Logic gates, visualized.

ComputersAug 2021

The three most basic building blocks atop the electrical signal are the logic gates. At its simplest, a logic gate is a device that mediates the flow of current from the source to the ground. This device is graphically expressed where the letter (a) is positioned in the diagram below:

A gate is made of transistors that take in an input and produce an output. If the lightbulb is on, that means we have a positive output, a truth, a one. If the lightbulb is off, we have a negative, false, or zero output.

Buffer Gate

Take the simplest gate, the buffer gate. In order for the lightbulb to turn on, the input needs to tell the transistor to conduct the current to the ground. If the input is (1), the current will flow and the output will also be (1) – aka. the lightbulb will be on.

NOT Gate

The next simplest gate is the NOT gate. The NOT gate inverts whatever input it is given. If the input is (1), it allows the current to flow. Given that electricity always chooses the path of least resistance, it takes the shortest path down through the transistor. As a result, it will not go through the light bulb and the input will be (0). On the other hand, if the input is (0), the current is forced to take the longer path and therefore turn on the light bulb.

AND Gate

Moving on to the AND gate. Like the name indicates, the AND gate needs both the first AND the second input to be (1). If ant of its transistors is (0), the current will simply not flow through; producing a (0) output.

OR Gate

The OR gate only needs one of its inputs to be (1) in order for the current to flow. As long as one of the inputs is (1) the current flows through and the output is (1).

Truth Tables

These are the most simple operations in boolean logic: AND, OR, and NOT. These are commonly represented using the following graphical system and equivalent truth tables.

Finally, it's worth noting that even though AND and OR are expressed as having two inputs, they can actually have more than two inputs. That being so, their logic remains the same.

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