An ambient light meter

For our first example using analog inputs, I'd propose that you build an ambient light meter: a device that indicates the amount of ambient light it can sense from its environment in some way and shows it in a visual manner, which is a perfect beginning for your next burglar detector.

For the purpose of this example, we will use a very interesting electronic device called a photocell or light-dependent resistor. A photocell is simply a specific type of resistor that varies the resistance it offers according to the amount of light it receives from its environment, exhibiting a photoconductive behavior, that is, lowering its resistance as the light increases and vice versa, commonly ranging between a few ohms when exposed to a bright light and up to some mega ohms when totally in darkness.

In the following picture, you can see a typical photocell and the usual schematic symbols you can find to refer to it:

A real photocell and its most common schematic symbols

Unlike any...