Avionics: New indicator lights circuit board.

I decided to preemptively replace my Plane Power AL12-EI60 alternator at 513 hours.  It whines like a coyote whereas my backup B&C BC410-H is as quiet as a mouse.  Late last year, B&C released the SF601 internally regulated alternator which is what I went with.  

That new B&C alternator doesn't have an indicator output so I redesigned my indicator circuit board to make the ALT indicator a low voltage light and also made each flight flash when they are enabled (other than the pitot heat light, because that would be annoying) to help grab my attention.  The new board also uses quick-connect tabs to make servicing it easier and it has less exposed trace area.  And since the ALT light is now a low voltage light, I won't need that tiny circuit board I made to ensure the ALT light always indicated the status of which of the two alternators was selected.

Here's what the new board looks like in CAD (easier to make out the details with the CAD rendering than with an actual image of the board).  The other side of the board attaches to the LEDs and indicator test push button.  After I soldered it up, I sealed it with lacquer.

The following schematic outlines what I designed.  A 555 timer is used for a T_On of ~0.7 s and a T_Off of ~0.3 s.  Since the 555 can't source much current, it controls a high side PFET which connects and disconnects +12 V from the LEDs in concert with the 555's Q output.  The Canopy Open and Oil Pressure LEDs will thus blink when enabled (active low). 

Q2's base is pulled high by the PFET output which normally enables the ALT LED.  But a 13 V Zener diode with a few resistors will break down above about 13.7 V, causing Q1 to turn on which turns off Q2, which shuts off the ALT light.  Thus the ALT light will be off when the main bus voltage is about 13.7 V and higher and blinking when it's lower than 13.7 V, indicating a low voltage situation on the main bus.

A 1 μF bypass cap on the power input was necessary to clean up the 555's output.  And a 3.3 μF cap was necessary across the ALT LED since when the PFET turns off, the LEDs' power line drops below 13.7 V, causing a dim and momentary flash on the ALT LED even when the bus voltage is higher than 13.7 V.  That 3.3 μF cap across the ALT LED holds the voltage high enough for long enough during the T_Off phase that the LED won't briefly try to turn on.  Since it's only 3.3 μF, it doesn't cause the ALT LED to fade off when being illuminated.  Capacitors across the Canopy Open and Oil Pressure LEDs aren't necessary since when they are not illuminated, their cathodes are floating so there is no current path available.

As before, an "indicator test" input will momentarily turn on all LEDs to verify they work.  This would cause the Oil Pressure, Canopy Open and ALT LEDs to flash and the Pitot Heat light to turn on solid.

Video showing canopy being locked/unlocked with lights flashing.

Circuit board parts list:

• 555 timer:  Texas Instruments SE555DR
• Capacitor 3.3 μF:  Nichicon UVR2A3R3MDD
• Capacitor 1 μF:  Nichicon UPX1V010MPD
• Diodes:  Kyocera AVX SD1206S040S2R0
• PFET:  Infineon Technologies:  IPP120P04P4L03AKSA2
• Quick connect tab, 0.25":  Keystone Electronics 4902
• Resistor 4.7 k:  Panasonic Electronic Components ERJ-8GEYJ472V
• Resistor 10 k:  Panasonic Electronic Components ERJ-8GEYJ103V
• Resistor 270:  Panasonic Electronic Components ERJ-8GEYJ271V
• Transistor:  onsemi MMBT2222ALT1G
• Zener 13 V:  Microchip Technology 2EZ13D5

Avionics: Connected right EFIS to backup battery and installed switch.

Since I installed dual SureFly ignition in my airplane, having a fully electronic ignition means being thoughtful and deliberate about how those magnetos get their power.  Speficially, one needs a backup battery and a means to control how that battery is connected to the ignition and the main bus.  After designing and installing my backup battery system and realizing it has a lot more capacity than necessary for the SureFly, I connected my AHRS and left EFIS to the backup battery (shown towards the bottom of that post).  I did so through a locking DPDT panel switch that lets me disconnect the left EFIS from the both sources of power (primary and backup) simultaneously.

I decided to do something similar for the right EFIS.  I connected it to the backup battery also but instead, used a DP3T switch so I have the following three switch functions:  1) OFF, 2) primary only and 3) backup and primary.  For that switch, I used a NKK Switches M2044LL1W01-C to match the switch I have for the left EFIS.  Here is that switch newly installed on my panel.

