classroom concentration warning / CO2 sensor / display with USB-C and wifi

Simple CO2 sensor with bright color display to help ventilate classrooms. Can be used to improve concentration performance of its inhabitants, and to help reduce risk of distribution of airborne life-threatening phenomena. The concept is if the display turns yellow or red one should ventilate the room (open window, or ‘raam’ in dutch, hence ‘operame’). 

Board kit with dead-simple throughhole soldering components, incl fragile and quite sensitive plug-and-play NDIR-based CO2 sensor sold (low cost) by https://shop.operame.nl/home/25-operame-kit.html

Hardware

The hardware is based on a TTGO T-Display and MH-Z19 CO2 sensors or variants thereof. The sensor operame uses can be bought from several sources. Schematics and operation can be found at link below. A regular USB-A to USB-C cable and simple ikea USB-A charger is enough.

Using and placing the sensor 

Best to keep the completed sensor assembly on a remote place like on top of a cabinet, where it can see air but is not too close to humans. Do not place the sensor on a desk, humans breathing out CO2, an thus elevating the displayed 'average' CO2 level in the room prematurely.

https://operame.nl/Hardware

Software

Open source, see https://operame.nl/Software

The software user-interface is very limited and in normal operation just the PPM is displayed in bright colors, with different background depending on level:

• White: general message during boot etc
• Red: error
• Yellow: Warning, too high level
• Green: Good Co2 level, no need to open a window
• Blue: wifi accespoint

Operation of sensor

After first turn-on, one might have to wait a few hours or sometimes even days to stabilise to CO2 base-value. The firmware in the actual sensor (hosted under the sensorcap) apparently needs a regular evacuated room state, where the base level is recorded and is used to determine elevated levels later on a day. The firmware on the TTGO display module just reads out the sensor, displays it and optionally exposes a wifi interface (use the two buttons).

Workings of NDIR CO2 sensors

There are several ways to measure CO2. For the very low levels needed to measure human concentration ability and background, the levels are in units of PPM, which stands for parts-per-million. Typical value outside is around 350 to 380 ppm. Indoor levels might be higher, above a thousend it will get really problematic.

It is quite hard to measure such low levels reliabity, some sensors are just not suitable for levels below 500 ppm. More information on https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor