volts with a correspondingly lower output.  In the real aircraft, the DIM functionality (and Light Test) is controlled by a semi-mechanical system comprising relays and zener-type diodes that vary the voltage. 

Two Controlling Systems - your choice

The DIM and Lights Test functionality can be achieved in the simulator by using one of two systems - software or mechanical.

Software Controlled

The avionics suites developed by Prosim-AR, Project Magenta and Sim Avionics have the ability to conduct a full Lights Test in addition to allowing DIM functionality.  However, depending upon the hardware used, the individual Push-To-Test function of each annunciator may not be functional.  The DIM functionality is controlled directly by the avionics suite software; it is not a mechanical system as used in the real aircraft.

In ProSim737 the DIM function can be assigned to any switch from the configuration/switches and indicators menu.  In Sim Avionics the function is assigned and controlled by FSUIPC offsets within the IT interface software.

Mechanically Controlled

I have chosen to replicate the Lights Test and DIM functionality in a similar way to how it is done in the real aircraft. 

There are no benefits or advantages to either system – they are just different methods to achieve the same result.

Two Meanwell power supplies are used to provide the voltage required to illuminate the annunciators.  A 28 volt power supply enables the annunciators to be illuminated at their brightest intensity, while the less bright DIM functionality is powered by a 16.5 volt power supply (or whatever voltage you wish).

A heavy duty 20 amp 12 volt relay enables selection of either 28 volt or 16.5 volts.

The DIM Board is surprisingly simple and comprises a single terminal block and a heavy duty 12 volt relay.  Wires are coloured and tagged to ensure that each wire is connected to the correct terminal

DIM Board

A small board has been constructed from ABS plastic on which is mounted a 20 amp 12 volt relay and a terminal block. The board, called DIM is mounted behind and beneath the MIP. This facilitates easy access to the required power supplies mounted within the Power Supply Rack (PSR). 

An important function of the DIM board is that it helps to minimise the number of wires required to connect the DIM functionality to the various annunciators and to the Lights Test switch.   

Interfacing and Connections

Prior to proceeding further, a very brief explanation is required to how the various panels receive power. 

Rather than connect several panels directly to a power supply, I have connected the power supplies to two 28 volt busbars - one busbar is located in the center pedestal and other is attached to the rear of the MIP.  The busbars act a centralised point from which power is distributed to any connected panels.  This allows the wiring to be more manageable, neater, and easily traceable if troubleshooting is required.

Likewise, there is a lights test busbar located in the center pedestal that provides a central area to connect any panel that is lights test compliant.  Without this busbar, any panel that was lights test compliant would require a separate wire to be connected to the Lights Test switch in the MIP. 

The below mud schematic may make it easier to understand.  To view the schematic at full size click the image.

Mud schematic.  Note that grey box should say 12 Volts - not 28 Volts

A 28 volt busbar located in the center pedestal is used as a central point from which to connect various panels to (lower pale blue box).  

The busbar is connected to the terminal block located on the DIM board.  Wires from the terminal block then connect to a 16.5 and 28 volt power supply located in the PSR (orange boxes). 

The relay is also wired directly to the terminal block on the DIM board and a single wire connects the relay with the Lights Test switch located in the MIP (green box). 

From the Lights Test switch, a single wire connects with the lights test busbar located in the center pedestal (pale blue box).  The purple box represents any panel that is Lights Test compliant - a single wire connects between a panel and the lights test busbar.

Although this appears very convoluted, the principle is comparatively simplistic.

How it Works

When the Lights Test switch is toggled to the DIM position the relay is closed.  This inhibits 28 volts from entering the circuit, but allowing 16.5 volts to reach the 28 volt busbar (located in the center pedestal); any annunciators connected to this busbar will now only receive 16.5 volts and the annunciators will glow at their lowest brightness level.  Conversely, when the switch is toggled  to BRT or to Lights Test, the relay opens and the busbar once again receives 28 volts.

Which Annunciators are Connected to DIM Functionality

The annunciators that connect with the DIM board are those in the fire suppression panel, various panels in the center pedestal, the forward and aft overhead, and in the MIP.  If further annunciators in other systems require dimming, then it is a matter of connecting the appropriate wires from the annunciator to the 28 volt busbar, and to the and lights test busbar, both of which are located in the center pedestal.

  • The nomeclature for the 12 Volt relay is: 12 V DC coil non-latching relay part number 92S7D22D-12 (Schneider Electric).