 

Here is the pinout and connection information for the new switch.

Here the switch for the left EFIS, which I added when I swapped over to fully electronic ignition and installed the backup battery.

These switches are really most useful for when I need to update the EFIS databases.  I no longer need to switch on the entire panel.

Avionics: Transponder upgrade.

I started off with the Garmin GTX 330ES transponder.  It did what it was supposed to do, mostly staring back at me with with a "1200".  It also provided TIS-B to my GTN 650 and my GRT HXr EFIS.

What it didn't do was provide FIS-B to my GTN 650, although I have the SkyRadar DX providing that to my HXr screens.  But, wouldn't it be nice to use the 650 for FIS-B info too?  Enter my new transponder, the GTX 345.

Functionally, the user experience between the two is essentially identical.  The 345 has a white display rather than the green display of the 330ES.  It can stream TIS-B and FIS-B info via its Bluetooth transceiver (to a limited group of Garmin-only apps, all but one of which costs money) and includes an integrated AHRS (available only to apps that must be purchased).  It's also 1.01" less in length and 0.03" shorter than the 330, thus requiring a tray swap.  And cruelly, it has a different pinout scheme than the 330 family.  So it's not a plug-and-play replacement.

Here's the new wiring map I made for the 345.  It requires an ethernet connection to the 650 (what Garmin calls HSDB, or high speed data bus), something the 330 family didn't offer.  I also wanted to connect an additional RS-422 line to my left HXr EFIS and a RS-232 line to my right HXr EFIS.  This is for both redundancy and testing to see if the 345 can send full TIS-B and FIS-B info to the EFISs.

 

Just for giggles, here's what the wiring change would look like if I were to rewire my current 330 harness for the 345.  As you can see, it's a mess.

So rather than risk breaking things, I decided to fashion a little dongle that rewires things for me.  I had to buy some parts (62-pin female d-sub receptacle, hood and female HD pins) and keep the dongle as short as possible so as not to cause undue strain on the existing wiring.  I also needed to pull new wires for the ethernet connection to the 650 and through to the EFISs to support the additional RS-422 and -232 connections previously described.  Here's the dongle.

Here's the 345's new tray in place.  Since it's 1.01" shorter than the 330ES' tray, I had to drill new holes for the forward screws.  Thus, I could not attach the tray to the forward pair of tray supports angles I put in back in 2016 (bottom of this post).

Here's the 345 in place (EFIS was off, so no SAT and DALT info available) and my happy 650 with FIS-B info.

Avionics: Final panel image.

The next posts to my blog should detail final assembly of the airplane in the hangar.  I don't know when that will be because I don't yet have a hangar.  However, since my last post detailed the wiring of the airframe, I wanted the top post to be something more aesthetically pleasing, especially since this post will be up for a good while.  So this post will show my panel for the last time.  The next posts should be of an aircraft with wings and a tail attached.

If you're interested in reading about the design of my panel, you can go here.  Here are the 12 posts relating to my panel.

Lit up, camera flash, day time.

Lit up, no camera flash day time.

No juice.

Lit up, camera flash, night time.

Lit up, no camera flash, night time.

LED strip full brightness, white only, night time.

LED strip full brightness, red only, night time.

LED strip full brightness, white and red lights, night time.

Here's the layout (includes replacing the GMA-200 with a GMA-245 and replacing the TruTrak ADI2 with a Konardia Horis 57).

Panel: Airspeed indicator purchased.

I purchased my Winter 7 FMS 523 airspeed indicator.  It's a 360 degree, 2.25", 0-200 kts, zero up indicator.  I had them place the following markings.

• V_NE  200
• V_A   130
• V_FE  100
• V_X    70
• V_S1   62
• V_S0   51

A shot of the panel essentially complete.

Avionics: Nav/Strobe and landing/taxi lights circuit

Here are the posts pertaining to nav-strobe lights.

