Introduction

The 74LS138 is a 3-to-8 line decoder/demultiplexer IC. In this experiment, we will explore how to use the 74LS138 to decode binary inputs into one of eight outputs. This IC can also be used as a demultiplexer by directing an input signal to one of several outputs.

Materials Required

• 74LS138 IC
• 3 binary input switches
• LEDs to display outputs (Y0 to Y7)
• Breadboard and connecting wires
• Power supply (5V)

No Ads Available.

Circuit Diagram

The following is the circuit diagram for the 74LS138 decoder/demultiplexer experiment:

Pin Configuration

The 74LS138 IC has 16 pins. Below is the pin configuration:

• Inputs: A, B, C (3-bit binary inputs)
• Outputs: Y0 to Y7
• Enable Inputs: G1, G2A, G2B

Steps for the Experiment

1. Place the 74LS138 IC on the breadboard.
2. Connect the binary input switches (A, B, C) to the input pins of the IC (pins 1, 2, and 3).
3. Connect each output pin (Y0 to Y7) to individual LEDs. These will represent which output line is activated based on the binary input.
4. Set the enable pins: connect G1 to logic HIGH (5V) and G2A, G2B to logic LOW (GND).
5. Connect the power supply (5V) to the IC’s VCC pin and ground the GND pin.
6. Power the circuit and set different combinations of binary inputs to observe the activated output.

Explanation

The 74LS138 decodes a 3-bit binary input into one of eight outputs. The binary input (A, B, C) determines which output (Y0 to Y7) is set to LOW (active). All other outputs remain HIGH. The 74LS138 is an active LOW decoder, meaning that only one output goes LOW based on the input combination.

Results and Observations

When you toggle the binary inputs (A, B, C), you should observe the following behavior:

• For binary input 000 (A=0, B=0, C=0), output Y0 will be LOW, and all other outputs (Y1 to Y7) will be HIGH.
• For binary input 001 (A=0, B=0, C=1), output Y1 will be LOW, and all others will be HIGH.
• This pattern continues until binary input 111 (A=1, B=1, C=1), where Y7 will be LOW.

Demultiplexer Mode

The 74LS138 can also act as a demultiplexer. In this mode, a single input signal is routed to one of the eight output lines based on the binary input. This is useful when you want to direct a signal to one of several destinations.

Conclusion

In this experiment, we used the 74LS138 to decode binary inputs into one of eight outputs and demonstrated how it functions as both a decoder and a demultiplexer. Understanding how to use decoders and demultiplexers is essential for designing complex digital systems, such as memory address decoding and data routing circuits.