Basic Logic Gates and Truth Tables
Boolean variables take values 0 or 1. The basic operations are AND as multiplication, OR as Boolean addition, and NOT as complementation; a circuit output is evaluated gate by gate.
Why this shows up in the exam
Identifying a gate from its truth table · Evaluating a multi-gate circuit · Relating an LED state to a binary output
Learn the idea
A truth table defines a logic gate by listing its output for every binary input combination. AND requires all inputs high, OR requires at least one high, and NOT reverses one input. NAND and NOR are the complemented versions of AND and OR.
🧠 Memory hook: AND means all; OR means any; a bubble means invert.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Y_AND = A B; Y_OR = A + B — Boolean expressions for two-input AND and OR gates
- Y_NAND = overline(A B); Y_NOR = overline(A + B) — complemented AND and OR outputs
How to approach it
- 1Label every intermediate gate output
- 2Evaluate one input row at a time
- 3Translate LED glow using the actual active-high or active-low connection
Common slip-ups that cost marks
- •Using ordinary arithmetic addition for Boolean OR
- •Ignoring an inversion bubble
- •Assuming LED glow always represents logic 1 without checking wiring
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A transistor has common-emitter current gain beta = 50. If base current is 20 microA, find collector current.
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