• 12-Aug-13, initial thoughts
• 17-Aug-13, lenses cut 
• 9-Sep-13, lights ordered
• 7-Jan-14, wiring completed
• 30-Apr-17, mounts fabricated 
• 16-May-18, first night flight

The navigation and strobe lights can be operated independently, as can the landing and taxi lights.  So there are four possible combinations to run them:

	Nav/Strobe	Landing/Taxi
1	Both off	Both off
2	Nav on. Strobe off.	Taxi on. Landing off.
3	Nav off. Strobe on.	Taxi off. Landing on.
4	Both on	Both on

I don't anticipate needing to use combination 3 (in red) for both sets of lights.  And, in the interest of consolidating panel space, I wish to control the remaining combinations, 1, 2 and 4 (in green), with only one switch for each light set.  An example of how to do that is shown below.

The diode is the key to this arrangement.  When the taxi light is selected, the diode prevents the landing light from being powered (option 2).  However, when the landing light is selected, the diode permits powering the taxi light (option 4).  In this way, one can select "off", "taxi" or "taxi+landing" with a single switch (or "off", "nav" or "nav+strobe").  The issue with using a diode is that its forward voltage drop causes heat generation when taxi+landing is selected.  This is especially an issue with a high current taxi light, like mine and a non-issue with the nav/strobe lights (due to their lower current demand).

Initially, I used a high current Schottky with a V_f  of 0.58 V (somehow I managed not to take a picture of that setup).  However, with the taxi light pulling 7.3 A when set to "high", that means the diode will create more than 4 W of heat.  And it sure did!  That was too much heat (again, this wasn't an issue with the nav/strobe lights since the navigation lights pulled only about 400 mA).

A more efficient approach would be to use a high-sided FET in a switching configuration, like shown below.  The PFET will only be switched on when its gate is effectively brought to ground.  This occurs when the landing light (or strobe) is powered on by the switch (not shown). When that happens, the NPN is switched on since V_BE>0.7 V.  With the transistor on, its V_CE is brought from +12 V to effectively ground.  With the FET's gate now at ground, V_GS is effectively -12 V, switching the FET on so that the +12 V at the source is now presented at the drain, where the taxi light is (or nav).  The taxi light (or nav) is then powered on with the landing light (or strobe).

Since R_DS for a FET is tiny (in my case, less than 0.004 Ohms!), there is a negligible amount of heat generated, even with more than 7 A being pulled through for a taxi light (which amounts to less than 30 mW of heat in the PFET).  Finally, a diode is placed to provide a path for the collapse of the magnetic field in case of any inductive reactance in the load (a.k.a., a flyback diode). 

With my planned approach determined, next I needed to design the circuit board appropriate for the OTTO K2 DPDT switch I planned to use (left).  I wanted my circuit board to plug in directly to the K2.  Here is a diagram I made to better show the K2's terminal connections (right).

My schematic is below.  It's a bit messy to present.  The K2 switch terminals are represented in the upper center.  Each K2 terminal needs a female tab so it can plug in to the switch.  In additional, each terminal needs a male tab to receive the wires (e.g., ground, power, landing and taxi or ground, power, nav and strobe).  The switch is DPDT, so that left and right lights can be on separate fuses (or wing and tail nav/strobes).  A single NPN transistor controls both the left and right PFETs.  In case the fuse blows on either side, the NPN is powered from both inputs, so a blown fuse will not result in the PFET changing its state and turning on or off a taxi light (and with 20 k-ohms between each powered input, neither side can power the other through that connection, should a fuse blow).  Finally, should the circuit fail, the lights are still controllable mechanically through the switch:  If a PFET remains off, then either lighting position can be selected, so either 'landing or taxi' or 'nav or strobe'.  If a PFET remains on then either 'taxi or landing+taxi' or 'nav or nav+strobe' can be selected.

Next I needed to design the circuit board so that I can just plug it in to the back of the K2 switch.  Using a micrometer and the K2 specs sheet, I mapped out the dimensions necessary.  Since space was limited due to the proximity of other switches on my panel (left), the board needed to be about the same width as the switch itself.  That criterion also necessitated using surface mount components.  The right image shows the board design.

Here are 3D rendered images of the board.

 

 

The boards were then manufactured.  The top image shows the front and back of the board (actually, the right board is an earlier revision, but it's similar enough to my final design).  The bottom images show the front (left) and back (right) after I soldered one up. The two red jumper wires were necessary since, when designing the board above, I ran out of room to lay down associated traces.  Lastly, the observant will notice that the PFETs are held down by LP4-3 rivets (hey, it's an airplane, right?).

