You can get cheap NDIR based sensors from aliexpress (around $20) such as MH-Z14...

		anfractuosity on June 24, 2019 \| parent \| context \| favorite \| on: Ask HN: What do you do with your Raspberry Pi? You can get cheap NDIR based sensors from aliexpress (around $20) such as MH-Z14A, which I've used with a Pi, via the UART interface. I just hooked it up to the 5V line on the Pi pin header and then used the Pis UART lines for the data, as that runs at 3.3V.