This Reflective IR Sensor is a simple plastic casing with two elements - an IR LED and an IR phototransistor. You can control the IR LED and turn it on to bounce IR off objects to determine their reflectivity. White & light colored stuff will bounce the light, so  you can detect it. Black & dark colored stuff will absorb the IR light so that it isn't detected. Likewise, if something isn't in the way of the sensor, it won't trigger either. These sensors are handy when you want to detect if something has passed by the sensor, or a very...