Electronic systems: input, process, output devices, logic systems

Systems & Control – IGCSE Design & Technology (0445)

1. The Design Cycle – Full Product‑Design Focus

2. Common‑Content Summary – What the Syllabus Expects

Area	Key Requirements (IGCSE)	How the notes address it
Design brief & specifications	Clear constraints, performance criteria, safety & sustainability targets.	Section 1, step 2 & 5.
Research & analysis	Existing products, materials, health & safety, environmental impact.	Step 3 includes these points; added safety & sustainability notes in Sections 4 & 5.
Idea generation & selection	Sketches, CAD, decision‑making tools.	Step 4 & 6; CAD mentioned in communication.
Making & testing	Prototype construction, testing against specifications, evaluation.	Steps 7 & 8 with worked examples.
Health & safety	Risk assessment, safe use of tools, electrical safety.	New dedicated sub‑section in 4 & 5.
Sustainability & societal impact	Energy efficiency, recyclable materials, ethical considerations.	Embedded in design brief, research and reflection.
Communication	Technical drawings, schematics, reports, oral presentation.	Step 9; examples of circuit diagrams and mechanical drawings.

3. Specialist Option – Systems & Control

3.1 Overview of an electronic system

Electronic systems sense, process and act on information. They are normally divided into three functional stages:

1. Input – devices that detect a physical quantity and convert it into an electrical signal.
2. Processing – circuitry or software that interprets the signal and decides what to do.
3. Output – devices that turn the processed signal into a useful form (light, sound, motion, communication).

3.2 Input devices

Conversion of a physical phenomenon into an electrical signal.

Category	Typical devices	Example use (IGCSE)
Mechanical	Push‑buttons, limit switches, potentiometers	Potentiometer as a speed controller for a DC motor.
Electrical	Voltage sensors, current transducers (shunt, Hall‑effect)	Hall‑effect sensor to detect motor shaft speed.
Optical	Photo‑resistor (LDR), photodiode, IR proximity sensor	LDR to switch lights when ambient brightness falls.
Magnetic	Hall‑effect switch, reed switch	Reed switch to detect a door’s magnetic closure.
Digital	Keyboard, touch‑screen, rotary encoder	Rotary encoder for user‑selected menu options.

3.3 Processing units

Type	Key features	Typical IGCSE example
Microcontroller	CPU, RAM, flash, built‑in I/O ports; programmed in C/Arduino.	Arduino UNO controlling a temperature‑controlled fan.
Microprocessor	Requires external memory & I/O; higher processing power.	Raspberry Pi running a Python script for image recognition.
Programmable Logic Controller (PLC)	Robust, industrial‑grade, ladder‑logic programming.	PLC controlling a conveyor‑belt safety system.
Discrete logic circuit	Individual TTL/CMOS ICs (AND, OR, NOT, etc.).	555‑timer based flashing LED circuit.

3.4 Output devices

Category	Typical devices	Example use (IGCSE)
Visual	LEDs, seven‑segment displays, LCD panels	LCD to show temperature reading.
Audible	Piezo buzzer, small speaker	Buzzer for alarm when a sensor is triggered.
Mechanical	Relay, solenoid, DC/stepper motor, servo motor	Servo to open a latch when a code is entered.
Communication	Serial port, Bluetooth/Wi‑Fi module, IR transmitter	Bluetooth module to send sensor data to a smartphone.

3.5 Logic systems – Boolean algebra

Decisions are made using Boolean logic. The basic two‑input gates are shown below.

