The OR gate

The OR gate

The OR gate is the simplest of the logic gates in wires. The picture below shows an OR gate fed by two signals. The gate consists of five copper cells in a ‘plus’ shape. The signals shown are arranged so that all four possible input combinations to the gate occur in sequence. The combination ‘00’ arrives at the gate’s inputs at generation 0, ‘01’ at generation 6, ‘10’ at generation 12, and ‘11’ at generation 18.

It should be clear from the picture how the gate works: essentially, the two input wires are just joined together. However, there is one subtlety: there is what might seem to be an unnecessary extra copper cell making up the left-hand arm of the plus. Consider what happens when an electron arrives at one input but not at the other. Without the extra cell, the signal would propagate through the gate and back up the other input wire. It would then annihilate the next electron it met coming the other way, which is obviously not desirable. With the extra cell, a row of three electron heads appears in the middle of the gate, and so the signal does not propagate.

Next we shall look at an exclusive-OR gate.

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