  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.
