Master Warning / Caution Dimmer Board & Trim Interface Board

Garmin Boxes

I plan to use the Garmin GEA 24 box as part of my avionics system, who's primary funciton is to allow for the input and display of sensors within the G3X system; such as flap position, fuel quantity, trim positions, as well as all the FWF sensors (EGT/CHT/MAP/RPM) etc. 

An addtional function of the system, is the ability to drive a Master Warning / Master caution light. This works by grounding pin 44 / pin 45 when a red or yellow message appears on the G3X screen. The system will alternate the ground to flash the external light, to serve as a visual indicator of a system message which needs your attention (just like a big jet!). 

To stop the flashing and accept the message, you can press on the message on the screen and the light will go out. Alternately, you can press a button to ground a discrete pin, which does the same function (you configure this in the G3X settings). I am using pin 40 / pin 41 to accomplish this. You can see the wiring of the J244 connector below. 

I plan to accomplish the lighting of the warning light, and the cancelling of the warning button using the same device. This lighted switch has a clear bezel on the front, and allows me to print a clear text label to place inside the switch. The part number is LB25RKG01-5C12-JC for red, and LB25RKW01-5D12-JD for amber. These also have 0.110" x 0.020" tabs, so i can use these crimp terminals (instead of solder). 

The TLDR here: I need to run power to this light, then send the ground to the GEA 24 which will ground the applicable pin to turn the light on, in the event of a red or yellow message. This means that these lights will recive full 12 volts all the time, and be unable to be dimmed at night. 

Enter another box 

I also plan in installing the Garmin GAD 27 box - this provides a bunch of system functions such as flap control, trim mixing, landing light wig wag etc, but importantly allows for 3 seperate dimming circuits. 

Dimming Control Inputs

The dimming circuit has two seperate components - the first is the dimming input. This is provided by some 10k potentiometers, input into pins 39-41 on the big J271 dsub connector. To provide these inputs, i plan to use P0915N-FC15BR10K potentiometers. These are linear and operate over a 300 degree arc. The panel mount in 9/32" holes and have PCB pins at the back. 

I could have just used some normal 10k pots and soldered the leads to them, but i thought this was a much nicer solution (and allowed me to practice using KiCAD and JLPCB and i got to learn what a gerber is!). 

The pots solder to the home made PCB at 1" pitch, and the output to the GAD27 is via a 9 pin dsub soldered to the board. The circuit is simple - it takes the 12v power from pin 37 on the GAD 27, and returns ground from the pots to GAD27 pin 38. Each pot centre pin is wired to pins 39-41 respectively. A wiring diagram for the board is below. 

Test mounted in some angle.

The board mounts through the panel using the pot mounting threads. With the 3 pots it is very secure.

Dimming Outputs

The second part of the GAD 27 dimming circuit, is the dimming outputs themselves. These are PWM based and work by grounding the respective pins 45-47. For a light which is on full brightness, the ground would be connected all the time. When it was in a dimmed state, the ground would pulse on and off and such a high rate, that we perceive the light as being dimmer. Actually, it is pulsing on and off faster than our eye can see. 

TLDR: The dimming works by pulsing the GROUND side of the light - not the power side. 

The Problem

The problem come therefore with the interaction of these two boxes. 

On one hand, we have the master warning light recieving power from the bus, then sending its ground to the GEA 24 in order to be activated by the G3X system. 

On the other hand, we have the GAD 27 needing to receive this same ground wire, in order for the dimming circuit to be completed. 

We can't have our cake and eat it too - or can we? 

What we need to do, is send the ground from the light to the GAD 27 so it can be dimmed. This means we need to be able to send 12v to the power side of the applicable light, whenever the GEA 24 master warning / caution activate pin goes to ground. 

The Solution

The solution is the humble 2N4403BU bipolar junction transistor (BJT). What is a transistor? 

It is basically like a tiny switch, or more like a relay. They have three pins - the COLLECTOR, the EMITTER and the BASE. They come in two varieties, NPN or PNP which describe how they function. 

For a NPN transistor, current will flow from the collector to the emitter, when a positive voltage is applied to the base. 

For a PNP transistor, current will flow from the emitter to the collector, when a GROUND is applied to the base. This sounds exaclty what we need! 

It should be noted that the above transisor is rated for 600ma only. So this system would not be suitable for an incandescant globe - but most LED's would be ok. The NKK switch lights i have selected draw 20 ma.

The idea is this - we make a board which uses PNP transistors. We send power from the BUS to the EMITTER of the transistor. We can then ground the BASE of that transistor using the ground signal sent from the GEA 24 when a warning / caution message appears on the G3X. This will allow current to flow out of the COLLECTOR of the transistor which we can send to the master warning / caution light. The ground of this light is then sent off to the GAD 27 dimming cuircuit, and viola! We have a dimmable master warning / caution light. 

Putting it all together

I designed the board below and sent it off to JLCPCB. It has 6 channels, and allows for the powering of the master warning / master caution switch lights, as well as 4 other panel lights (should i need these). 

The board works by taking 12volts in on pin 1, which is wired to each transistor's EMITTER. It then has 6 activate pins, which when grounded will ground the BASE of their respective transistor. Once these are grounded, power will then flow out on the applicable matching COLLECTOR pin to the light. The board has a 'test' funciton, which when this pin is grounded, grounds the BASE of all 6 transistors, and sends power out on all 6 output pins (COLLECTOR). This allows for a 'push to test' fucntion (The diodes are there to stop the ground for a single light, turning on all the lights. The resistors are there because google told me to put them there.)

The input pin / output pins are paired as follows:

• Master Warning - Ground 13 - power out on 12
• Master Caution - Ground 6 - power out on 4
• Light 1 - Ground 14 - power out on 11
• Light 2 - Ground 7 - power out on 3
• Light 3 - Ground 15 - power out on 10
• Light 4 - Ground 8 - power out on 2

Actually Building It

Once i got the boards and my digikey order back, it only took about an hour to solder it together and test it out! If i were doing it again, i would get a smaller tip for my soldering iron. The transistor legs were tiny, and the pads close together. I bridged across the first one and had to de-solder it and try again. I also installed one diode backwards, which i found in testing and rectified. 

So tiny - i cleaned and inspected every solder joint using a loupe.

This shows the scale of the board - i could have made it even smaller but i thought this was a good size to work with. 

Oops - you don't want to do this!

Much better.

That is not my thumb - that is my pinky finger for a scale comparison!

The completed board, minus the dsub (can you see the problem?)

The dsub is both electrially and mechanically attached to the board. 

The one diode i installed backwards. oops.

As a summary of how this board functions, i made a short explainer:

The board will eventually be mounted on a rib using these types of standoffs - these go into the 4mm holes on the board, and mount in a 3/16" hole on a rib or somewhere convenient.