Post-submission Field Notes

A project log for 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

David Matthew MooneyDavid Matthew Mooney 10/05/2020 at 18:490 Comments

08-14-2021: I replaced the black, 2.5-mm mic cable with a blue, 3.5-mm cable for better part availability and better visual contrast with the black straps.

08-07-2021: P100 masks have reappeared on the shelves of my local builder supply store. $60.00 CDN, $48.00 US.

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 a streak of dust on the gasket and hearing a difference in the sounds the two sides made when inhaled through individually.

03-12-2021: Coming soon: automatic volume control.

02-20-2021: I have successfully field tested a version of the intercom system that has the speaker box and intercom box taped together for convenience. Weight: 315 g. There is now only one cable that matters, which improves safety by reducing the opportunities for cables getting caught on things, and there is now no possibility of getting two cables mixed up when connecting the system. The photo gallery has been updated.

02-07-2021: Build details of microphone board: The plastic sheath around the microphone capsule was reinforced and waterproofed at the base with hot glue. Here is what the conductor side of the board looks like, to save you some time if you decide to build the project:

The small holes are on 1/10" centers. The large holes are 9/10" apart. Sound reproduction quality is sensitive to the position of the mic capsule inside the mask. A previous build placed the mic slightly higher and had worse essing.

01-09-2021: Flared exhalation tube, January experience

The backpressure from the exhalation is still too much under winter conditions when the cloth mask can get wet from condensate after about 15 min of use outdoors. Using the next-larger size of funnel (having a diameter of 8 cm rather than the existing 6 cm) to create the flared section seems advisable. However, I have not tried this yet and it's not clear that a typical cloth mask would be big enough to go over it. (With a larger flared section, additional hold-down points will also be necessary, probably four in all.) PS 01-13-2021: A blue, flat-type mask works well with an 8-cm dia funnel. Make and model tried: KATA KTD003. PS 01-23-2021: This type of mask seems not to block up with condensate as badly as the others I have tried. Therefore, I am undecided if a larger funnel is necessary. PS 02-06-2021: It' s looking like: borderline "not necessary" with the cited make and model of fabric mask but take some spares with you when you go out. 


Whenever I leave the house, I take with me a 500-mL spray bottle of hand sanitizer (not the goopy kind), which dispenses a generous volume of liquid at each squeeze of the handle. Outside, I promptly sanitize my hands, mask project, or purchases whenever I am the least bit doubtful as to their safety. PS 04-23-2021: I also carry a length of paper towel in my pocket to deal with episodes of itch torture. The paper towel is carefully folded to keep a non-contaminated surface inside. PS 05-16-2021: My spray bottle is now a homage to WWII bomber nose art. Medium: marker pen with a covering of scotch tape. ("Lady Luck" isn't on there because I didn't have the necessary skill.) 


The mask is now sufficiently comfortable to double as a balaclava on cold days. Haven't tried it as a balalaika or baklava. But interesting.


I added a flared section to the end of the exhalation tube to decrease the back pressure during exhalation. The result was a great improvement in comfort and convenience, but I fear I will not look very fashionable. The flare was made from a 6-cm dia plastic kitchen funnel with the stem cut off. The seam was sealed with a glue gun. This modification has been tested indoors but not outdoors, so stay tuned. PS 12/18/2020: Tested the project outdoors today in -4 deg C weather and no problems. Very little condensate formed inside the mask, probably due to better ventilation. This project is looking done.


The adapter between the external lugs on the mask and the patch cable has been redesigned for better durability and to allow semi-permanent attachment to the mask. The gallery has been updated to show the new adapter in place and below is a template that can be used to fabricate it. 


If you are in a high-stakes situation like asking a pharmacist for a specific pharmaceutical, do not use the intercom; write your message down and press it on the transparent barrier for the person to read. (I carry a pocket notebook and pen, which actually have many uses when out of the house.)


I have a tendency to lean in to my listener if I am not being understood while using the intercom, which breaks social distancing rules. Turning down the volume usually works better because that gives less distortion. I have to remind myself that the intercom is not a bullhorn, and it is not karaoke, it is just a voice replacement and it doesn't have to be any louder than my normal voice.


I am treating all condensate that forms inside the P100, which is plenty, as if it were full of SARS-CoV-2. Options seem to be: after use, drain mask with due care, blot the inside, dispose of the wet paper towel carefully, spray mask inside and out, and hands, with A) 70% isopropanol, or B) 99% isopropanol.


We have integration!

And the first field test thereof: A trip to the local cop shop by public transport to file a crime report ("No incident should be considered too small to report."), but that is a story for another day. My thoughts:

1) Intercoms are not magic. It seems that even the commercial units have limitations. When using this intercom, you need to speak up and you need to speak slowly and clearly. I understand from reddit that Walmart employees should do the same.

2) Now that the cold weather is here, condensate is rapidly forming on my cloth mask, making exhalation difficult. When that happens, I just slide the mask up or down to put a dry spot over the end of the exhalation tube. Five minutes later, it's time to shift it again. This is only an issue in cold but crowded places like bus shelters, where I can't simply remove the outer mask.  We'll see what a January in this town throws at me.


The tube for the exhalation valve kept falling out, so I fixed it as shown. The white blob is putty, which I added to prevent any air leakage. (The blob on the near side cannot be seen in this shot.) The wire is 20-gauge, tin-plated copper (header wire). Holes were drilled with a 3/64" twist drill. ( But see the "Clarifications" log.) I am solving the understandability problem and the exhalation-filtration problem on separate masks, and will eventually integrate the results.

When my doctor saw me wearing the dual mask arrangement, he said, "I like your mask." Twice.

I got a COVID-19 exposure alert on my phone this month and had to go and get tested, then self isolate until the results were ready. Result: Negative. Thanks to Bill & Sal for bringing me groceries.

I have tried 3 different cloth masks with the exhalation tube. Two were OK but the third offered too much resistance to airflow. One needed a rubber band to cinch it around the tube, two didn't.


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. PS 11-02-2020: False alarm. People can only understand me while I wear a P100 mask with the intercom switched off if I am saying something they hear every day, such as "Bag with that," or "Medium, double-double." If I try to say something unusual, such as "What do you think of Tolkien?" I am likely to hear: "Couldn't say; I've never toked."


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. PS 01-17-2021: After enlarging the mask area 2.6-fold  with a 6-cm diameter flared section, I did not notice an obvious improvement in cough containment, in the field, in January.


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? PS 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. Ideally, the tube should be made of the same plastic as the valve, to prevent the friction fit from loosening due to differential thermal expansion/contraction due to seasonal changes.


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? PS 01-17-2021: better yet, use modern low-drift op amps. 


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.


Spend some time getting the volume right; more is not always better if it leads to distortion.  The word "hour" is especially hard to get out clearly, but if you can, you're good to go. 

Reproduction fidelity continues inconveniently low under adverse but common conditions of background noise. Can someone point out a way to modify my circuit or physical build for higher fidelity? Am I flogging a dead horse in sticking with analog?

When you go out in this mask, take a conventional cloth mask with you in case of a malfunction or an encounter with a situation where only conventional masks are permitted or a situation of too much background noise. 

Op amps are temperature-sensitive; expect to have to turn up the volume on hot days to be understood and to have to turn it back down again on cold days to avoid feedback.


 I added a bypass capacitor to the LM386 chip to increase the amplification ten-fold, which is essential for dealing with noisy environments with even spotty success. The circuit diagram has been updated.