And Logic

Last time we talked logic we looked at switches and the way that input and output were related by the opening and closing of the “gates” – see this here.

In conventional logic, things are a little more complicated. You can think of a logic gate like a gatekeeper to a fort, where you have to meet the requirements to be allowed to pass. If we take an AND gate, the most simple gate, you can think of it in terms of taking offerings (or bribes, if you want to look at it that way) to the person on the gate.

If you bring something in your right hand AND something in your left hand, you will be let through by the greedy gatekeeper. And it’s the same with these gates. The only thing is, they don’t accept bribes. But they do love electric current. So a high electric current through both inputs should do the trick.

So we can again write a truth table, so that we can see the outcomes of all the inputs. Rather than switch on and switch off here, we’ll use low current and high current. Low currents aren’t any good for bribing with, so they are sometimes also denoted with a 0, and high currents with a 1. Outputs can be written in either form as well, but we’ll use high for a successful outcome.

Input 1	Input 2	Output
low	low	low
high	low	low
low	high	low
high	high	high

You might notice that these are the very same outcomes as for the two switches in the circuit we used last time!