Experimental characterization of off-the-shelf LEDs as photodetectors for waking up microcontrollers

The use of LEDs as photodetectors has been subject of study for a long time. However, LEDs manufacturers do not provide information about the response of LEDs to the light. It is essential for the designers of electronic devices to know what the behavior of LEDs is so as to develop new applications in this area. In this work, we propose the use of commercially off-the-shelf LEDs, illuminated by the flashlight of a smartphone, as a contactless wake-up system for microcontrollers that are in a sleep mode. In particular, the generated LED voltage is proposed as the wake-up signal. So, in order to choose the most suitable LED and depending of the main interfering light, the currentvoltage (I-V) characterization of different kind of LEDs is performed for two scenarios: outdoors and indoors. It is demonstrated that placing a resistor in parallel with the LEDs allows a better discrimination between the LED voltage coming from the flashlight and that from the interfering lights. Results show that red and blue LEDs are more appropriate for outdoor and indoor applications, respectively.