Below are schematics for the "Montara Lighthouse Fresh Air Detector", which displays ambient CO2 levels in a human-friendly format.
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.
The first schematic shows the basic voltage-supply and signal-conditioning circuits as simply as possible:
Here is the same 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, the same schematic labeled for assembly: