Definition
A diode is a semiconductor device that allows current to flow easily in one direction only.
Forward bias: the diode is connected so its depletion region (a thin zone at the junction with almost no free charge carriers) narrows and eventually disappears as voltage increases, letting current flow freely.
Reverse bias: the diode is connected the opposite way, so the depletion region widens instead, blocking current flow almost completely — practically no current passes.
Remember
A diode can be permanently damaged if connected in reverse bias with a very high voltage, because beyond the diode's rated breakdown voltage the widened depletion region can no longer hold back the charge, and a sudden large reverse current flows and overheats the junction — this shows a diode isn't a perfect one-way valve at any voltage, only within its rated physical limits.
Definition
An LED is a diode that emits light when forward biased: as electrons cross the junction and recombine with holes (missing electrons), each recombination releases energy as a photon of light rather than heat, the way an ordinary diode or resistor would dissipate it.
The semiconductor material used sets the exact amount of energy released per recombination, and so the colour (wavelength) of light emitted — different materials are chosen to produce red, green, blue, or other LED colours.
Remember
An LED glows brighter with more forward current (more electron-hole recombinations happening per second means more photons emitted per second) and dimmer with less — this is exactly the property exploited in adjustable-brightness displays, which vary the current (often by rapidly switching it on/off, a technique called PWM) to control perceived brightness.
Definition
An analog signal varies continuously over a range of values (e.g. a smoothly changing voltage). A digital signal only takes a small number of distinct values (in binary electronics, just two: HIGH/1 and LOW/0).
An analog-to-digital converter (ADC) samples a continuously varying analog signal at regular intervals and converts each sample into a digital (binary) number representing its value — this is how a microphone's continuous sound wave becomes digital data a computer can store and process.
Remember
Digital systems stay more reliable than analog ones over long-distance transmission because a digital receiver only has to distinguish between two possible states (HIGH or LOW) — small amounts of noise picked up along the way rarely flip a clear HIGH into a LOW or vice versa. An analog signal's exact value matters, so the same noise directly and permanently distorts the received value with no way to tell noise from signal.
Definition
A byte is made up of bits in Boolean/digital logic — each bit is a single binary digit ( or ).
Definition
Boolean logic works with only two values, true () and false (), and combines them using logical operations — it underlies every digital circuit and computer decision, from a simple light switch to complex processors, because any decision that reduces to yes/no conditions can be built from these basic operations.
Definition
NOT gate (inverter): one input, output is the opposite of the input (, ). Despite its simplicity, it's essential wherever a circuit needs to act on the absence of a condition rather than its presence — a huge share of digital decision-making is really just checking whether something is NOT true.
AND gate: output is only when both inputs are — used wherever an action should happen only if every one of several conditions is met.
OR gate: output is when at least one input is .
NAND gate (NOT+AND): output is only when both inputs are ; otherwise .
NOR gate (NOT+OR): output is only when both inputs are ; otherwise .
Remember
A security or alarm system often uses NAND gates rather than plain AND or OR gates because many safety conditions are naturally phrased as 'trigger the alarm unless every sensor confirms safe' — a NAND gate's output is HIGH (alarm can trigger) in every case except when all its inputs are HIGH (fully safe), matching that logic in a single gate.
Example 1 — NAND gate and LDR burglar alarm
Explain how a NAND gate and a light-dependent resistor (LDR) can be combined to design a simple burglar alarm.
- An LDR's resistance is high in darkness and low in light; wired into a potential divider, it gives a LOW input signal when light falls on it (e.g. an intruder breaking a light beam aimed at it) and a HIGH signal in the dark, or vice versa depending on the circuit design.
- Feed this LDR-based signal, along with a second input from a control/arming switch, into a NAND gate.
- The NAND gate's output goes HIGH (triggering the alarm/buzzer) whenever the two inputs are not both HIGH — arranged so that this happens exactly when the light beam to the LDR is broken while the system is armed.
Answer: The NAND gate's output switches the alarm on precisely when the LDR's light-based input signal changes, i.e. when an intruder crosses the beam
