A bit of thought led him to build a reflective sensor which can measure ... drop in intensity is picked up by the phototransistor. [Arao] made the system rock-solid by rolling an LM358 op-amp ...

All it uses is a TSL261 Light-to-Voltage sensor connected to an Arduino with an Ethernet shield ... something like the Pinoccio microcontroller system can allow for devices to be turned off ...