Gate	Symbol	Truth table (A, B → Q)
AND		0 0 → 0 0 1 → 0 1 0 → 0 1 1 → 1
OR		0 0 → 0 0 1 → 1 1 0 → 1 1 1 → 1
NOT (Inverter)		0 → 1 1 → 0
NAND		Same as AND, then inverted.
NOR		Same as OR, then inverted.
XOR		0 0 → 0 0 1 → 1 1 0 → 1 1 1 → 0

Complex functions are built by combining gates. Example:

Q = (A AND B) OR (¬C)

3.6 Basic electronics refresher (essential formulas)

Concept	Formula / Key point	Worked example
Ohm’s Law	V = I R	V across 220 Ω at 20 mA → V = 0.02 A × 220 Ω = 4.4 V
Power	P = V I = I²R = V²/R	P = 0.02² × 220 ≈ 0.088 W
Series resistance	R_eq = ΣR	220 Ω + 1 kΩ = 1.22 kΩ
Parallel resistance	1/R_eq = Σ(1/R)	Two 1 kΩ → R_eq = 500 Ω
Resistor colour code	Band 1 & 2 = value, Band 3 = multiplier, Band 4 = tolerance	Red‑Violet‑Yellow‑Gold → 27 × 10⁴ Ω ± 5 % = 270 kΩ
Capacitive reactance	X_C = 1/(2πfC)	100 µF at 50 Hz → X_C ≈ 31.8 Ω
555 timer (astable)	f = 1.44 / ((R₁+2R₂) C)	R₁=1 kΩ, R₂=10 kΩ, C=0.01 µF → f ≈ 13 kHz
Ideal op‑amp (comparator)	V_out saturates to supply rails when V⁺ ≠ V⁻.	LM324 used to detect > 2.5 V sensor voltage.

3.7 Health & safety for electronic work

• Electrical safety – never work on a live circuit, use insulated tools, check for correct voltage rating.
• ESD protection – wear an anti‑static wrist strap, work on a grounded mat when handling ICs.
• Component handling – avoid bending leads, store in anti‑static bags.
• Fuses & protective devices – include a 500 mA fuse on a 5 V supply to prevent over‑current.
• Personal protective equipment (PPE) – safety glasses, closed‑toed shoes.

3.8 Sustainability in electronic systems

• Choose low‑power components (e.g., CMOS over TTL).
• Design for easy disassembly – use screws instead of permanent solder where possible.
• Prefer recyclable materials (e.g., aluminium casings, biodegradable PCB substrates).
• Consider end‑of‑life: provide a schematic for safe disposal of batteries and hazardous components.

4. Mechanical & Structural Concepts (Specialist – Structures focus)

4.1 Loads, reactions & equilibrium

• Point load – force applied at a single location.
• Distributed load – force spread over a length (e.g., uniform weight of a beam).
• Support reactions – forces/moments developed at supports; calculated using ΣF = 0 and ΣM = 0.

4.2 Moments (torque)

Moment M = F × d, where *d* is the perpendicular distance from the line of action of the force to the pivot.

4.3 Levers

Class	Effort arm	Load arm	Typical example
1	Both sides of fulcrum	Both sides of fulcrum	Seesaw
2	Between fulcrum & load	Load outside fulcrum	Wheelbarrow
3	Effort outside fulcrum	Load between fulcrum & effort	Fishing rod

4.4 Gear trains

• Gear ratio  =  N_driven / N_driver
• Speed ratio  =  1 / gear ratio
• Torque multiplication  =  gear ratio (ignoring losses)

4.5 Belts & pulleys

• Linear belt speed = π D × rpm (D = pulley diameter).
• Tension must satisfy T₁/T₂ = e^{μθ} (θ in radians, μ = coefficient of friction).
• Choose V‑belts for high‑speed, toothed belts for no‑slip power transmission.

4.6 Friction & lubrication

• Static friction ≥ μ_s N, kinetic friction = μ_k N.
• Lubricants (oil, grease) reduce μ, protect against wear and corrosion.

4.7 Joints & material properties (brief)

• Fixed joint – no relative movement; used in frames.
• Pin joint – allows rotation; common in mechanisms.
• Material selection – consider strength, stiffness, density, cost, recyclability.

4.8 Stress & strain (basic)

Stress σ = F/A (N / mm²). Strain ε = ΔL/L. Young’s modulus E = σ/ε.

