To summarize, infrared close proximity sensors are an effective alternative to some other electronic sensors and mechanical switches in order to detect when an object has been dropped at a given ...

The part list consists of one Arduino, two resistors, and one IR emitter. That’s right, an emitter. When an LED (IR or otherwise) is reverse biased it can act as a light sensor.

If you believe that a PM2.5/10 sensor can also detect O1, NO2, SO2 then go right ahead and market it as an “Air Quality” sensor. I look forward to the law suits.