Rather than just have a row of labeled indicator lights, I wanted to make a custom annunciator panel. Initially I thought this would be a relatively easy process, but as I started to think more about what I wanted it to do, it got more complicated. The main purpose of an annunciator panel is to indicate when important functions are enabled (i.e. fuel pump, pitot heat), warn when functions are enabled that might not be appropriate for all stages of flight (i.e. speed brake, starter engaged) and make a big racket when things have gone wrong (i.e. doors ajar, alternator out, get that gear down!). I broke these down to green, yellow and red indicators. I wanted an audible alarm when any of the red indicators were lit. Finally, I wanted to be able to test all the indicators and alarm with one button. Easy enough, right? Well, all that requires a bit of electronics. I won’t go into great detail, but in order to have a single-button push-to-test, you need to use some diodes to allow all devices to be tested and still have proper operation. Another issue was that several of the indicator sources pulled to ground while others gave +12V to indicate on. Basically, the push-to-test circuit connected all the indicators to ground through a diode. For the circuits that had a +12V signal, a transistor was used to convert this to a ground signal. This might make sense if you look at the schematic.
In the image gallery below, you’ll see the process I went through to create the annunciator panel. First tested a few ideas using a breadboard then drew up a schematic. Then, etched a circuit board using a photo-resist technique. Using this technique, a transparency is made of the circuit and used to mask the photo resist on the board. After illumination, the board is briefly rinsed with a dilute sodium hydroxide developer solution. This rinses away any resist that saw light. So, you’re just left with the pattern of your circuit on the copper. Then, a ferric chloride solution is used to etch away all copper not covered by the photo resist. So solder flows nicely on the copper, the board is then treated with a tinning solution that turns all the traces silver. After that, I diced the board into two and drilled the holes for all the components. The components were soldered on, and it was ready to use! I used two LEDs per indicator for redundancy. They were wired in parallel, so if one fails the other will remain lit.
I used the same CO2 laser process to make the bezel and lenses for the indicator. I created a front bezel that defined the lenses, and a clear piece of plastic behind this that had the indicator labels. I sandblasted the lens so that it diffused the light from the LED’s.
The audible alarm was created using a Piezo electric buzzer that used one of the unswitched audio inputs of the audio panel as ground. This creates a pretty annoying alarm both in the cabin, but also through the headset. I added a mute switch, as taxiiing out with the doors open would get pretty annoying with the alarm blaring away. I’ll have to make sure to include disabling the mute switch in my takeoff check list.
