Electronic Systems: Process (Microcontrollers & Timers)

This lesson covers the process stage of electronic systems — the components that receive input signals, make decisions and control outputs. This is part of AQA GCSE Design and Technology (8552), Section 3.1.4.

---

Recap: The Systems Approach

Stage	Function	Examples
Input	Detects environmental changes	LDR, thermistor, switch, microphone
Process	Makes decisions based on input	Microcontrollers, timers, comparators
Output	Produces a response	LED, motor, buzzer, speaker

The process stage is the "brain" of the system — it determines what happens in response to the input signal.

---

Microcontrollers

A microcontroller is a small, programmable computer on a single integrated circuit (IC). It contains a processor, memory and input/output pins, and can be programmed to perform a wide range of tasks.

Popular Microcontrollers for D&T

Microcontroller	Description	Programming Language
Arduino Uno	Widely used in education; many shields and sensors available	C/C++ (Arduino IDE)
BBC micro:bit	Designed for education; built-in sensors, LEDs, Bluetooth	MakeCode (block-based), Python
Raspberry Pi Pico	Affordable, powerful microcontroller board	MicroPython, C/C++
PIC microcontroller	Industry-standard; used in commercial products	C, Assembly
Arduino LilyPad	Designed for wearable e-textiles; can be sewn onto fabric	C/C++ (Arduino IDE)

How a Microcontroller Works

1. Input signals (from sensors, switches) are read through input pins.
2. The program (stored in memory) processes the input data using logic and calculations.
3. Output signals are sent through output pins to control LEDs, motors, buzzers, etc.

The diagram below shows the internal blocks of a typical microcontroller (e.g. Arduino or micro:bit) and how a program flows from sensor input to output device:

graph TD
    S["Sensor / Switch"] --> IP["Input Pins"]
    IP --> CPU["CPU\n(executes program)"]
    MEM["Memory\n(stores program & variables)"] --> CPU
    CPU --> MEM
    CPU --> OP["Output Pins\n(digital / PWM)"]
    OP --> LED["LED / Motor / Buzzer"]

Example: Automatic Night Light

INPUT:  LDR connected to analog input pin
PROCESS: Microcontroller reads LDR value
         IF light level < threshold THEN
           Turn LED ON
         ELSE
           Turn LED OFF
         END IF
LOOP

Advantages of Microcontrollers

Advantage	Explanation
Programmable	The same hardware can perform completely different tasks by changing the software
Compact	A single chip replaces dozens of discrete components
Cheap	An Arduino Nano costs ~£3; a PIC chip costs ~£1
Accurate timing	Can generate precise delays and pulse-width modulation (PWM) signals
Multiple I/O	Can handle many inputs and outputs simultaneously
Reprogrammable	Code can be updated and improved without changing hardware

Disadvantages of Microcontrollers

Disadvantage	Explanation
Programming skill required	Users must learn to write or modify code
Processing limitations	Simple microcontrollers cannot run complex software (e.g. video processing)
Power consumption	Continuously running a microcontroller drains battery power
Debugging difficulty	Errors in code can be hard to find and fix

AQA Exam Tip: In D&T exams, microcontroller questions often ask you to describe how a microcontroller could be used to control a simple product (e.g. a greenhouse heater, a security alarm). Structure your answer as: input → process (what the program decides) → output. Use the terms "input pin", "output pin" and "program" in your answer.

---

Programmable Components

Microcontrollers can control many types of output based on programmed conditions:

Programming Concept	Description	D&T Application
IF...THEN...ELSE	Makes a decision based on a condition	If temperature > 25°C, turn fan ON, else turn fan OFF
Loops (WHILE, FOR)	Repeats actions continuously or a set number of times	Continuously read a sensor and update a display
Variables	Store values that change during the program	Store the current temperature reading
Analog input	Reads a range of values (0–1023 on Arduino) from a sensor	Read exact light level from LDR
Digital input	Reads ON or OFF (1 or 0) from a switch	Detect if a button is pressed
PWM (Pulse Width Modulation)	Rapidly switches an output on and off to simulate a variable voltage	Control motor speed, LED brightness

Flowcharts for Program Design

AQA expects you to be able to read and create flowcharts to represent microcontroller programs.

Symbol	Meaning
Oval (rounded rectangle)	Start / End
Rectangle	Process (an action, e.g. "Turn LED on")
Diamond	Decision (a question, e.g. "Is temperature > 30°C?")
Parallelogram	Input / Output
Arrow	Flow direction

AQA Exam Tip: In the exam, you may be asked to complete or draw a flowchart for a simple control system. Practice drawing flowcharts for common scenarios: automatic street light, greenhouse heater, security alarm, traffic light sequence.

---

Timer Circuits (555 Timer)

The 555 timer is one of the most widely used integrated circuits in electronics. It can be configured to operate in two main modes:

Monostable Mode (One-Shot)

• Produces a single timed pulse when triggered.
• The pulse duration is set by a resistor (R) and a capacitor (C).
• Formula: $t = 1.1 \times R \times C$t=1.1×R×C

Application	How It Works
Hand dryer	Triggered by a button press; runs for a set time then stops
Staircase light timer	Light turns on when switch is pressed; turns off after ~60 seconds
Alarm sounding duration	Alarm sounds for a set time after being triggered

Astable Mode (Oscillator)

• Produces a continuous square wave — the output switches between ON and OFF repeatedly.
• The frequency and duty cycle are set by two resistors and a capacitor.

Application	How It Works
Flashing LED	LED blinks on and off at a set rate
Tone generator	Drives a buzzer or speaker to produce a beeping sound
Clock pulse	Provides a regular timing signal for other circuits

555 Timer vs Microcontroller

Feature	555 Timer	Microcontroller
Complexity	Simple — requires a few external components	More complex — requires programming
Flexibility	Limited to timing and oscillation	Can perform any programmable task
Cost	Very cheap (~£0.20)	Cheap but more than a 555 (~£1–5)
Ideal for	Simple timing tasks	Complex, multi-function systems

---

Comparator Circuits