LAB #6: SEVEN-SEGMENT DISPLAY

OBJECTIVE

Build a circuit that converts a BCD value into a 7 bit output that can be used to display the decimal equivalent on a 7 segment display decoder.

MATERIALS

· Digital prototyping board

· 74xx138 Decoder IC

· 74xx21 AND

· 74xx20 NAND

· 74xx11 AND

· 74xx10 NAND

· 74xx04 NOT

· Common anode 7 segment display

INTRODUCTION

Often, numbers must be displayed by a computer for the user to see. However, humans are not used to reading the binary numbers that computers work with. Therefore, it is beneficial to output values in decimal. A 7-segment display is a popular form used to display a digit. This lab will have the student build a circuit that takes in a binary number from 0-9 (BCD) and then displays the decimal equivalent on a 7-segment display. The table below provides the pin-out for the LSD5061-11.

7 segment displays have 7 light emitting diodes (LED’s), one for each segment. In order for an LED to give off light, a certain amount of voltage and current must be supplied. A common anode 7 segment display ties all of the + voltage sides of the LED’s together (the commons). These commons are tied to 5 volts. In order to turn on a specific segment, you must supply a low voltage (logic 0). These 0’s will be supplied from your logic gates.

Note that a logic 0 from your gate (around 0 volts) and 5 volts on your anode would create a 5 volt drop over the LED’s. This is too much. Tie the outputs of your logic gates to the specific input using a resistor. Some of the voltage will then drop over the resistor, protecting your LED. Experiment with a variety of resistors (100, 330, 1k). What happens to the intensity of the light given off by the LED as the resistance increases? Why do you think this happens?

PROCEDURE

1. Create a truth table for your 4-bit input and the corresponding output necessary to light the proper segments of the display. For example, let the input be represented by wxyz:

w	x	y	z	S1	S2	S3	S4	S5	S6	S7
0	0	0	0	0	0	0	0	0	0	1

Since 000 is decimal 0, all the outer LED’s should be lit, the middle LED should be off.

Once you create the rows in a truth table for the other 9 combinations (1-9, assume 10-15 are

don’t cares for outputs), find the function for each segment.

2. Depending whether you are dealing with more 0’s or 1’s, choose NAND or AND gates to implement each segment function.

3. Wire the input DIP switches to the decoder input. Note that you have 2 3X8 decoders. You will need to use the inputs and enable lines to combine them in order to implement these 7 functions (since they require a 4 bit input).

4. Wire the NAND or AND gates to the proper outputs of the decoders in order to implement the functions for each segment.

5. Wire the outputs of the gates to the proper pin on the 7-segment display (remember to use resistors instead of jumpers).

6. Attach 5V to pins 3 and 8.

7. Demonstrate numbers 0-9 to instructor

NOTE

All circuits MUST be drawn and simulated using LogicWorks as part of your prelab work. You should come to lab with all of your circuits built and ready to be tested/debugged. The lab time should be used to demonstrate the proper operation of your circuits to the lab instructor.