Measuring the capacitor charge and discharge with a LED and a smartphone

In this article, we present a simple, inexpensive, and effective method for measuring the capacitor charge and discharge processes using a light-emitting diode (LED) and the light meter of a smartphone. We propose a simple circuit in which the LED's brightness is linear on the capacitor's voltage, allowing us to use the smartphone to monitor the capacitor state accurately. The method is tested experimentally, giving highly satisfactory results. Its exceptional combination of accuracy, minimal requirements, and ease of setup makes it an excellent way to introduce undergraduate students to the concepts of electricity and electronics in any educational setting.