The problem with simple transistor circuits is that any circuit with a transistor in it isn’t all that simple. And frankly, I don’t know how much you know about circuits; I’m guessing it ranges from “nothing at all” to “teach your grandmother to suck eggs why don’t you.” At the risk of boring the latter crowd we’re going to give this a slow and superficial treatment. Let’s start with a circuit that’s just about as simple as I can make it. So simple it doesn’t even have a transistor in it!

I’d make this circuit more interesting but I don’t know the symbol for ‘electric chair’.