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 and a piece ...

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.

When the Pi is shut down in software, the Arduino can sense that the Pi isn’t on any more and disconnect the power. The remote control is used to turn the Pi on.