I am currently teaching my daughters Digital Electronics, ages 9 and 12. In doing so, I wanted to introduce them to CMOS logic, since I feel that a hands-on approach is always better than just doing theory.
I purchased a few NMOS and PMOS through-hole type transistors intending to demonstrate basic logic gates such as NOT (1 NMOS, 1 PMOS), NOR and NAND (2 NMOS and 2 PMOS). That worked great, and they could reason about why the gate worked the way it does.
Next up is XOR and XNOR, but it stumbled me a bit. I know how to make a circuit using nine transistors, but looking online I've seen 6-transistor and even 3-transistor solutions. However, these assume you can change width and length, so these are solutions meant for ASIC designs, not for through-hole components.
So, my question is – is there a way to demonstrate XOR/XNOR gate logic with off-the-shelf CMOS/PMOS transistors using only a few transistors? Six transistors would be all right, eight slightly worse yet doable.
Perhaps I should just skip XOR/XNOR and move on to latches and flip-flops?
Best Answer
As always, Wikipedia ia a good place to start and here's a CMOS circuit
and 6 transistors is what you asked for. Of course, it's not the most obvious of circuits. I'd recommend that it's time to transition to CMOS ICs, with the 74HC series my preference. You can make an XOR gate from
Since a 74HC00 package conveniently has 4 NAND gates, it's a good place to start. Get them to build up one XOR, and then show that 4 of those fit in a single 74HC86 package, and you're on your way to teaching them about the benefits of integration, starting with SSI (small scale integration).