Radio and Electronics Cookbook

59

Digital logic circuits

Introduction

Logic circuits form the backbone of even the most advanced computer, yet their basic operation can be demonstrated by a couple of switches, a battery and a bulb.

Logic using switches

Everyone reading this article will look at Figure 1 and know immediately how it works and be able to write down something like ‘When switch A and switch B are closed, the light will come on’. Without knowing it, you have written down a logic statement involving the so-called AND operation; the light comes on only when switches A AND B are ON. Below the circuit in Figure 1, is a table showing the only possible positions of the two switches and the state of the bulb for each position. This is called a truth table, and is ...