When I first started out on this project, I figured that using actual Mooney aircraft rocker switches would be the best way to go – what’s more realistic than the real thing? And they’re just switches – how expensive can they possibly be? A search online with a few Mooney parts suppliers quickly brought me back to reality:

$314 for one switch?!

So let’s see here… roughly $300/switch, multiplied by 16 (all the rocker switches in the main and overhead panels)… $4,800, just for switches!

Now, I can sort of see where they’re coming from since these are actually circuit breakers, not just on/off switches… but $300 each is still highway (taxiway?) robbery. Gotta love the “aviation grade” tax.

Next step was to find other breakers that looked just like these that I could get engraved or marked correctly, but even after looking at virtually every major circuit breaker manufacturer, I couldn’t find a damn thing. The only switches that looked anything remotely like these were the Sensata/ETA and Klixon breakers, both of which were aviation specific, and again, exorbitantly expensive.

So after ruling out rocker-type breakers, I started looking at normal (non-breaker) rocker switches. The Honeywell AML series was a great candidate: readily available, not insanely expensive (roughly $20-25/switch), and the AML54 switch operators (covers) could be easily sourced from the fine folks at Aircraft Engravers at a very reasonable cost. Needless to say, they did a pretty awesome job:

What I’ll probably do to implement the circuit breaker behavior is add an extra row or two of ordinary breakers to the panel’s circuit breaker panel corresponding to each of the rocker switches. This way if there’s a short circuit in, say, the pitot heat – it’ll pop an ordinary round breaker as opposed to automatically flipping the rocker switch. If anyone has any other ideas for how I could make an ordinary rocker switch flip on its own, let me know. 🙂

One additional feature I wanted to implement in the sim was backlighting (yes, yes, I know.. the M20 doesn’t have backlit switches!), but at low brightness settings the backlighting will help accentuate the engraved text which appears slightly gray because of the underlying texture of the switch operator (and backlighting just looks ridiculously cool – this is a case where I’m willing to deviate from my goal to make this as systems-accurate as possible). The AML24 switch can accommodate two 5V LED bulbs, the AML92 series, to light up the top and bottom of the switch. So I ordered two of the AML92EWL (the 5V white LED bulbs) to get a sense for how they look…



See anything wrong with this picture?

So… these bulbs are, well… kind of orange. I honestly thought there was something wrong with the bulbs (counterfeit parts from China?) at first, so I got a hold of Honeywell’s technical support and sent them these pictures. They verified that, yes, this is indeed the expected color of the supposedly-“white” bulb!

The second problem with these bulbs is that they’re outlandishly expensive, on the order of $25 a bulb, meaning $50 per switch. This again felt like highway robbery, considering that I can get a generic white 5mm LED for pennies, and those would do a perfectly fine job lighting up my engraved switch operators. However, there’s no way to get a generic LED in the bulb socket within these switches… or is there? This started my weeks-long adventure to try and adapt an AML switch to use ordinary LEDs…

The Honeywell AML24 switch innards, with two internal bulb sockets.

The Honeywell $25 yellow-tinted proprietary bulb.

A 10-cent white LED – if only I could use this thing somehow…

Attempt #1: Jam the leads in the bulb socket. As expected, this utterly failed. The “claws” of the bulb socket were already spaced too far apart, and refused to grab the thin LED leads.

Attempt #2: Put a big blob of solder on the LED leads, and jam that in the socket. This sort of worked, but it would easily wiggle loose if the switch just barely shook. Not quite reliable enough…

Attempt #3: Put solder paste on the bulb socket, carefully position the LED on the socket, and heat up the socket from the outside. The solder paste did melt from the heat of the soldering iron on the external connector tab, but the melted paste had this tendency to flow everywhere, mostly deep into the innards of the connector and not on to the LED. This was also super messy and ruined one of my switches. Ugh.

Attempt #4: Make a mold out of clay, and cast a connector out of solder in the same shape as the bulb. This was one of my more outlandish ideas that turned out to be an utter flop. The solder just didn’t want to flow into the mold and keep its shape, and the bulb shape is just too tiny to be able to do this with any degree of precision.

Attempt #5: Fifth time’s the charm? When I was measuring the bulb with a pair of calipers during one of my previous attempts, I stumbled across an interesting dimension:

The thickness of the bulb is precisely 2 mm… and it’s possible to get printed circuit boards manufactured in that thickness. Could I just make a PCB that’s the exact shape of the bulb, with exposed pads where the bulb’s contacts are, and jam that in the slot?

After a few hours of tinkering around in EAGLE, I designed a board with the required dimensions (5.2 mm x 8 mm x 2 mm), two exposed pads, and two holes to accommodate a standard 5 mm LED:

About a week after sending the Gerber files to a PCB manufacturer in China, I got back a bag of 50 really tiny PCBs. I had the exposed pads plated with immersion gold (ENIG) to prevent corrosion, but since even ENIG has a limited shelf life, I’ll probably put a thin layer of solder on these pads to further prevent any corrosion. Another option is to get them manufactured again using hard gold (similar to what’s used in audio jacks and the like), but that might be cost prohibitive. We’ll see…

The final result! Note the exposed pads are on both sides of the board (same as the bulb).

A generic white 5 mm LED soldered to my connector.

And after soldering bulbs to the connector, here’s the final result:

A perfect fit, and held solidly by the internal bulb socket.

Success!

They fit perfectly in the slot, don’t move around at all, and illuminate the switch with a solid bright white. I’ll probably investigate some other LED options that are more diffuse, but otherwise these look great!

If anyone wants to use these connectors for your own project, I’ve attached the Gerber files here – you’re welcome to send them off to your PCB manufacturer of choice. (These are provided “as-is” with no warranty of any kind – use at your own risk).

–Alex