NOT gate using a NOR gate: two ways, live
A NOR gate becomes a NOT gate in two ways: tie both inputs together, Y = NOR(A, A) = NOT A, or tie one input to 0, Y = NOR(A, 0) = NOT A. The live circuit below builds both side by side.
NOR outputs 1 only when both inputs are 0. With both inputs joined to A, that happens exactly when A = 0, so the output is NOT A. With one input held at 0, that input never blocks the output, and the gate again outputs 1 only when A = 0. Note the difference from the NOT gate using a NAND gate: a NAND's spare input is tied to 1, a NOR's to 0, because each must be held at the value that lets the other input decide the output. Electrically, tying the inputs together doubles the load on whatever drives A, while tying one to ground leaves A driving a single input. This inverter is the building block for every other gate made from NOR: the OR gate from two NORs is a NOR followed by one of these, and the AND gate from three NORs inverts both inputs first. All of them side by side are on the NOR universal gate page.
Open this circuit in the lab →
What you need
- 1× NOR: Y1 = NOR(A, A): both inputs tied to A
- 1× NOR: Y2 = NOR(A, 0): one input tied to ground
Step by step
- Place an input
Aand probesY1andY2. - Wire A to both inputs of the first NOR gate and probe its output as Y1.
- Wire A to one input of the second NOR gate and ground (0) to the other, and probe it as Y2.
- Toggle A: both outputs are always the opposite of A.
NOT from NOR truth table
| A | Y1 = NOR(A, A) | Y2 = NOR(A, 0) |
|---|---|---|
| 0 | 1 | 1 |
| 1 | 0 | 0 |
Both input rows are checked on the live circuit before the page is built. In the full NOR truth table, these are the rows A = B = 0 (output 1) and A = B = 1 (output 0) for Y1, and the rows with B = 0 for Y2.
You can also read the full theory in the lesson on the NOR gate. Prefer to build it yourself from scratch? Open the digiwleea lab and place the gates on the canvas, or start from the logic gate simulator.
More builds: Universal gates: NAND only, NOR only
All 246 build guides, by topic
Each of these is one step from a transistor to a working CPU. Build every gate from CMOS transistors and work all the way up in the free digiwleea lab, with the free theory reference alongside.
Open the NOT gate from a NOR gate in the lab →Frequently asked
How do you make a NOT gate using a NOR gate?
Connect both inputs of the NOR gate to the signal, or connect one input to the signal and the other to logic 0 (ground). Either way the output is the inverse of the input.
How many NOR gates make a NOT gate?
One. A single NOR gate with its inputs tied together, or with one input held at 0, is an inverter.
Why is the spare NOR input tied to 0 and not 1?
A 1 on any NOR input forces the output to 0 whatever the other input does, so tying it to 1 gives a constant 0. Tied to 0, the spare input has no effect and the output is NOT A.
Why is NOR called a universal gate?
Because NOT, OR and AND, and therefore any logic function, can be built from NOR gates alone. The NOT gate made from one NOR is the first step: OR is a NOR plus this inverter, and AND is a NOR of the two inverted inputs.