### Logic Gates

#### Diodes

A device that can control the direction of the flow
of a current is a diode; it is made of semiconductor materials such as silicon.

As you can see, the position of the diode can turn the current on
or off.

That means, a diode can be used as a one way switch: if the wider end of
it faces a positive terminal of a battery, it lets current through or else it
cuts the current off.

Based on this system, a couple of switches are made
and they are called **logic gates.**
The input of these gates can be combinations of **'On'** or** 'Off'**
states and they are known as **'1'**s or **'0'**s. These are binary digits, that has been the
basis of the ** Binary System.** The output can also be**
'0'**s or **'1'**s. That means, we can
express the output of these systems in terms of**
'1'** and **'0'** - binary digits.

In
other words, the output of the logic gates can be used to interpret the numbers
in binary form. This is the birth of computer systems; they recognize only** '0'** s and **'1'**s.

The following animations show the major logic gates,
their input's and output's.

#### Logic Gates

**OR Gate**

Truth Table |

Left | Right |

Top | Bottom | Output |

0 | 0 | 0 |

0 | 1 | 1 |

1 | 0 | 1 |

1 | 1 | 1 |

**AND Gate**

Truth Table |

Left | Right |

Top | Bottom | Output |

0 | 0 | 0 |

0 | 1 | 0 |

1 | 0 | 0 |

1 | 1 | 1 |

**NOT Gate**

Truth Table |

Input | Output |

0 | 1 |

1 | 0 |

**NOR Gate**

Truth Table |

Left | Right |

Top | Bottom | Output |

0 | 0 | 1 |

0 | 1 | 0 |

1 | 0 | 0 |

1 | 1 | 0 |

**NAND Gate**

Truth Table |

Left | Right |

Top | Bottom | Output |

0 | 0 | 1 |

0 | 1 | 1 |

1 | 0 | 1 |

1 | 1 | 0 |

#### Practice Exercises

Construct a truth tables for each the following circuit of logic gates; then, check them with the corresponding animation that follows:

**E.g.1**

Since there are **three** inputs, the possibilities are, 2^{3} = 8. Here is the animated answer.

**E.g.2**

Since there are **four** inputs, the possibilities are, 2^{4} = 16. Here is the animated answer with some of the possibilities - not all.

**E.g.3**

Since there are **three** inputs, the possibilities are, 2^{3} = 8, in this case too. Here is the animated answer with all the possibilities.

#### Uses of Logic Gates

Logic gates are used in almost every electronic
device today that we take for granted; the following animation shows some of the role
played by them in the things that we use everyday.

**Washing Machine**

Have you noticed when the motor is turning on? the
right temperature, right water level and of course, the closure of the door -
for obvious reasons.

**7-digit Decoder**

This is how a calculator displays the digits.

Do you want to practise what you learn? Here is a book for you: