Extra-concentrated carbon dioxide marks air which has already been breathed, so if that number is low enough the air must be safe from respiratory contagions. Below are schematics for the "Montara Lighthouse Fresh Air Detector", which displays ambient CO2 levels by flashing LEDs in a "neurosafe" way optimized for the human attentional system: slow, dim green is "safe," fast bright red is "warning."
Here is a couple-minute video of it working.
These circuits are released under the Neurosafe Moral License, which allows free copying, use, and construction for educational and personal/community use. The probably-unenforceable moral license asks that for any Neurosafe product sold or rented, ten percent of the purchase price be held in escrow for donation to an organization dedicated primarily to defending the human nervous system.
Schematics and assembly pictures are shown directly below, suitable for experienced hobbyists, with a parts list at the end. I would welcome a cleaner, clearer set of instructions suitable for kids, so they could build air-safety monitors for their own families.
Above: The first schematic shows the basic voltage-supply and signal-conditioning circuits as simply as possible:
Above is the same supply/conditioning schematic, now with names for the components (for the parts list) and breadboard socket rows (the breadboard's left column of thirty five-pin rows is labelled a1-30; the right column is f1-30).
The second schematic shows the five-axis flasher circuit as simply as possible. The Lighthouse contains two such circuits: on to show the sensor level S+, the other to show its derivative dS/dt.
Again above, the same schematic labeled for assembly.
Below are several pictures of assembled circuits, enough redundancy to make sure they're all the same, and all visible.
Here is how to make a "lighthouse" topper from foam-core:
Parts List
| R1 | 1k |
| R2 | 1k |
| R3 | 1k |
| R4 | 5k |
| R5 | 1.5k |
| R6 | 470k |
| R7 | 470k |
| R8 | 10k |
| R9 | 10k |
| R10 | 100k |
| R11 | 1M |
| R12 | 100k |
| R13 | 100k |
| R14 | 5.1k |
| C1 | 1 uF |
| C2 | 4.7 uF |
| C3 | 4.7 uF |
| C4 | 4.7 uF |
| D1 | Shottky diode |
| T1 | 2N3904 NPN |
| T2 | 2N3904 NPN |
| T3 | 2N3906 PNP |
| T4 | 2N3904 NPN |
| Sensor | MH-Z19 carbon dioxide sensor |
| NOTE: Parts, e.g. from Amazon, can be fake. | |
| Timer chip | NE555 |
| Quad OpAmp | LM324 |