Dual 2-input multiplexer
Did you know that a switch that works just like a double pole toggle switch can be made by combining NAND gates?
That's what we're going to build right now. Switches of this kind are used widely for digital signal switching.
When you finish assembling the project, set select switch up, and press S1 and S2 at the same time. This results in LED 1 and LED 2 lighting up.
Now set the select switch down and press S3 and S4 at the same time. LED 1 and LED 2 blink ON and OFF again, but this time they do not light up even if you press S1 and S2. Can you guess what this means?
It means that the A1, A2 circuits and the B1, Β2 circuits are switched over.