1Build details for mask intercom
The speaker is protected from disinfection solutions by plastic film taped inside the enclosure, and the speaker is clamped onto the film using mirror brackets bent to fit in a drill-press vise, to provide further mechanical stabilization. (I disinfect the speaker and the mask with 70% isopropanol spray.) The white strap on the speaker box is a creative use of straps sold for holding the corners of unfitted sheets on mattresses. The other straps come from a spare P100 mask. (At the start of the project, I had 3 identical P100s, all bought before the pandemic. They cost the equivalent of $47 US apiece in Ontario in fall of 2019.)
The microphone board is secured inside the mask with two stainless bolts that also serve as the signal conductors. There are 3 nuts on each bolt, all stainless. The first secures the external brass connection lug, the second seats on the white nylon mask insert with a stainless lock washer underneath, and the third is an acorn nut that seats on the internal signal lugs, which I place on top of the mic board. I also include a rubberized-fiber washer under the board to prevent short circuits. No messy protective coatings were applied except around and under the microphone capsule; I am putting all my faith in stainless steel. However, I am not quite satisfied with the amount of mechanical stability this arrangement of the nuts gives me. The alternative seems to be an additional nut, to place a nut just above and below the hard nylon insert to clamp it tight, but this would have required somewhat longer bolts, which I had difficulty finding. In use, the acorn nuts need re-tightening now and then.
When drilling the holes in the mask for the microphone bolts, there is a risk of the elastomer ("rubber") part wrapping itself around the drill bit and coming away in a big chunk. Not good, considering what we are about here. This happened to me when drilling from the inside out but not when drilling from the outside in. The trick may be to go slow and keep the rubber pressed firmly to the nylon insert while drilling, which is not an issue if drilling from the outside in. It's a female dog to fully separate the rubber and nylon parts so they can be drilled/punched separately. Remember to leave sufficient flat area around the hole to seat a future nut, especially inside, and don't leave any turnings between the rubber and nylon parts.
The mic is protected from humidity by a collet pried out of a PL-3020 push coupling, 3/8-inch OD (Watts or Sioux Chief), with the teeth clipped off. The top of the collet was covered in a layer of double-sticky carpet tape followed by a layer of plastic food wrap. I left a space of 1/16" between the film and the microphone capsule. Without this space, reproduction quality is unsatisfactory.
The battery is lithium-ion type, 9V. It connects to the intercom box with an adapter that plugs into a barrel jack on the box. The wires of this adapter tended to break at the point where they enter the 9-V clip, so I reinforced this spot with epoxy and a zip tie. A better solution is to use an adapter with a molded, hard-plastic 9-V clip such as Adafruit 80, not a vinyl-covered clip.
A problem with the P100 is that there is no filtration on the exhalation valve, so others are not protected if the wearer is infected by SARS-CoV-2 in spite of taking unusual precautions. You can mitigate this by wearing a second mask over the P100. The second mask is of fabric and hooks around your ears per convention. Make sure the cloth mask covers the exhalation valve. The project log has more to say on this.
To make the little brass adapter (pictured), I removed the business end from a quick disconnect and soldered it to an 1/8" tang, 5/16" long, protruding from a brass cutout. The brass stock was .03" thick and a template for it is provided in the project log.