Electronic Systems: Input-Process-Output, Units and Measurements
Welcome to your revision guide for Electronic Systems! This chapter forms a core part of Unit 1: Technology and Design Core Content in your CCEA GCSE course. Unit 1 is assessed in your 1 hour 30-minute written exam and makes up 25% of your total GCSE grade.
Don't worry if electronics seems a bit technical at first glance. Once you understand the basic building blocks and how electricity flows, it becomes just like solving a fun, logical puzzle. Let's break it down step by step!
---1. What is a System? (The Universal Model)
In Technology and Design, a system is defined as an assembly of parts that work together to perform a specific task.
Every single electronic system, whether it is a simple bedside lamp or an automatic street light, follows the exact same universal block diagram:
Input \(\rightarrow\) Process \(\rightarrow\) Output
Analogy: Think of making toast! The Input is pushing down the toaster lever. The Process is the internal timer counting down while heating elements turn on. The Output is hot, crunchy toast popping up!
The Three Stages Explained
1. The Input Stage:
Input devices are sensors or switches. They sense changes in the physical environment (such as light levels, temperature, pressure, or movement) and convert that physical energy into an electrical signal.
CCEA Required Examples:
• LDR (Light Dependent Resistor): Senses light levels.
• Thermistor: Senses temperature changes.
• Switches: Turn circuits on or off mechanically (e.g., Push switches, Toggle switches, and Micro-switches).
2. The Process Stage:
The process stage acts as the "brain" of the circuit. It takes the electrical signal from the input, makes a decision, and controls what happens next. This usually involves control circuits or integrated circuits (ICs).
CCEA Required Examples:
• Resistors: Used to protect delicate components or set up potential dividers.
• Transistors: Act as electronic switches or amplifiers.
• Logic Gates: Make logical decisions based on high or low signals (specifically AND, OR, and NOT gates).
3. The Output Stage:
Output devices take the processed electrical signal and convert it back into a physical action that humans can see, hear, or feel.
CCEA Required Examples:
• LED (Light Emitting Diode): Produces light efficiently.
• Bulb / Lamp: Produces light.
• Buzzer: Produces sound.
• Motor: Produces rotary motion / movement.
• Seven-segment display: Displays numbers or characters visually.
Key Takeaway: Always remember the flow: Sensors (Input) \(\rightarrow\) Brain / Decision (Process) \(\rightarrow\) Action (Output).
---2. Units and Measurements in Electronics
When measuring electrical quantities, you must use standard International (SI) units and correct scientific prefixes. The table below shows the key physical quantities you need to know for your exam:
• Voltage | Symbol: \(V\) | Unit: Volts | Abbreviation: V
• Current | Symbol: \(I\) | Unit: Amperes (Amps) | Abbreviation: A
• Resistance | Symbol: \(R\) | Unit: Ohms | Abbreviation: \(\Omega\)
• Power | Symbol: \(P\) | Unit: Watts | Abbreviation: W
• Capacitance | Symbol: \(C\) | Unit: Farads | Abbreviation: F
Understanding Unit Prefixes
In electronics, numbers can be extremely large (like resistance) or very tiny (like capacitance and current). We use standard prefixes to make these numbers easier to write:
• Mega (M): Multiplies by \(10^6\) (\(1,000,000\))
Example: \(1\text{ M}\Omega = 1,000,000\text{ }\Omega\)
• Kilo (k): Multiplies by \(10^3\) (\(1,000\))
Example: \(1\text{ k}\Omega = 1,000\text{ }\Omega\)
• Milli (m): Multiplies by \(10^{-3}\) (\(0.001\) or divide by \(1,000\))
Example: \(1\text{ mA} = 0.001\text{ A}\)
• Micro (\(\mu\)): Multiplies by \(10^{-6}\) (\(0.000001\) or divide by \(1,000,000\))
Example: \(1\text{ }\mu\text{F} = 0.000001\text{ F}\)
Top Tip for Conversions:
• To go from mA to A: divide by \(1,000\).
• To go from k\(\Omega\) to \(\Omega\): multiply by \(1,000\).
3. Core Formulae & Calculations
You will be required to carry out calculations in your Unit 1 examination. Always write down the formula first, substitute the numbers with correct units, and include the final unit symbol!
