TFA Dostmann make an “Airco2ntrol mini” which is USB powered (and you can pull the readings over USB too) and has a nice little red-amber-green light system on

TFA Dostmann make an “Airco2ntrol mini” which is USB powered (and you can pull the readings over USB too) and has a nice little red-amber-green light system on the front (as well as the LCD) for easy reference. It’s about £85. It has a dual NDIR sensor (not sure which one exactly) which the article says are usually the most accurate type of consumer-grade sensors. I’ve had mine for years, very pleased with it… just sits on my desk and lets me know when it’s time to open the windows.