Welcome to Computer Control Systems & Flowcharts

Have you ever wondered how an automatic door knows when to slide open, how a microwave counts down precisely, or how a modern home thermostat keeps a room at the perfect temperature? All of these everyday devices rely on Computer Control Systems.

In this chapter of your CCEA GCSE Technology and Design Core Content, you will discover how microcontrollers make decisions, how sensors collect information from the physical world, and how to design logical flowcharts to bring automated systems to life. Don't worry if flowchart programming sounds a little daunting at first—once you learn the standard shapes and rules, it works just like drawing a visual recipe or a step-by-step game plan!

1. The System Architecture: How Computer Control Works

Every automated computer control system follows a simple universal model known as the Input → Process → Output cycle:

Input → Process → Output (with Feedback)

A. Inputs (Sensors and Transducers)

An input device detects physical changes in the environment and converts them into electrical signals that a microcontroller can understand.

Common input devices include:
• Push Buttons / Switches: Provide a simple digital signal (pressed/open, on/off).
• LDR (Light Dependent Resistor): Senses light levels (e.g., used to switch on street lights at dusk).
• Thermistor: Senses temperature changes (e.g., used in central heating systems).
• Reed Switch: Detects magnetic fields (e.g., used in window and door alarm sensors).

B. Process (The Brain)

The process unit is the decision-maker. In modern technology projects, this is usually a microcontroller or a PIC (Peripheral Interface Controller). It reads the electrical signals from the input sensors, follows a pre-programmed set of instructions, and decides what actions need to take place.

C. Outputs (Actuators and Indicators)

An output device takes the electrical signal from the microcontroller and converts it into physical movement, light, or sound.

Common output devices include:
• LEDs (Light Emitting Diodes): Visual warning lights or status indicators.
• Buzzers: Sound alerts (such as a microwave beep or car reversing sensor).
• DC Motors: Provide continuous rotational movement (such as driving wheels or fans).
• Solenoids: Create quick linear (push/pull) movement (used in electric door locks).
• 7-Segment Displays: Show numerical values (such as timer countdowns or scores).

D. Feedback & Closed-Loop Control

In an open-loop system, the process carries out an action regardless of the result (e.g., a simple toaster heats bread for a set time, even if it burns).
In a closed-loop system, feedback is introduced. The system constantly monitors its own output using sensors and feeds that information back into the processor to make automatic adjustments.

Real-World Analogy: Think of a central heating system. The thermistor (sensor) checks the room temperature. When the room reaches \(21^\circ\text{C}\), the microcontroller turns off the heater. When the temperature drops, the sensor detects this drop and turns the heater back on. This continuous checking loop is feedback.

Key Takeaway: All computer control systems operate on the chain: Input → Process → Output. When sensors monitor output actions and feed data back to regulate the system, it becomes a closed-loop system.

2. Standard Flowchart Symbols & Conventions

In CCEA examinations, flowcharts must be drawn using standard symbols (BS 4058 / ISO standards). Each symbol has a strict, specific meaning, so using the correct geometric shape is essential.

1. Terminator (Start / Stop)

Shape: Oval / Rounded rectangle (stadium shape).
Function: Marks where the program begins and where it ends.
Rule: Every flowchart must begin with a Start box. A continuous looping program might not have a Stop box, but any finite program ends with a Stop box.

2. Process / Output Command

Shape: Rectangle.
Function: Executes an instruction, changes an output state, sets a delay, or updates a variable.
Examples:
• Output 1 = ON
• Motor ON
• Wait 5 seconds
• Count = Count + 1

3. Decision / Input Test

Shape: Diamond (rhombus).
Function: Asks a question or checks the state of an input sensor.
Examples:
• Is Switch 1 ON?
• Is Temperature > \(25^\circ\text{C}\)?
• Count = 5?
Crucial Rule: A decision diamond must always have at least two labeled exit paths—typically labeled "Yes" and "No" (or "High" and "Low").

4. Input / Output (Data)

Shape: Parallelogram.
Function: Used when the system reads data from external input sources or sends data out.

5. Subroutine / Macro

Shape: Rectangle with double vertical lines on the left and right sides.
Function: Calls up a separate, reusable block of code that carries out a specific task (for example, a complicated flashing sequence) before returning to the main program.

