2. Introduction to Logic Gates

Description: 

In this experiment we are going to examine logic gates such as the AND gate and implement a few different circuits on our breadboards.

Note: The full set of videos is presented on the YouTube channel: DerekMolloyDCU

Introduction

In your kit there is a huge selection of ICs. This video discusses the different packaging of ICs and then examines the use of a few ICs to create a basic logic system. The ICs in your kit are fairly delicate - please take the following steps when wiring your circuits:

• • Do not connect the power until you are finished wiring the circuit

  • Check the pin diagram carefully to make sure that you are connecting power to the correct pin and that you are grounding the correct pin.

  • Avoid touching the pins in case that static discharge from your hands damage the ICs.

  • Make sure that the legs are positioned exactly over the pins of your breadboard before applying pressure.

  • Make sure that you power the chip!

Background

We are going to use the "74" family of chips for this experiment. This family includes the 7408 AND gates as can be seen in figure 1. This figure illustrates the various packages that the IC is supplied in. We are going to use the DIP package (as seen in a), which usually contains an 'N' at the end of the identifying IC number. I have provided an image of the SOIC version for comparison - most of the space inside such an IC is taken up by the wires that connect the pins to the die. The die itself is tiny - as can be seen in Figure 2. Press on any image for a larger version.

 (a)

 (b)

 (c)

Figure 1. (a) The 7408 (Quad 2 input AND gates); (b) A close up of the SOIC package in the video; (c) The SOIC package with an adapter - (Click on the images for larger versions)

Figure 2. Illustrates the inside of an IC like the 74 series, which has been cut down to expose the die itself.

A NOT gate using a single Transistor

First we create a basic NOT gate using only a single transistor and a few resistors. This demonstrates a very basic implementation - the actual implementation involves several transistors; however, this should be sufficient to demonstrate how a gate could be created using transistors.

First Attempt at wiring an AND gate in a DIP package

In this video I discuss the different packaging types for our ICs and then I wire up a 7408 (Figure 3 below gives the pin diagram for the 74 series ICs we are using here) using some LEDs and a couple of push buttons. It doesn't work - but why?

Figure 3. Some of standard 74 series gates that we are going to use in our experiments (from the notes used)

Fixing the AND gate circuit

This video shows how to address the problem in the previous video by using the push buttons in pull-up and pull-down configurations. 

Question for you:

Question: Implement a 2-input AND gate using NAND gates only and implement a 2-input AND gate using NOR gates only. Do out a truth table for an AND gate and determine what configuration is required to get the same logical outputs. Hint: you will require more than one gate in each configuration. You should not mix NAND and NOR gates, rather design two independent circuits that both implement a 2-input AND gate. The pin diagram for the 7400 (NAND) and the 7402 (NOR) gates are given at the top of Figure 3.

Solution: The video below gives my solution to this question.