I recently had to build an infrared close proximity sensor that could tell if something was dropped in a parcel delivery drop box. After some searching, I ended up using a microcontroller ... the time ...

When an LED (IR or otherwise) is reverse biased it can act as a light sensor ... normally be when using a more common modulated IR receiver module. All of the Arduino code you need to get ...

By reading different values from the sensor, we can detect if is it light , dark or a value between them. Another element that we are going to use on this experiment is Piezo Buzzer ... functions ...

using the average of several measurements, he ends up with a decent result. Happily, [Fernando] has released the code, and it should be easy enough to repurpose it with a more accurate sensor.