EE40 Summer '04, Lab13

Introduction

This lab is going to be a simple lab about logic circuits. You will use two kinds of integrated circuits: 7404 (4 inverters) and 7400 (4 nand gates). Here are the data sheets: 7400 and 7404.
We are going to build the driver circuit for one segment of a display. A seven segment display is shown in figure 1.

Figure 1

We want to build a driver for S3. We have done this in class already. In the case of three inputs, the truth table for S3 is the following:

a b c | S3
0 0 0 | 1
0 0 1 | 0
0 1 0 | 1
0 1 1 | 1
1 0 0 | 0
1 0 1 | 1
1 1 0 | 1
1 1 1 | 1

Also in class we have computed a minimized version of S3 which turns out to be:
S3 = b + a'c' + ac

Since we have only inverters and nand gates, we have to implement this function using the available logic gates. We note that (using De-Morgan laws):
S3 = b + a'c' + ac = (b'(a'c')')' + ac = b'(a'c')'(ac)'
So the circuit looks like the one in figure 2.

Figure 2

We have used 4 inverters and 3 nand gates. We run out of inverters but we have one more nand gate available. So we can use a nand gate to build an inverter. Also we want to display the result with an LED. We will then connect the output of the the circuit in figure 2 to the input of the circuit in figure 3.

Figure 3

Depending on the power supply, choose R in order to have 5mA in the LED.

Pre-lab

No pre-lab is required. I basically did the pre-lab

Lab

Build the circuit. You need two chips: one 7400 and one 7404. Follow the TA's instruction for connecting power supply etc. Verify it's behavior: verify every entry of the truth table.

Report

Write the usual summary of your experiments and answer to the following questions.
Q1)If you had only 3 nand gates, how would you have connected the LED and the resistor to S3' in order to obtain the same result?