ECE216 | Exp4 | Combinational logic by implementing the boolean Function using decoder

Aim:

To design a circuit to implement Boolean functions using Decoders. Apparatus required: IC 74138, 7404, 7432,7400,7408,7410 and LEDs. 

Learning Objectives: 

This experiment enables a student to learn: 

How to realize the functionality of a 3-to-8-line active low Decoder viz. 74138 IC. That is on setting the two-active low and one active-high enable inputs to the proper level, one can verify that one and only one of the eight active low outputs are asserted based on the values assigned to three select inputs.

Theory: 

IC 74138 works as a 3-to-8 active low decoder, based on the values assigned to three select inputs of the three enable inputs, G1 must be made high value while G2A and G2B must below. The eight active-low inputs (Y0 to Y7) correspond to eight max terms (M0 to M7) or in other words, components of the corresponding minterms m0-m7. For example, Y0 = component of C B A = C+B+A.

Truth table for 3 to 8 decoder:

Procedure:

1. At first, go through the structure of 74138. Then apply high-level voltage to VCC and apply low-level voltage to GND and also apply high-level voltage to G1
2. Next, apply low-level voltage to all the three select inputs (C B A). Now check that Y0 is at a low state. Other outputs are at a high state
3. Apply low-level voltage to C and B and apply high-level voltage to A. Now check that Y1 is at a low state. Other outputs are at a high state
4. Apply low-level voltage to C and A and apply high-level voltage to B. Now check that Y2 is at low state. Other outputs are at a high state
5.  Apply low-level voltage to C and apply high-level voltage to B and A. Now check that Y3 is at a low state. Other outputs are at a high state
6. Next, apply high-level voltage to C and apply low-level voltage to B and A. Now check that Y4 is at low state. Other outputs are at a high state
7. Apply high-level voltage to C and A and apply low-level voltage to B. Now check that Y5 is at a low state. Other outputs are at a high state
8. Next, apply high level voltage to C and B high and apply low level voltage to A. Now check that Y6 is at a low state. Other outputs are at high state.
9. Next, apply high-level voltage to all the select inputs (C, B, A). Now check that Y7 is at a low state. Other outputs are at a high state

Circuit Diagram:

Observations:

Observation Table: Truth Table for 3:8 decoder 

Results:

Learning Outcomes:

Students will be able to understand the analysis and synthesis of logic functions with the help of decoders. 9. Precautions 1. Do not press the IC on the breadboard until pins are aligned with pours. 2. Make connection properly. 3. There should not any short circuit in the circuit. 4. Avoid the heating of IC.