# Building a 3-input NAND or NOR gate with 2-input NAND or NOR gates - how do I approach questions like these?

For NAND, I am doing a truth table for it and then truth tables for all the possible combinations, but as you can see the process is very long and I am still yet to get an answer. Same goes for NOR gate. Any help is appreciated. (Answer welcome :P)

As equations.

$\overline{ABC} = \overline{(AB)C} = \overline{\overline{\bar A+\bar B}\cdot C}$

• Wow that's cool – studious Jan 16 '16 at 21:58
• It's also something that should have been covered in Boolean Algebra 101. – Ignacio Vazquez-Abrams Jan 16 '16 at 21:59
• First week of the semester and so on – studious Jan 16 '16 at 22:00
• Though to build that from NAND/NOR gates would take four gates in total. It can be done with just three gates. Notice that the $(AB)$ is a 2-input AND gate, which is equivalent to $\overline{\overline{AB}}$ which is a 2-in NAND gate followed by an inverter (another 2-in NAND with both inputs tied together). So $\overline{ABC} = \overline{\overline{\overline{A \cdot B}}\cdot C}$ – Tom Carpenter Jan 17 '16 at 3:53
• Is there any other way to do this other than this? – studious Jan 18 '16 at 14:02

An alternate approach is given.

simulate this circuit – Schematic created using CircuitLab

The schematic represents the function: $$Y = \overline{ABC} = \overline{\overline{\overline{AB}} C}$$

• How did you find Step 2 from Step 1? – studious Jan 18 '16 at 14:02
• @studious $Y=\mathtt{nand(A,B,C)} = \mathtt{not\{and(A,B,C)\} = \\ not\{and(and(A,B),C)\}=nand\{and(A,B),C\}}$ – nidhin Jan 18 '16 at 16:34

Note that in the above answers, there are three propagation delays for A and B, but only one for C. Therefore, if propagation delays matter, put two nand gates in series in line C, wired as inverters.