Avionics: Backup alternator installed

When I decided to modify my engine and cowl to effectively be the IO-390-EXP119, I thought it was a good time to install a backup alternator.  I decided on the B&C BC410-H with the LR3D-14 external regulator.  This alternator sits on the vacuum pad of the engine and has an external regulator.  As such, it doesn't provide much output until the engine is spinning at and above 1600 RPM.

I opted to place the regulator below my now-antiquated SkyRadar DX.  Yes, I installed it in a non-preferred orientation.

Here it is installed before applying some anti-chafing measures and reinstalling the alternator filter I once had on.  The bolt underneath the tach drive cap is nearly impossible to access. It requires a lot of thought and a thin socket on a low-profile swivel ratchet.

The backup alternator's cable is the same gauge as the primary's.  It connects directly to the primary alternator by routing in parallel with the latter's cable.  I did this so that I didn't need to bring the secondary's cable through the firewall.

Here it is installed with the alternator filter and the anti-chafing measures.  I intend to wrap the filter's wire in its own anti-chaffing rubber tube.

With two alternators and only one alternator indicator LED on the panel, I needed to design a circuit that could let that one LED work for both lights.  It's not straightforward since the primary alternator has its indicator on when the alternator is either providing low output or no output (e.g., when its field supply is switched off).  The secondary alternator only lights the LED when 1) its field supply is on and 2) if it's output is low.  So the circuit needs to allow the primary alternator to control the indicator LED only when the secondary alternator is not selected.  And the indicator output lines from both alternators need to be isolated from the other.  The circuit is basically a pair of transistors that switch the panel LED as described above.  

 

With this circuit, when ALT2 (backup alternator on) is selected, Q1 turns on, which turns off Q2.  When Q2 is off, the panel LED is disconnected from ALT1's indicator output.  When ALT2 is off, Q1 turns off which turns on Q2.  When Q2 is on, ALT1's indicator output (which is active low) is connected to the panel LED.  Not shown is ALT2's indicator output (also active low) connected to J3 (via an isolating 1N5817 diode).  This way, the panel LED indicates ALT1's status only when ALT2 isn't selected.  Otherwise, it shows ALT2's status.  This means the panel indicator light will turn on when no alternator is selected since ALT1's indicator output is pulled low when ALT1 is off.  Arguably, I don't need R1, but I prefer not to have Q1's base float when ALT2 is off.

 

This is what the circuit board looks like before the final piece of heat shrink went on it.  For my own records:  Red=ALT2 Select, Red/Green=ALT Field, White=Panel LED, Green=ALT1 Indicator Output.

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I replaced the original Otto K1 switch (model K1ABAPCABA) that I was using for turning on the primary alternator field with model K1ABFCBCFA which is incandescent (couldn't source the LED version) and has red and green lights.  The green side is for selecting the primary alternator whereas the red side is for the backup alternator.

I also ran a wire directly from the battery to under the panel through one of the passthroughs that had extra space after my effective EXP119 conversion.  I figure I could later add a dome light for the cabin that doesn't require the "Master" to be engaged.  Update 5-Apr-24:  I added a footwell light.