5. Worked examples (integrated with design cycle)

5.1 Structure – Simple beam with a centre point load

Problem: A 0.8 m wooden beam, simply supported at both ends, carries a 50 N load at its centre. Determine the reaction forces at each support.

1. Σ F_y = 0 → R_A + R_B − 50 = 0 → R_A + R_B = 50 N.
2. Take moments about A: R_B × 0.8 − 50 × 0.4 = 0 → R_B = 25 N.
3. Substitute: R_A = 50 − 25 = 25 N.

Both supports share the load equally (25 N each). This calculation demonstrates the use of equilibrium equations – a required skill for the “Structures” focus.

5.2 Mechanism – Gear ratio and speed reduction

Problem: A motor drives a 12‑tooth pinion that meshes with a 36‑tooth gear attached to a wheel. The motor runs at 3000 rpm. Find the wheel’s rpm and the torque multiplication (ignore losses).

• Gear ratio = N_driven / N_driver = 36 / 12 = 3 : 1.
• Speed ratio = 1 / gear ratio → wheel rpm = 3000 / 3 = 1000 rpm.
• Torque multiplication = gear ratio → wheel torque = 3 × motor torque.

Link to the design cycle: the gear train would be selected during the “Research & analysis” stage and its dimensions finalised in “Develop specifications”.

5.3 Electronics – OR‑gate alarm circuit

Task: Design an alarm that sounds a buzzer when a door sensor (D) **or** a window sensor (W) is opened. Use only OR and NOT gates, and limit the buzzer current to 20 mA from a 5 V supply.

1. Logic – Define “closed” = 0, “open” = 1.
   Alarm output A = D OR W.
2. Gate implementation – 74HC32 (quad OR) IC.
   Pin 1 = D, Pin 2 = W, Pin 3 = A.
3. Driver stage – OR gate cannot source 20 mA, so use an NPN transistor (2N2222).
   Base resistor R_B = (5 V − 0.7 V) / I_B. Choose I_B = 2 mA → R_B ≈ 2.15 kΩ → use 2.2 kΩ.
4. Current‑limiting for the buzzer – Desired I_buzz = 20 mA.
   Assume buzzer forward voltage V_F = 3 V.
   R_limit = (5 V − 3 V) / 0.02 A = 100 Ω.
5. Circuit connection – OR output → base via 2.2 kΩ. Emitter to ground. Collector to one side of buzzer; other side of buzzer to +5 V through 100 Ω.

Truth table for the alarm:

D	W	A = D OR W
0	0	0 (silent)
0	1	1 (buzzer on)
1	0	1 (buzzer on)
1	1	1 (buzzer on)

6. Device summary – quick reference

Device	Function in a system	Typical IGCSE example
Push‑button	Input – binary on/off signal	Start/stop control for a motor.
Potentiometer	Input – variable voltage proportional to position	Speed controller for a fan.
Photo‑resistor (LDR)	Input – light level detector	Automatic night‑light.
Arduino UNO	Processing – programmable microcontroller	Temperature‑controlled heating element.
74HC32 OR gate	Processing – discrete logic	Alarm logic in example 5.3.
LED	Output – visual indication	Power‑on indicator.
Buzzer	Output – audible alarm	Door‑open warning.
Servo motor	Output – precise angular movement	Locking mechanism for a safe.
Bluetooth module (HC‑05)	Output – wireless communication	Send sensor data to a phone app.

7. Quick‑check checklist for the exam

• Can you write a clear design brief and measurable specifications?
• Have you considered health & safety and sustainability at each stage?
• Do you know the function and example of at least three input, three processing and three output devices?
• Can you draw the symbol and truth table for AND, OR, NOT, NAND, NOR and XOR?
• Are you comfortable converting a real‑world problem into a Boolean expression and then into a gate diagram?
• Can you calculate load reactions, gear ratios, and current‑limiting resistors?
• Is your final communication (schematic, diagram, report) neat, labelled and includes a reflection on societal impact?