6. Flow Lines (Direction of Control)

Shape: Straight lines with clear directional arrowheads.
Rule: Flow lines show the exact direction of program execution. They must point directly into the top or sides of symbols and must never be left hanging without an arrowhead.

Memory Tip: Remember "Diamonds make Decisions, Rectangles do Real work!"

3. Flowchart Programming Logic & Control Structures

Now that you know the shapes, let's explore how to connect them to create working program logic.

A. Time Delays (Wait Commands)

Microcontrollers run instructions in fractions of a millisecond. If you want an LED to flash noticeably, you must tell the processor to pause between states using a delay inside a Process box:

1. LED = ON (Rectangle)
2. Wait 2 seconds (Rectangle)
3. LED = OFF (Rectangle)
4. Wait 2 seconds (Rectangle)

B. Continuous Polling (Input Loops)

When waiting for a user to press a button, the system repeatedly checks (polls) the input in an infinite loop until the condition changes:

• Decision Diamond: "Is Push Switch ON?"
• "No" Branch: Loops straight back up to just above the decision diamond to ask the question again.
• "Yes" Branch: Moves downward to trigger the next action (e.g., turning on a motor).

C. Counting and Iteration (Loops with Counters)

If you need an action to repeat a specific number of times (e.g., beep a buzzer 3 times), you use a counter variable:

1. Initialise: Count = 0 (Process rectangle)
2. Action: Buzzer ON → Wait 1s → Buzzer OFF → Wait 1s
3. Increment: Count = Count + 1 (Process rectangle)
4. Check: Count = 3? (Decision diamond)
• If No: Loop back to step 2.
• If Yes: Move forward to the next part of the program.

D. Software Logic: Combining Inputs (AND vs. OR Logic)

In microcontrollers, you can build logic gates directly into your software by chaining decision diamonds:

AND Logic (Series Decisions):
Both conditions must be met for the action to happen.
Example: A safety press requires both Button A AND Button B to be pressed simultaneously.
• Test 1: Is Button A pressed? → If No, loop back. If Yes, move to Test 2.
• Test 2: Is Button B pressed? → If No, loop back. If Yes, turn on the press motor.

OR Logic (Parallel Decisions):
Either condition being met will trigger the action.
Example: An emergency stop alarm triggers if Button A OR Button B is pressed.
• Test 1: Is Button A pressed? → If Yes, activate alarm. If No, move to Test 2.
• Test 2: Is Button B pressed? → If Yes, activate alarm. If No, loop back to start.

Key Takeaway: Two decision diamonds placed in series create AND logic. Two decision diamonds where either "Yes" route leads to the action create OR logic.

4. Common Exam Mistakes & How to Avoid Them

Examiners frequently highlight the same simple mistakes on Unit 1 exam papers. Avoid these traps to protect your marks:

1. Missing Directional Arrowheads:
Drawing plain lines without arrowheads leaves the path ambiguous. Always draw clear arrowheads showing the exact flow of control.

2. Unlabeled Decision Branches:
Every exit line leaving a diamond decision box must be clearly labeled "Yes" or "No" (or "High" / "Low"). Leaving them blank will lose marks immediately.

3. Dead Ends (Hanging Lines):
Every flow line must lead somewhere! Ensure your lines reconnect cleanly back into the main flow above a box or terminate neatly into a Stop oval.

4. Combining Questions and Actions in One Box:
Never write "Check switch and turn motor on" in a single rectangle. You must first test the switch with a Decision diamond (Is Switch ON?) and then execute the output with a separate Process rectangle (Motor ON).

5. Re-entering at the Wrong Point in a Loop:
When looping back to repeat an action, make sure your return line enters the flow line above the target box, not directly into the middle of another process.

5. Quick Revision Summary Checklist

Before heading into your CCEA GCSE Unit 1 exam, make sure you can answer YES to all of the following:

• Can I identify standard input transducers (LDR, thermistor, push switch, reed switch) and output actuators (LED, buzzer, motor, solenoid)?
• Can I clearly explain the difference between open-loop and closed-loop control systems?
• Do I know the exact shape and role of all 6 flowchart symbols (Terminator, Process, Decision, Input/Output, Subroutine, Flow lines)?
• Have I labeled every decision exit path with "Yes" and "No"?
• Can I draw software AND and OR logic using decision diamonds?
• Did I include arrowheads on every single line?