And finally, here are two boards plugged in to the back of both my nav/strobe and landing light K2 switches.  After running the taxi+landing lights on for 15 minutes, the PFETs do not get noticeably warm (same for the nav/strobes, however they have much lower average current so it wasn't an issue).  Success!

As an aside, the wigwag line for my lights is active low.  Yet I wanted the panel switch to light up when wigwag was selected.  So I needed to use an independent rather than dependent light OTTO K1. To make sure the light turned on when wigwag was selected, I had to switch in the ground to the K1's light.  However, when the switch was off, the wigwag line would be connected to the ground of the K1's light.  Unfortunately, the wigwag line floated low enough to cause the OTTO K1 switch to illuminate slightly when wigwag was off (i.e., the landing light control line was sinking current from the switch's light).  Can't have that!  Hence the simple isolating Schottky diode on the switch.

In the above image, you can see that diode encased in a few layers of heat shrink tubing (it's the clear heat shrinked discrete on the ride side).  Below shows a representation of the circuit for the wigwag switch.  To ensure that wigwag line is pulled low enough when the switch is thrown, that diode I chose has a V_f of 450 mV.

For the curious, as in the case of when I designed my panel, I am a proponent of open-source software.  For schematic generation and board layout, I used KiCAD.  And there are a myriad of so-called "board houses" that manufacture circuit boards when provided with the CAD files for one's design.  Then it's just a matter of whipping out the soldering iron and affixing the components.

 

Parts list:

• Diode:  Kyocera AVX's SD1206S040S2R0
  
• PFET:  Infineon's IPP80P03P4L04AKSA2
• Quick Connect male:  Keystone Electronics' 4902
• Quick Connect female:  Keystone Electronics' 3528
• Resistor:  Panasonic's ERJ-8GEYJ103V
• Schottky diode:  On Semiconductor's 1N5817
• Transistor:  On Semiconductor's MMBT2222ALT1G
   

Avionics: Indicator lights circuit.

Update 6-Jun-25:  This circuit was updated.

 

I desperately wanted to have a "push-to-test" button for my stack of four indicator lights, only because it's cool.

Since the lights are all active low, that switch would need to apply ground to the cathode of each LED light (as the anode side of the LEDs would all be tied to +12 V).  However, it's poor form to also pull low each indicator output line from each instrument (they may not be designed to handle being pulled low when they're floating).  So one has to isolate the output lines when the switch pulls the lights low.  Below shows that simple circuit.

The lights are shown as LEDs in the above.  When the switch pulls the cathodes low, it does that through a stack of diodes that prevents the indicator output lines (coming in from the right) from being pulled low too.  This way the LEDs all turn on but the indicator output lines can float happily.

Next I needed to design the circuit board so that I could just solder it in directly to the back of the lights without additional connectors.  Since I knew the precise spacing I had on my panel that I designed, that was simple enough.  Using surface mount diodes kept the board's footprint small.

Here is the board soldered up.  You'll notice I have two "spare" boards in case I need to replace an indicator light (since that would require destroying the installed board to remove a light).

And here it is soldered to the back of the instrument panel lights.  It's unassuming and out-of-the-way.

Finally, here is what happens when you "Push-to-Test".

As an aside, I'll probably remove one of the TAWS indicators in favor of an oil pressure of EIS (engine information system) warning.  I may also remove the other TAWS indicator in favor of an "Alternator" light.

Update 15-Dec-16:  I changed the indicators as described above.  The TAWS indicators are now "Alt" and "Oil Press". 

For the curious, as in the case of when I designed my panel, I am a proponent of open-source software.  For schematic generation and board layout, I used KiCAD.  And there are a myriad of so-called "board houses" that manufacture circuit boards when provided with the CAD files for one's design.  Then it's just a matter of whipping out the soldering iron and affixing the components.

Avionics/Panel: Lit up in airframe.

After spending 20+ hours routing wires and connecting them in the airframe, I hooked up the panel to some juice and fired the bad boy up.  Though the EIS, switches and indicators are not yet wired up, the whole shebang still looks like a party.

I have at least 20 hours of wiring remaining before it's done as I need to fashion the wing and tail harnesses and tidy the mess up.

You can read a reverse-chronology of my panel design and construction.  After learning what the cost is to have a panel professionally designed, built and wired, I opted to do the whole thing myself.