CO2 "Clock" Meter
Last week I met with some folks (business, government, research) who are interested in monitoring CO2 levels in their businesses, government offices, and schools. The reason has to do with measuring ventilation in crowded conditions and relates to COVID-19. The idea is that if exhaled CO2 can be adequately vented out of a space, any virus-laden aerosols will be vented out, too. Human exhaled breath consists of 4% (40,000 ppm) CO2. The normal outdoor level is closer to 400 ppm. By measuring CO2 levels in a closed space, such as a classroom or restaurant, one can get an understanding of the ventilation system's efficacy.
Anyway, I took my little Air Quality Monitor with me as a demo. It was favorably received, but there was some discussion about a unit that could be read at a distance. I suggested something with a clock face and a pointer that could be read from afar. The group liked the idea, so I promised to build one. Here's the result, next to the smaller meter:
The bezel and lens came from a wall clock I bought at the local hardware store. I really lucked out: it was nearly perfect for this project. I decided to use a Parallax Servo360 to drive the pointer, due to its spot-on accuracy and repeatability. The bezel was not deep enough to accommodate the servo in a direct-drive mode, so I came up with a way to mount it sideways, which I posted about here:
I was able to use the same PCB I designed for the small AQ Monitor, which you can see in the guts view here:
It just lacks the TFT display and substitutes a pair of servo headers for the push-button switch. The faceplate is 1/8" MDF with a color-printed dial attached to it. The faceplate, with 12 spacers, is held in place against the glass by four spring clips that came with the clock. You can see one or two behind the vertical supports for the back panel.
Everything is controlled by a Propeller FLiP module, which plugs into the rear of the PCB:
I had to modify the FLiP by removing one of the two layers of insulation from its pins, so the pins would be long enough to reach the connector contacts on the other side of the PCB.
The back panel fastens to its supports with four screws and includes a hang hole and a guide for the USB cable:
This has been a very satisfying project, and I'm anxious to demo it for the folks I met with. First, though, I need a different USB cable -- one with a right-angle micro-B connector, since I did a rather poor job of positioning the PCB, and a straight cable won't fit. I'm also getting some white acetal sheets to cut the pointer from. The one in the photo is black with white paper stuck to it.