BELOW:  A rather haphazard video showing the two brightness levels.  The example shows the annunciators in the OEM Fire Suppression Panel (FSP).  The clicking sound in the background is the Lights Test switch being toggled from BRT to DIM and back again.  Note that the colour of the annunciator does not alter - only the intensity (brightness).  The colour change in the video, as the lights alter intensity, is caused by a colour temperature shift which is not visible to the naked eye but is recorded by the video.

 

DIM functionality test

 

Glossary

  • Annunciator - A light that illuminates under set conditions.  Often called a Korry.

  • Busbar - A bar that enables power to distributed to several items from a centralised point.

  • Mud Schematic - Australian colloquialism meaning a very simplistic diagram (often used in geological mapping / mud map).

  • Push-To-Test Function - All annunciators have the ability to be pushed inwards to test the circuit and to check if the globe/LED is operational.

  • OEM - Original Equipment Manufacturer aka real aircraft part.

  • Panel/Module - Used interchangeably and meaning an avionics panel that incorporates annunciators.

  • Toggled- A verb in English meaning to toggle, change or switch from one effect, feature, or state to another by using a toggle or switch.

OEM 737-800 Lights Test Toggle Switch - Wired and Installed to MIP

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OEM Lights Test Switch (before cleaning...) One switch comprising several switches

The lights test is an often misunderstood but simple procedure.  The light test is carried out by the crew before each flight to determine if all the annunciators are operating correctly (illuminating).  The crew will toggle the switch upward to lights test followed by a routine scan of each annunciator on the overhead, center pedestal and instrument panel.  An inoperative light may preclude take off.

The lights test switch is a three-way switch which can be placed (and locked) in one of three positions; it is not a momentary switch.  Toggling the switch upwards (lights test) illuminates all annunciators located in the MIP, forward and aft overhead and fire suppression panel (wheel well annunciator may not illuminate), while the central position (BRT) provides the brightest illumination for the annunciators (normal operation).  Toggling the switch downwards activates the DIM function dimming the brightness by roughly half that observed when the toggle is in BRT mode.

Depending upon which manufacturer’s Main Instrument Panel (MIP) you are using, the toggle switch may not function this way.  For example, Flight Deck Solutions (FDS) provide a three-way momentary toggle which is not the correct style of switch.  You should not have to hold the toggle to light test as you make your pre-flight scan.  The real toggle switch in the Boeing 737 aircraft is not a momentary switch.

Anatomy of the Toggle Switch

The OEM Light Test switch may appear to be a ‘glorified’ toggle switch with an aviation-sized price tag; however, there is a difference and a reason for this high price tag.  

The switch although relatively simple in output, encompasses 18 (6+6+6) high amperage individual switches assigned to three terminals located on the rear of the switch.  Each terminal can be used to connect to a particular aircraft system, and then to each other.  This allows the toggle switch to turn on or off multiple aircraft systems during the light test. 

The purpose of these multi-terminals is to allow the toggle switch to cater towards the high amperage flow of several dozen annunciators being turned on at any one time during the lights test, in addition to generators and other aircraft systems that are not simulated in Flight Simulator.  In this way, the switch can share the amperage load that the annunciators draw when activated during the light test.

The switch can control the annunciators (korrys) for the MIP, forward overhead, aft overhead, fire suppression panel and any number of modules located in the center pedestal.  

OEM Lights Test switch.  The appearance of the OEM switch is not dissimilar to a normal toggle switch; however, the functionality is different in that there are a number of terminals on the rear of the switch to allow multi-system connection

Terminals, Interfacing and Connection

To determine the correct terminals to be used for the light test is no different to a normal toggle-style switch. 

First, ascertain which of the six terminals correlate to the switch movement (toggle up, center and down).  The three unused terminals are used to connect with other systems in the real aircraft (not used in Flight Simulator).

To determine the correct terminals for wiring, a multimeter is set to conductivity (beep) mode.  Place one of the two multimeter prongs on a terminal and then place the other prong on the earth (common) terminal.  Gently move the toggle.   If you have the correct terminal for the position of the toggle, the multimeter will beep indicating an open circuit. The toggle switch does not require a power source, but power is required to illuminate the annunciators during the lights test.  

For an overview of how to use a multimeter see this post - Flight Deck Builders Toolbox - Multimeter.

Daisy Chaining and Systems

Any annunciator can be connected to the light test function, and considering the number of annunciators that the light test function interrogates, it is apparent that you will soon have several dozen wires that need to be accommodated. 

