CLO2: Implement basic boolean expressions using logic gates and MSI.
To understand the working of the 74LS138 3-to-8 line decoder and implement it using a digital logic trainer board, breadboard, and TTL power supply.
74LS138 3-to-8 line decoder IC
Breadboard
Digital logic trainer board (with LEDs and switches)
TTL power supply (5V DC)
Connecting wires
The 74LS138 is a 3-to-8 line decoder IC from the TTL logic family. It takes a 3-bit binary input and activates one of the eight outputs. This IC is widely used in memory address decoding and demultiplexing applications. The outputs are active-low, which means they will be LOW when selected.
The 74LS138 has three input lines (A, B, C), three enable inputs (G1, G2A', G2B'), and eight output lines (Y0 to Y7). The outputs are active low. The truth table for the 74LS138 is as follows: The outputs are H when any of the enable inputs is not active. G1 is active high, G2A and G2B are active low. The active output is L.
ABC|G1 G2A G2B|Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7|
000| 1 0 0 | 0 1 1 1 1 1 1 1|
001| 1 0 0 | 1 0 1 1 1 1 1 1|
010| 1 0 0 | 1 1 0 1 1 1 1 1|
011| 1 0 0 | 1 1 1 0 1 1 1 1|
100| 1 0 0 | 1 1 1 1 0 1 1 1|
101| 1 0 0 | 1 1 1 1 1 0 1 1|
110| 1 0 0 | 1 1 1 1 1 1 0 1|
111| 1 0 0 | 1 1 1 1 1 1 1 0|
000| 1 0 0 | 1 1 1 1 1 1 1 1|
xxx| x x 1 | 1 1 1 1 1 1 1 1|
xxx| x 1 x | 1 1 1 1 1 1 1 1|
xxx| 0 x x | 1 1 1 1 1 1 1 1|
...
74LS138: 3 to 8 Decoder
/Demultiplexer
+---u---+
A ---| 1. 16 |--- Vcc
B ---| 2 15 |o-- Y0
C ---| 3 14 |o-- Y1
G2A'--o| 4 13 |o-- Y2
G2B'--o| 5 12 |o-- Y3
G1 ---| 6 11 |o-- Y4
Y7 --o| 7 10 |o-- Y5
Gnd ---| 8 9 |o-- Y6
+-------+
The decoder works only when G1 is HIGH and G2A, G2B are LOW. Depending on the combination of inputs A, B, and C, one of the outputs will go LOW.
Circuit Setup: Place the 74LS138 IC on the breadboard. Connect the VCC pin to +5V and GND pin to ground.
Enable Pins Configuration: Connect G1 to +5V and G2A, G2B to ground to enable the decoder.
Input Connections: Connect inputs A, B, and C to the switches on the digital logic trainer board.
Output Connections: Connect outputs Y0 to Y7 to LEDs on the digital logic trainer board through 220Ω resistors.
Power Supply: Provide 5V DC power to the circuit using the TTL power supply.
Testing: Change the input combinations using the switches and observe which output LED lights up (active-low means the LED connected to the output that is LOW will turn off, and others remain on).
Verification: Verify the results with the truth table.
Use the 74LS138 to implement the 3 input Boolean function F(A, B, C)=∑m(0,1,3,6,7)
Connect the outputs Y2, Y4, Y5 through 1N4148 diodes to form a wired-AND configuration.
The combined output represents the Boolean function F.
Connect the LED to visualize the output.
Active-Low Outputs: When the selected output goes LOW, current flows through the diode and turns OFF the LED, indicating F = 0.
Power Connections: Connect Vcc (Pin 16) to +5V and GND (Pin 8) to Ground.
Enable Pins:
Connect G1 to 5V (HIGH).
Connect G2A' and G2B' to Ground (LOW).
Input Selection: Connect DIP switches for inputs A (Pin 1), B (Pin 2), C (Pin 3).
Testing: Switch through input combinations and observe the LED.
+5V +5V
| |
+---u---+ | /
A ---| 1. 16 |--- Vcc ----+ \
B ---| 2 15 |o-- Y0 /
C ---| 3 14 |o-- Y1 1N4148 \
G2A'--o| 4 13 |o-- Y2 --|<|-----+--OLED
G2B'--o| 5 12 |o-- Y3 |
G1 ---| 6 11 |o-- Y4 --|<|-----+
Y7 --o| 7 10 |o-- Y5 --|<|-----+
Gnd ---| 8 9 |o-- Y6
+-------+
By completing this experiment, students will understand the functionality of the 74LS138 decoder and how to use it for address decoding. They will also learn to read truth tables and implement digital circuits using a breadboard and digital logic trainer board. The experiment also demonstrates how to use a decoder for Boolean logic implementation.