Basic logic gates

Basic logic gates

1. What Basic Logic Gates Are

Logic gates = building blocks of digital circuits that perform Boolean operations on binary inputs (0=low, 1=high) to give one binary output. Basic gates (fundamental) are AND, OR, NOT; from them all others are built.

2. Definitions, Symbols, Truth Tables, Boolean Expressions

Gate	Symbol (ANSI)	Boolean	Truth Table (2 inputs $A,B$)
AND	D-shaped with flat input

| $0·0=0, 0·1=0, 1·0=0, 1·1=1$ |
| OR | Curved input, pointed output |

(+) is OR, not addition | $0+0=0, 0+1=1, 1+0=1, 1+1=1$ |
| NOT (inverter) | Triangle with bubble |

| $0→1, 1→0$ |

Working: AND = series switches (both must be ON), OR = parallel switches (either ON), NOT = inverting switch.

3. Timing and Voltage Levels

• TTL: $0$ =0–0.8V, $1$ =2–5V
• CMOS: $0$≈0V, $1$≈$V_{DD}$ (3.3/5V)
• Positive logic (as above); negative logic swaps.

4. Worked Checks

(a) AND: A=1,B=0 ⇒ Y=0 ; OR same inputs ⇒1

(b) Need to implement Y=A·B + \bar{C}? Needs AND, OR, NOT.

(c) Waveforms: AND output high only when both high; OR output high when any high.

(d) NAND etc. are universal but made from basic (next topic).

5. Exam Points

• List AND, OR, NOT with symbol, truth table, Boolean — 5-7 marks, very frequent (almost certain to ask basic gates).
• Show $Y=A·B$, $Y=A+B$, $Y=\bar{A}$ — 3-5 marks, frequent.
• Give switch analogy — 3 marks.
• Mention voltage levels — 3 marks.