Electronic Components & Devices
Input Components and Devices
Input devices convert a physical quantity or user action into an electrical signal. The OCR specification covers several switch types: the Single Pole Single Throw (SPST) switch simply opens or closes one circuit; a push-to-make switch connects the circuit only while pressed; a push-to-break switch disconnects the circuit while pressed. A tilt switch contains a small ball bearing that completes or breaks the circuit when tilted — used in safety cut-offs. A reed switch is a magnetically operated switch: two metal contacts inside a glass capsule close when a magnet is brought close — common in door and window alarms. A Quantum Tunnelling Composite (QTC) switch uses a material whose resistance changes when compressed, enabling pressure-sensitive switching without moving parts.
Sensor input devices include: a Light Dependent Resistor (LDR), whose resistance falls as light increases — used in automatic lighting circuits; a photodiode, which conducts in proportion to incident light — faster and more precise than an LDR; an NTC thermistor (Negative Temperature Coefficient), whose resistance falls as temperature rises — used in temperature measurement and control; a pressure sensor, which converts mechanical force into an electrical signal; an infrared sensor, which detects infrared radiation — used in motion detectors and remote controls; and smart (WiFi-enabled) sensors, which connect to a network to transmit data remotely. Touch screens are also listed as input devices — they detect the position of a finger or stylus on a display surface.
Process Components and Devices
Process components manipulate signals between input and output. Amplifiers increase signal voltage or current. Counters count pulses or events. Timers generate a timed output after a set delay (e.g. the 555 timer IC). Latches store a binary state — once triggered, they hold their output until reset. Pulse generators produce repetitive clock signals. Analogue to digital converters (ADCs) sample an analogue input and convert it to a binary number — essential for connecting sensors to microcontrollers. Logic functions (AND, OR, NOT, NAND) are digital process components whose outputs depend on combinations of binary inputs. You must know their truth tables and be able to apply them singly and in two-level combinations.
Input switches: SPST, push-to-make, push-to-break, tilt, reed, QTC. Input sensors: LDR, photodiode, NTC thermistor, pressure sensor, infrared sensor, smart sensor. Output devices: lamps, LEDs, LED displays, LCDs, buzzers, piezo sounders, motors. Drivers/interface: NPN transistor, Darlington driver, relay. Passive: resistors (fixed, variable), diodes, capacitors (polarised, non-polarised).
Output Components and Devices
Lamps produce light from current; they require relatively high current and are less efficient than LEDs. Light Emitting Diodes (LEDs) are semiconductor devices that emit light when forward biased — low current, long life, available in many colours. LED displays (7-segment and dot-matrix) are arrays of LEDs configured to display numbers, letters, and symbols. Liquid Crystal Displays (LCDs) use liquid crystal cells to modulate light — very low power, used in clocks, meters, and screens. Buzzers use an electromagnetic mechanism to produce sound when current flows; piezo sounders use a piezoelectric disc that vibrates when an alternating voltage is applied — smaller and more power-efficient than buzzers. Motors convert electrical energy into rotational mechanical energy.
Drivers, Passive Components, Power Supplies and Wiring
Higher-power output devices need driver components because microcontrollers supply only a few milliamps. An NPN transistor amplifies current — a small base current switches a much larger collector-emitter current. A Darlington driver pairs two NPN transistors for very high current gain. A relay closes a separate high-power switching contact using a small electromagnet current, electrically isolating the control circuit from the load. Passive components need no supply: fixed resistors set resistance; variable resistors adjust it; diodes pass current in one direction only; capacitors store charge — polarised (electrolytic) types must be connected the right way round, non-polarised types can go either way. Power supply options: batteries (portable DC); photovoltaic cells (renewable, light-dependent output); supercapacitors (rapid charge/discharge, limited energy storage); mains adaptors (stable DC from AC mains). Wiring: single-strand (solid core) holds shape — ideal for breadboard; multi-strand (flexible) bends without breaking — used where connections must flex.
NPN transistors and Darlington drivers switch high-current loads using a small control current. Relays provide electrical isolation between the control circuit and the load — essential when switching mains-voltage devices from a low-voltage microcontroller. Polarised capacitors must be connected with the correct polarity or they can be damaged.
NTC stands for Negative Temperature Coefficient. As temperature rises, more electrons gain enough energy to become charge carriers, so more current flows for the same voltage — meaning resistance is lower (R = V ÷ I). This resistance change is used in a voltage divider circuit to produce a varying voltage that a microcontroller reads and compares with a threshold to trigger a heating or cooling output.
Battery: portable but must be replaced or recharged — good for handheld devices, poor for long-term field sensors. Photovoltaic cell: renewable and maintenance-free in daylight, but output varies with light and a buffer store is needed for night use. Supercapacitor: charges and discharges very rapidly, long cycle life, but stores far less total energy than a battery — suited to brief backup or energy harvesting. Mains adaptor: reliable, constant output, but needs a socket — ideal for fixed, indoor devices only.