2-line to 4-line decoder
Here's a circuit that has two terminals with four different combinations of 1 and 0 levels (2x2 = 4).
The 2-line to 4-line decoder we're going to experiment with here can output according to each of these four input levels. With this decoder, we can find out the kind of input applied simply by watching the LEDs.
Wire the project, turn power ON, and see how this decoder works when you press and release S1 and S2. You'll notice that LED 1 lights up when S1 and S2 are both OFF.
Press S2, and LED 2 lights up. Now press S1, and LED 3 lights up. When you press both S1 and S2, LED 4 lights up.
Did you notice that only one LED lighted whichever switch you pressed?
If you didn't, press S1 and S2 while watching the LEDs closely, and you'll see that it's true.