Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

## Thursday, 3 November 2011

### Simple Logic

Imagine a field, with sheep in. But don’t count them, I don’t want you falling asleep! The field has a fence, and the fence has two gates. If we’re trying to keep the sheep in, we need to make sure that both the gates are closed! Simple.

Now change the field for a circuit that looks like this:

So, you can see that the sheep, I mean the electrons, can only flow around the circuit (rather than escaping? electrons don’t do that…I see my analogy has fallen apart somewhat) when both gates, or switches, are closed. And when this happens, the light goes on.

This is the simplest form of logic system, and the different outcomes we can expect are written as a truth table. We can build a system to test this, or just use our brains, because we know that for a current to flow, we need an unbroken circuit.

Truth tables, despite having such a friendly name, can be a little confusing, because instead of using OPEN and CLOSED for the gates, they use 0s and 1s. 0 means that the gate is open, think of it as no electrons getting through. And so 1 means that the gate is closed. When it comes to the output, 0 means NO LIGHT in our bulb, and 1 means LIGHT.

 Gate A Gate B Output 0 0 0 1 0 0 0 1 0 1 1 1

This is the simplest logic gate that you can make, but more complicated gates exist when you want your electronic circuits to work in a more complex way…I’ll be adding another instalment on logic soon!