Rather than think of individual annunciators, it is easier to relate a group of like-minded components as a system.  As such, depending upon your simulator set-up, you may have the MIP annunciators as one system, the overhead annunciators as another and the fire suppression panel and modules mounted in the center pedestal as yet another.  If these components are daisy chained together (1+1+11+1+1=connection), only one power wire will be required to be connected at the end of the array.  This minimises the amount of wire required and makes connection easier with the toggle switch.

Two Methods to Connect to the Switch

There are two ways to wire the switch; either through the flight avionics software (software-based solution), or as a stand-alone mechanical system.  There is no particular benefit to either system.  The software solution triggers the Lights Test by opening the circuit on the I/O cards that are attached to the computer, while, the mechanical system replicates how it is done in the real Boring aircraft.

Switch in-line (software connection using ProSim737)

The on/off terminal of the toggle switch is connected to a Leo Bodnar card or other suitable card (I use a Flight Deck Solutions System card), and the card’s USB cable connected to the main computer.  Once the card is connected, the avionics suite software (ProSim737) will automatically register the card with to allow configuration.  Depending upon the type of card used, registration of the inputs and outputs for the card may first need to be registered in Windows (if using Windows 7 type into the search bar joystick and select calibration).

To configure the toggle switch in ProSim737, open the configuration/switches tab and scroll downward until you find the lights test function.  Open the tab beside the name; select the appropriate interface card (Leo Bodnar card) from the drop down menu and save the configuration.  

ProSim737 will automatically scan the interface cards that are installed, and if there is a card that has a power requirement, such as a Phidget 0/16/16 card (used to convert OEM annunciators, modules and panels), the software will make a connection enabling the lights test to function.

Considering the connection is accomplished within the ProSim737 software, it stands to reason the lights test will only operate when ProSim737 is open.

To illuminate the annunciators when the switch is thrown, a 28 volt power supply will need to be connected to the annunciators either separately or in a daisy chain array.

OEM aviation relay mounted in center pedestal

Stand-alone (mechanical connection)

The second method, which is the way it is done in the real aircraft, is to use an OEM 50 amp 6 pull/6 throw relay device. 

Depending upon the type of relay device used (there are several types), it may be possible to connect up to three systems to the one relay.

Lights Test Busbar

Although the Lights Test switch has the capacity to connect several systems to the switch itself, it would be unmanageable to attempt to connect each panel to the lights test switch.

To solve this issue a centrally-placed aviation-grade relay has been used in association with a busbar.

A benefit of using an OEM relay and busbar is that the relay acts as a central point for all wires to attach.  The wires from the various systems (panels, korrys, etc) attach to the busbar which in turn connects to the various posts on the relay.

The relay will then open or close the relay enabling power to reach the annunciators (via the busbar) when the switch is positioned to Lights Test.

The stand-alone system will enable the lights test to be carried out without ProSim737 being open.

Although the relay is not large (size of a small entree plate), it can be problematic finding a suitable area in which to mount the relay where it is out of the way.  A good location is to mount the relay inside the pedestal bay either directly to the platform floor or to a wooden flat board that is screwed to the lower section of the center pedestal.

Using the DIM Functionality (toggle thrown downwards)

This post has only discussed the lights test.  The DIM switch is used to dim the OEM annunciators (korrys) for night work. 

 

Diagram 1: basic overview to how the oem lights test toggle is connected

 
 

diagram 2: flow schematic between oem light test toggle and annunciators

 

Landing Lights, But No Overhead - Go Flight Module

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The 737 Project has been reasonably well mapped out and the work schedule revolves around specific aircraft systems. I try to finish work on one system (for example, the automation on the throttle) before moving to the next.

One of the provisos when I started this project was that I wanted the simulator to be working during construction. I was afraid that without using the simulator the project would loose traction.

The overhead is not scheduled for sometime (it is one of the last systems to completion).  In the interim, I am using the Sim Avionics virtual overhead, and moving a mouse around a screen is hardly realistic.

There are a number of switches on the forward overhead panel that can be easily mapped and configured to a toggle switch, and the switch mounted somewhere within the simulator.  Although unrealistic it is a better option than the mouse.

I have several GoFlight modules from my earlier simulator that were gathering dust; therefore, I decided to use two Go Flight T8 button modules to act as a interim overhead. I use the module to turn on and off the various lights - such as landing, taxi, navigation, collision and beacon.  The toggles also operate the anti-ice, CDU mode, seat belts, no smoking lights, yaw dampener and re-circulation fans.

The modules have been mounted into the center pedestal.  Although rather crude and certainly not realistic it works and fills the gap until the overhead is completed; making it easier to access the landing and a few other switches that are are used on every flight.