A. Ohm's Law
Ohm's Law connects voltage, current, and resistance:
\(V = I \times R\)
Rearranged versions:
• \(I = \frac{V}{R}\)
• \(R = \frac{V}{I}\)
Worked Example:
A circuit has a supply voltage of \(9\text{ V}\) and a resistor of \(450\text{ }\Omega\). Calculate the current in amperes.
1. State the formula: \(I = \frac{V}{R}\)
2. Substitute values: \(I = \frac{9}{450}\)
3. Final Answer: \(I = 0.02\text{ A}\) (or \(20\text{ mA}\))
B. Resistors in Series
When resistors are placed end-to-end in a single line (series), their total resistance is simply added together:
\(R_{total} = R_1 + R_2 + R_3 + \dots\)
Worked Example:
Calculate the total resistance when a \(1\text{ k}\Omega\) resistor is placed in series with a \(470\text{ }\Omega\) resistor.
1. Convert units to match: \(1\text{ k}\Omega = 1,000\text{ }\Omega\)
2. Apply formula: \(R_{total} = 1,000 + 470\)
3. Final Answer: \(R_{total} = 1,470\text{ }\Omega\) (or \(1.47\text{ k}\Omega\))
C. Calculating a Protective Resistor for an LED
LEDs are delicate and will burn out if connected directly to a power source. A protective resistor placed in series absorbs the extra voltage and limits current.
\(R = \frac{V_{supply} - V_{LED}}{I_{LED}}\)
Where:
• \(V_{supply}\) = Total power supply voltage (in Volts)
• \(V_{LED}\) = Voltage dropped across the LED (in Volts)
• \(I_{LED}\) = Operating current of the LED (in Amperes)
Worked Example:
An LED requires \(2\text{ V}\) and a current of \(20\text{ mA}\) to operate safely. It is connected to a \(9\text{ V}\) battery. Calculate the required value of the series protective resistor.
1. Convert current to Amperes: \(20\text{ mA} = 0.02\text{ A}\)
2. Apply formula: \(R = \frac{9 - 2}{0.02}\)
3. Calculate: \(R = \frac{7}{0.02} = 350\text{ }\Omega\)
4. Final Answer: \(R = 350\text{ }\Omega\)
4. Circuit Symbols and Conventions
According to Appendix 3 of the CCEA specification, you must be able to recognize and neatly draw standard graphic symbols:
• Battery / Cell: Drawn with a long thin line (positive terminal, \(+\)) and a shorter, thicker line (negative terminal, \(-\)).
• Switches:
- SPST: Single Pole Single Throw (a simple on-off switch).
- PTM: Push to Make (switch contacts close only while pressed).
- PTB: Push to Break (switch contacts open only while pressed).
• Logic Gate Shapes:
- AND Gate: Shaped like a straight-backed letter 'D'.
- OR Gate: Curved back with a pointed tip (rocket shape).
- NOT Gate: Triangle pointing right with a small invert circle at the tip.
• Polarity: Components like LEDs and Electrolytic Capacitors must be connected in the correct direction (anode to positive, cathode to negative) or they will not work correctly.
5. Examiner Pitfalls (Avoid These Common Mistakes!)
Examiners frequently report where students lose easy marks. Keep these points in mind:
• Incomplete Symbol Drawings: Never leave gaps between connecting wires and component symbols. Always ensure wire junctions are marked clearly with a neat connecting dot.
• Unit Forgetting / Conversion Errors: If an exam question gives a current of \(15\text{ mA}\), do not plug \(15\) directly into \(V = I \times R\)! Convert it to \(0.015\text{ A}\) first.
• Mixing up Logic Gates: Remember that an AND gate requires all inputs to be high (\(1\)) for a high output, whereas an OR gate requires at least one input to be high (\(1\)).
• LED Polarity: Make sure your LED symbol points in the correct direction of conventional current flow (arrow pointing towards the flat bar/negative side).
• Flowchart Precision in Control: When drawing decision diamonds in computer control circuits, you must clearly label exactly two exit branches: Yes and No.
Quick Summary Checklist
Before your exam, make sure you can:
1. Identify the Input, Process, and Output stages of any given system.
2. State the correct SI unit and symbol for Voltage, Current, Resistance, Power, and Capacitance.
3. Convert between Mega, Kilo, milli, and micro prefixes.
4. Use Ohm's Law (\(V = IR\)) and the LED protective resistor formula confidently.
5. Draw and identify standard Appendix 3 circuit symbols accurately.