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    Build details for mask intercom

    I bolted the project box to the exhalation valve with four 1.5-inch Robertson-head machine screws, 6-32 thread, 9/64” dia. These same screws fix the microphone board to the inside of the mask, fix the main circuit board inside the box, and provide conductors for the mic. I used strip-board for both boards. The screw heads were on the main-board end to conserve space, but this decision forfeits access to the printed conductor side of the board for post-build modifications. A wiser decision would have been to use 2”-long threaded rod with nuts on each end instead of screws, and half-inch nylon spacers between the board and the nuts to lift the latter high enough to clear the components. The same trick was used inside the mask to deal with a lack of flat spots on which to seat two of the nuts.

    To avoid loss of continuity with the mic due to corrosion, use a nut, connector coating (MG Chemicals), and rubberized-fiber washer, in that order, under the mic board on each of two screws, and apply anti-oxidation paste to the mic lugs (on top of the board) before tightening down the top nuts. Then seal with more connector coating. Use two generous coatings, with drying between, to make sure no water can get in.

    The centers of the bolt holes in the bottom of the intercom box form a 1.2" × 1.6" rectangle. I glued a cross-shaped spacer of ¼-inch acrylic to the box with epoxy to provide an escape route for the exhaled air. The bolt holes in the mask were carefully sealed with silicone caulking after assembly so that the mask continued to pass the suction test. After drilling, the mask was fully disassembled (a challenging task the first time, requiring the use my two largest jeweller's screwdrivers at once to remove each retaining ring) and cleaned to remove dirt and turnings that might otherwise cause air leaks. The speaker is protected from disinfection solutions by plastic film. I disinfect with 70% isopropanol spray. All exposed metal is protected from corrosion with connector coating. 

    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 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. In use, the battery just hangs from the short adapter. When not in use, I unplug the battery plus adapter and put it in my pocket. 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.

    Another 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. My build mitigates this by placing the intercom box over the exhalation valve to deflect air sideways into a cloth face-mask folded twice lengthwise and tied around the “waist” between the box and the mask. The cloth covers only the top and sides and is tied under the bottom. Unfiltered exhalation air is therefore directed downward, as with a face shield. This keeps the exhalation back-pressure low so as to avoid impairing understandability (otherwise, the nasals seem to drop out).

    A problem I am living with is that there is now no way to perform a pressurization test upon donning the mask; I have to be content with suction tests only. (An obvious solution would be to replace the cross-shaped spacer with a ring-shaped spacer, and leave a gap in it at the bottom that can be stopped with a finger. Ideally, the spacer should be designed to fit a future aerosol-removing means.) I have puffed finely ground stevia sweetener around the mask while inhaling through my mouth, with good results–almost no sweet taste. Huge taste without the mask.