A P100 Mask Intercom

A conventional P100 respirator upgraded for use as COVID-19 PPE by the addition of an intercom and a means of exhaled-air treatment

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This project is for people who already own a respirator that they can spare for a project. A P100 mask should provide the wearer greater security against infection by respiratory pathogens than a cloth mask (see: and it doesn't fog up your glasses. It is made of elastomer and hard nylon and seals without leaks if the straps are correctly adjusted and the height of the mask on the face is correctly adjusted. A suction test and a pressurization test (see: are needed each time the mask is put on. However, you can’t talk and be understood while wearing one. You have to remove the mask to talk–precisely the highest-risk situation, and talking seems unavoidable in social situations. I therefore fitted a home-built intercom box to a P100 that I already had and put an electret mic inside. The project (initial version) cost me about $30 US, main items.
  • 1 × P100 mask medium, 9200, McCordick
  • 1 × black project box Hammond 1593K
  • 1 × grey project box Hammond 1593J
  • 1 × speaker Mode 61-650-1
  • 1 × electret microphone Mode 72-160-1

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  • Clarifications

    David Matthew Mooney12/29/2020 at 18:51 0 comments

    ("Supplementary Data" to you science types.)

    Ensuring a good fit:

    In occupational settings, the all-important fit of the mask to the face must be professionally tested before use and every year thereafter.  Here is another link re fit testing (both last accessed 02-06-2021). I got my present mask at a hardware store outside of an occupational context but did a DIY fit test with powdered stevia sweetener puffed out of a rubber bulb just after giving the bulb a good shake. The stevia seems to lose its sweetness slowly after removal from the packet, so I grind it fresh for each test. Will try saccharin next. (02-20-2021: saccharin test passed, but self-administered.) NB: I have two years of experience in wearing a respirator occupationally. If you are new to this, you should practice a lot in a safe setting until you know what a good and bad face seal feel like and the moves you need to get a good one. My big discovery was that moving the mask up or down on my face often did the trick. If that didn't work, the straps were either too loose or too tight. (Tight straps can distort the face seal.)

    I learned how to wear a respirator in a paint shop where there are smelly fumes that you can smell if they leak into your mask. This facilitated the learning process and I am not surprised that OSHA provides a smell option for fit testing involving water solutions of isoamyl acetate;  using smell is so much more convenient than taste (no need for a "nebulizer" that often blocks up) and  makes a more stringent and foolproof test because to pass, the mask has to keep out individual molecules, which are vastly smaller than the tiniest particles of concern in infection control. You just swap organic vapor cartridges for filter cartridges for the duration of the test, then swap 'em back afterward.

    In cold weather, a mask leak can fog up my glasses and thus be detected easily if it occurs alongside my nose, which is a problematic area anyway. Fogged glasses can be  good for detecting leaks.

    Making a flared exhalation tube:

    I first prepared a 6-cm dia. plastic funnel by sanding the places where future hot glue would go. I then cleaned off the sandings. I cut off the hanger tab and then encircled the funnel with a bead of glue to help hold the filter. I then cut off the stem and drilled small holes near the bottom to pass the hold-down wire. I then assembled the flared tube on the mask and used hot glue to seal the seam between the funnel and the straight part of the tube (cut from a 1.5" dia. aspirin bottle along  the top and bottom of the label).  I then secured the ends of the hold-down wire with more glue. (See below). The straight part of the tube fits snugly into the exhalation valve without leaks, but getting it to go in required a combination of force, accurate alignment, and stuffing in the parts that boinked out. A lubricant might have helped; haven't tried it. Something else to try would be filing the end of the tube on a bevel.

  • Post-submission Field Notes

    David Matthew Mooney10/05/2020 at 18:49 0 comments

    07-16-2021: The intercom and speaker boxes are now fastened together with three 3/8'" dia. metal snaps rather than Gorilla tape. Much snappier! The main full-system picture has been updated.

    07-14-2021: I replaced the volume-control potentiometer with a single-turn trimpot  to keep the best setting from being altered by my accidentally touching the knob: much more convenient. Best setting = maximum loudness free from all distortion. The trimpot was simply glued with epoxy to the underside of the hole the potentiometer used to be in, and after adjustment, was covered by electrical tape. Now, if I need more volume, I'll just talk louder. I also replaced the toggle switch with a miniature slide switch to keep the intercom box from being accidentally powered on and running down the battery. I also made up a custom stereo mini jumper just long enough to connect the speaker box to the intercom box. I run the automatic volume control with the gain of the feedback amplifier turned all the way up (to 100x, because I am using a separate 1-kΩ input resistor, unlike what the circuit diagram shows), via a trimpot access hole (not visible in this shot), which is normally covered by a piece of electrical tape.

    07-02-2021: The project has been mechanically integrated into a one-piece system, and is now much easier to don and doff. Both the mask and the speaker/intercom box fasten behind the neck with the same motion. The cable unplugs and plugs in at the box, which is much easier than at the mask, and allows cable slack to be kept out of the way. Note the repairs to the head-straps. PS 07-05-2021: If I want to wear cabled ear-buds with this rig, I have to run the cables down my back before bringing them around to the front, or the mask tugs on the cables, threatening to pull the earbuds out, which can get pretty annoying.

    04-15-2021 and 06-23-2021: 1) It's looking like: I can repair holes in the mask with a tire repair kit, but not the dry-repair kind: tire-repair rubber cement seems to be required (I used "Certified" brand). For intensely curved areas, a piece of black neoprene rubber seems to work as a patch;  tire-repair patches are way too stiff for these areas. I just followed the tire repair kit instructions. My source of neoprene membrane was a "Gorilla Grip" glove, model C23403.  Use a generous amount of glue. After the repair, consistently performing a suction and pressure test upon donning the mask becomes doubly important. See picture (also note that the design of the microphone board has been improved). PS 07-17-2021: This repair lasted one month in midsummer with frequent outdoor use, then the patch had to be pulled off and replaced.

    04-15-2021: 2) After some use, the joint between the exhalation tube and the mask got loose, so I hot-glued it also, after sanding and cleaning the area (see picture).

    03-25-2021: Intercom with automatic volume control (AVC): Boom! Ready to go into the box. The Flying Spaghetti Monster (i.e., the first build) is getting kicked out. This will become the canonical version of the project if it passes field testing. <PS 03-30-2021: It passed.> <PS 04-23-2021: The AVC upgrade yielded a vast improvement in intelligibility, but speaking clearly and slowly and getting the volume control setting right are still important. I am now a firm believer in AVCs.>

    03-22-2021: The AVC-augmented intercom circuit is finished and working. Will post diagram soon. The blue button cuts in the AVC function so that I can easily tell if this function is working.

    03-21-2021: NB: Today I found a nick in the sealing ring on the right filter cartridge and a smaller nick on the corresponding mask-integrated sealing ring on the opposite side of the same gasket. Have replaced the cartridge with a hopefully intact spare and am crossing my fingers about the lesser nick. Obviously, the integrity of the seals is something you need to check regularly. The giveaways were seeing...

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  • 1
    Build 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. 

    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 section hacksawed out of a PL-3020 push coupling, 3/8-inch OD (Watts or Sioux Chief), with the wide end sanded flat with fine sandpaper. The wide end 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 electret mic leads were not bent before soldering to the mic board, and were clipped short after soldering. This was to facilitate future unsoldering and replacement. I made sure I could push the capsule out of its housing with push-pins inserted through the perf board as soon as the solder melted.

    The battery is nickel-metal hydride 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.

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