A P100 Mask Intercom

COVID-19 PPE taken to the next level. Anticipated to be good against aerosol transmission.

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This is for all you worker bees out there who already own a respirator. 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 rubber and hard nylon and seals to the face 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 are needed each time the mask is put on. However, you can’t talk and be understood while wearing one. (??? Lately, it seems that you can. Stay tuned.) 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. This pr
  • Post-09/07/2020 developments

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


    Lately, it seems that persons who deal with the public occupationally can understand me OK with the intercom switched off. How could I be so mistaken? I have clear memories of people telling me I sound like I'm speaking Mandarin if I tried to talk (English) while wearing a respirator. I now feel like a fool but I will be looking into this phenomenon. 


    The photos show how I am filtering the exhalation air from a P100. There is obvious backpressure on exhaling, but that just pressure-tests the mask + new filter with every breath. It's not disabling; I can climb 2 flights of stairs without stopping or getting winded. Moderate coughs can be filtered if the straps are tight, but not violent coughs or sneezes, which leak up the sides of the nose. Fortunately, I don't cough or sneeze a lot.

    10/19/2020: The gallery photos have been updated to show the new, tripartite intercom design.


    A) I have decided to redesign the project to mechanically separate the intercom box from the mask and to connect them with a short mono audio cable. This will recover the ability to do a pressure test and to properly clean the mask once in a while. This will also give me more options for thermostatting the circuitry with my body heat in the winter. 

    B) Without the intercom box present, a cloth mask can be worn over the exhalation valve to provide filtration (I have done it for the past two days with my spare P100), and it may seal better to the end of a simple, short tube than to the contours of a human face. Right now, this seal seems dicey because of interference from the filter cartridges, but it would be less dicey with a longer tube that pushes on the cloth mask more firmly. An exhalation-valve extender that friction-fits to the existing valve would make a great 3D printing project, but how long should it be? [20-10-2020: This extender would not be challenging enough to make a good 3DP project. All you need is a tube with an outside diameter of 1.5". I am assuming a model 9200, Workhorse brand.]


    I am still undecided on what approach to use to clean up the exhaled air stream. We are going after one-half micron particles with only exhalation force as the power source, with as little back-pressure as possible, and a compact, rugged design. Options seem to be: filter, Andersen Impactor, cyclonic cleaner, chemicals, and bubble tube. Some impactor effect is probably present already in the two right-angle turns the air must make while escaping.


    Another sub-project will be to thermally insulate the intercom box and the gap between it and the mask. This should reduce the variation in amplification due to changes in the weather that affect the temperature of the electronics. These changes can take you from understandable to not understandable from one day to the next. This can be fixed by readjusting the volume knob, but who wants to do that a lot? Electronic (partial) solutions such as nulling the LM741 would be something else to try.


    I am currently designing a lucite box to go around the intercom-mask junction and direct all exhaled air out one circular opening facing downward. This will immediately allow pressure testing of the mask fit and someday soon, the connection of an exhaled-air filter to the mask. Following the KISS principle, I had hoped that a simple cotton ball stuffed into a tube would suffice for the filter, but no luck. Cotton balls in tubes are remarkably hard to exhale through.


    Since I keep learning new, share-worthy things about this project due to the fact that I am using it every day, I decided that this project is "ongoing," not "finished," as originally advertised.

    The gain in the first stage of the circuit may have to be adjusted. While indoors, I set the volume-control knob to mid-range, then adjust the first-stage feedback trimmer-pot to get best volume + excellent fidelity.


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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.) The enclosure is a Hammond 1593J.

    The microphone board is secured inside the mask with two stainless, 32-8 x 3/4" 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 (lugs salvaged from large microswitches), 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 (taken from banana jacks), 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 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 1.5" square of two layers of film cut from a sandwich bag and held in place with a 7/16" O-ring. Before the film was put on, I glued a 5/16" dia × 1/16" thick spacer ring (improvised from a small split ring sold for keychains) on top of the microphone capsule with tire-repair rubber cement. Without this spacer, reproduction quality is unsatisfactory. After pushing the O-ring all the way down, I pulled it back up a little to give the film some slack; this also seems to help with sound fidelity.  

    The battery is lithium-ion type, 9V. It connects to the intercom box with a 2.5"-long adapter (Kitronik 4132, Sparkfun 09518, or similar) that plugs into a 5.5 × 2.1 mm, center-positive barrel jack on the box. The power seems to be good if I can hear a harsh noise burst on the "s" and "sh" sounds. (I understand that this is caused by the unavoidable nearness of my mouth to the mic.) If you do not hear this, try a new battery; this can markedly improve sound fidelity. The intercom box is a Hammond 1593K, contrary to what it says on the circuit diagram.

    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.

    The cabling is mechanically polarized to prevent mix-ups between the mic and speaker connections. A small, removable adapter connects the external signal lugs on the mask to the mini jack. This was a safety precaution. If something snags one of my cables, I want the cable to pull away instantly and not take my head with it. All plugs are therefore straight, not right-angled.

    THIS EXACT BUILD IS STILL BEING FIELD TESTED AS OF NOW (27 OCT 2020). However, it is based on a design that was field tested for 6 weeks and uses the same electronics. 

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