Unit 1: Digital circuit construction and verification
Hardware Lab-I (Digital Circuits & Logic Design) notes · PTU syllabus (BSIT206/BSBC307)
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Unit summary
This hardware lab builds and verifies digital circuits on breadboards and trainer kits: diodes, transistors and logic families, basic gates, adders and subtractors, code converters, decoders and encoders, flip-flops built from gates and ICs, and ring, asynchronous and synchronous counters. Each experiment lists the ICs, the procedure and what to verify.
After this unit you can
- Identify diodes, transistors and TTL and MOS logic families
- Verify truth tables of logic gates and arithmetic circuits
- Build code converters, decoders and encoders
- Build flip-flops and counters
PTU syllabus topics
- Introduction to diodes
- transistors and TTL/MOS technology
- truth table verification of basic logic gates
- arithmetic circuits (half/full adder and subtractor)
- BCD-to-binary and binary-to-BCD converters
- BCD-to-decimal and BCD-to-7-segment decoders
- BCD-to-decimal and octal-to-binary encoders
- R-S/J-K/Master-Slave J-K/D flip-flop circuits using gates and ICs
- ring counters
- asynchronous and synchronous counters
- 1
Identify the IC
e.g. 7408 AND, 7432 OR, 7400 NAND
- 2
Check pin diagram
Vcc on pin 14, GND on pin 7
- 3
Wire on the breadboard
- 4
Apply input combinations
- 5
Observe LEDs
Record the truth table
- 6
Compare with theory
Topic 1
Lab safety and procedure
- 1
Note the aim and components
- 2
Check IC pin diagram (pin 14 Vcc, pin 7 ground for 14-pin TTL ICs)
- 3
Wire the circuit on the breadboard or trainer kit
- 4
Connect +5 V supply and ground
- 5
Apply inputs with switches; observe LEDs
- 6
Record the truth table and compare with theory
- Precautions: switch off power while wiring, check the IC notch orientation, avoid loose connections, do not exceed 5 V for TTL.
Topic 2
Diodes, transistors and logic families
- Diode: allows current in one direction (forward bias); used in rectifiers and diode logic.
- Transistor (BJT): works as a switch — cut-off (off, logic 1 output in an inverter) and saturation (on, logic 0).
Building block
Bipolar transistors
MOSFETs
Supply
5 V
3–15 V
Power consumption
Higher
Very low
Speed
Fast
Comparable in modern families
Noise margin
Moderate
High
Topic 3
Verifying logic gates
| Gate | TTL IC |
|---|---|
| NOT (hex inverter) | 7404 |
| AND (quad 2-input) | 7408 |
| OR | 7432 |
| NAND | 7400 |
| NOR | 7402 |
| XOR | 7486 |
- Apply all input combinations (00, 01, 10, 11) and record outputs; also realise AND, OR and NOT using only 7400 NAND gates.
Topic 4
Adders and subtractors
- Half adder: XOR (7486) for sum, AND (7408) for carry. Full adder: two half adders and an OR gate, or the 4-bit adder IC 7483.
- Half and full subtractors: XOR for difference; NOT and AND for borrow.
- Verify all eight input combinations for full adder and full subtractor.
Topic 5
Code converters
- Binary-to-BCD and BCD-to-binary: realise using logic gates from K-map expressions or a 4-bit adder (add 6 correction for BCD).
- BCD-to-7-segment decoder (7447 for common-anode display): apply BCD 0000 to 1001 and observe digits 0–9 on the display.
- BCD-to-decimal decoder (7442): one of ten outputs goes low for each BCD input.
Topic 6
Encoders
- Decimal-to-BCD encoder (74147, priority): active-low inputs; outputs the BCD of the highest-priority active input.
- Octal-to-binary encoder (74148 or gates): eight inputs to three outputs.
Topic 7
Flip-flops using gates and ICs
- SR latch using NAND gates (7400): verify set, reset, no change and invalid states.
- JK flip-flop (7476 or 74112): verify toggle when J = K = 1.
- Master–slave JK: observe that output changes only at the clock edge, removing the race condition.
- D flip-flop (7474): output follows D at the clock edge.
Topic 8
Counters
- Ring counter: shift register (7495 or D flip-flops) with output fed back — a single 1 circulates.
- Asynchronous counter: JK flip-flops in toggle mode (7476) or the 7493 4-bit binary counter; observe 0000–1111 on LEDs; make a MOD-10 counter using 7490.
- Synchronous counter: common clock with AND gates driving J and K inputs; or IC 74163; verify the counting sequence.
Key terms
- Breadboard
- Board for building circuits without soldering
- TTL
- Transistor-transistor logic family
- 7-segment display
- Display of seven LEDs forming digits
- Priority encoder
- Encoder outputting the highest-priority input
- Ring counter
- Shift register circulating a single 1
Quick revision
- Experiment procedure and precautions; Vcc pin 14, GND pin 7.
- Diode and transistor switching; TTL vs CMOS.
- Gate ICs 7400, 7402, 7404, 7408, 7432, 7486.
- Adders (7483), code converters, 7447, 7442, 74147, 74148.
- Flip-flops 7474, 7476; counters 7490, 7493, ring counter.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Which IC contains quad 2-input NAND gates?
- Q2.What is the function of IC 7447?
- Q3.Distinguish TTL and CMOS logic.
- Q4.How is a half adder built from gates?
- Q5.Which IC is used for a decade counter?
- Q6.What is a ring counter?
Long-answer questions
- Q1.Verify the truth tables of all basic logic gates using ICs.
- Q2.Construct and verify a full adder and full subtractor.
- Q3.Construct a BCD-to-7-segment decoder circuit and explain its working.
- Q4.Construct JK and D flip-flops and verify their truth tables.
- Q5.Construct and verify asynchronous and synchronous counters.
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