7-Segment Decoder Driver and Display

Introduction

In this experiment you will demonstrate the operation of a decoder-driver circuit that accepts a binary or BCD input code and generates the 7 segment display signals to produce the numbers 0 through 9 and other characters.

Figure 1 shows a simplified block of the 74LS48 BCD to 7-Segment Decoder. The 74LS48 contains three main block circuits, a 7-segment decoder, a driver and a system of basic memory units. The basic memory unit is often called a latch or a flip-flop. The decoder outputs drive an encoder circuit made up of OR gates that generate the 7-segment code necessary to display the digits 0 through 9 and the letters a through f. The output devices are current driver transistors that supply the proper current to th e segments in the driver.

Figure 1

Equipment needed

Microprocessor power supply
Logic Probe
74LS00
74LS02
74LS04
74LS48 (46)
7-Segment LED

Figure 2

Part 1. -- Set-Reset flip-flop

1. Wire the latch circuit shown in fiqure 2. The Set (A) and Reset (B) are the inputs and C (L1) and C (L2) are the outputs.
2. Apply power to the circuit and create a truth table for S and R Inputs and C and C outputs.
3. Wire the latch circuit shown in figure 3. Repeat steps 1. and 2. for circuit 3. These new outputs are labeled D and D. Why do we call this circuit a basic memory unit? What happens to the outputs when S and R both 0? Refer to the textbook (Katz) for a discussion of flip-flops (chapter 6).

Part 2. -- (7-Segment Decoder-Driver and Display)

1. Construct the circuit shown in figure 4. Use the TTL handbook to verify the correct conections. The pin connections for the 74LS48 and the 7-Segment Display are shown in fiqure 5.
2. Calculate the value of the resistor between the 74LS48 and the 7-seg LED.
3. Apply power to the circuit. Create a truth table for figure 4. Do the LEDs L1-L4 which output the binary word agree with the output of the 7-Segment LED? What does the 7-Segment LED read in binary states 1010-1111? What do you think the LT, RBI and BI/ RBO pins do?

Figure 3

Figure 4

Figure 5

Check the MAN74A documentation.

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