Welcome to Electrical Components, Resistors, and Switches!
Welcome to this core section of GCSE Technology and Design (Unit 1)! Electronics might seem daunting at first with all its symbols, numbers, and colour codes, but it is actually just like a puzzle where every piece has a clear and logical job. In this topic, we will explore the fundamental building blocks of circuits: how electricity flows, how we control it using resistors, and how we direct it using switches.
Whether you are designing an automatic nightlight, a motor-reversing buggy, or a home security system, understanding these components is essential for your exam and your practical design work.
1. Fundamental Circuit Concepts and Units
Before we look at specific components, let's understand the three core electrical quantities that appear in almost every circuit calculation.
Current, Voltage, and Resistance
1. Current (\(I\)): Current is the rate of flow of electric charge around a circuit.
• Measured in Amperes or Amps (\(\text{A}\)).
• In low-power electronics, it is often measured in milliamperes (\(\text{mA}\)), where \(1\text{ A} = 1000\text{ mA}\).
2. Potential Difference / Voltage (\(V\)): Voltage is the electrical push or force that drives the electric charge through the circuit.
• Measured in Volts (\(\text{V}\)).
3. Resistance (\(R\)): Resistance is the opposition to the flow of electric current. It limits how much current can pass through.
• Measured in Ohms (\(\Omega\)) or kilo-ohms (\(\text{k}\Omega\)), where \(1\text{ k}\Omega = 1000\text{ }\Omega\).
Everyday Analogy: Think of electricity flowing like water in a pipe. Voltage is the water pressure pushing the water, Current is the volume of water flowing past every second, and Resistance is a narrow restriction in the pipe slowing the water down.
Ohm's Law
Ohm's Law connects these three key quantities in a simple mathematical relationship:
\(V = I \times R\)
You can rearrange this equation depending on what you need to calculate:
• To find Current: \(I = \frac{V}{R}\)
• To find Resistance: \(R = \frac{V}{I}\)
Exam Tip & Common Pitfall: Always check your units before calculating! If current is given in milliamperes (\(\text{mA}\)), divide by \(1000\) to convert it to amperes (\(\text{A}\)). If resistance is given in kilo-ohms (\(\text{k}\Omega\)), multiply by \(1000\) to convert it to ohms (\(\Omega\)). Forgetting to convert units is one of the most common ways students lose marks in Unit 1.
Measuring Electricity: Ammeters and Voltmeters
When drawing circuit diagrams or setting up a test circuit, you must connect measuring instruments correctly:
• Ammeter: Measures current. Must be placed in series (directly in line with the component) so that the current flows through it.
• Voltmeter: Measures potential difference. Must be placed in parallel (connected across the component) to compare the electrical potential on both sides.
Key Takeaway: Current (\(\text{A}\)) is measured in series by an ammeter; Voltage (\(\text{V}\)) is measured across a component in parallel by a voltmeter; \(V = I \times R\).
2. Resistors and Resistive Devices
Resistors are passive components designed to provide a specific amount of resistance. They are widely used to control current levels and drop voltage to safe operating levels (such as protecting an LED from burning out).
The 4-Band Resistor Colour Code
Small through-hole resistors have colour-coded bands printed on their bodies to indicate their resistance value in ohms (\(\Omega\)) and their manufacturing tolerance.
• Band 1: First significant digit.
• Band 2: Second significant digit.
• Band 3: Multiplier (the power of 10 to multiply by: \(10^n\)).
• Band 4: Tolerance band (how accurate the resistor is: Gold = \(\pm 5\%\), Silver = \(\pm 10\%\)).
Colour Values:
• Black: \(0\)
• Brown: \(1\)
• Red: \(2\)
• Orange: \(3\)
• Yellow: \(4\)
• Green: \(5\)
• Blue: \(6\)
• Violet: \(7\)
• Grey: \(8\)
• White: \(9\)
Worked Example:
What is the value of a resistor with the bands Yellow, Violet, Red, Gold?
• Band 1 (Yellow) = \(4\)
• Band 2 (Violet) = \(7\)
• Band 3 (Red) = Multiplier of \(10^2\) (or add \(2\) zeros, which means \(\times 100\))
• Band 4 (Gold) = \(\pm 5\%\) tolerance
Value: \(47 \times 100 = 4700\text{ }\Omega\) (or \(4.7\text{ k}\Omega\)) with \(\pm 5\%\) tolerance.
Preferred Value Series (E12 and E24)
Manufacturers do not produce resistors in every conceivable whole number value. Instead, they produce standard ranges called preferred values.
• E12 Series: Designed for resistors with \(\pm 10\%\) tolerance. There are 12 basic values per decade: \(1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2\) (and multiples of 10, such as \(120\text{ }\Omega\), \(4.7\text{ k}\Omega\), \(68\text{ k}\Omega\)).
• E24 Series: Designed for resistors with \(\pm 5\%\) tolerance. It provides 24 values per decade, offering closer intervals between standard values.
Exam Tip: If an exam question asks you to choose a standard commercial resistor for a circuit design after calculating an odd number (like \(312\text{ }\Omega\)), you must select the nearest standard E12 value (e.g., \(330\text{ }\Omega\)).
Resistor Combinations: Series and Parallel
Resistors in Series
When resistors are connected end-to-end in a single line, the total resistance increases because the current has to pass through each obstacle one after the other.
\(R_{\text{Total}} = R_1 + R_2 + R_3 + \dots\)
Resistors in Parallel
When resistors are connected side-by-side across the same two points, the total resistance decreases because the current has multiple paths to flow through.
For two resistors in parallel, you can use either formula:
\(\frac{1}{R_{\text{Total}}} = \frac{1}{R_1} + \frac{1}{R_2}\)
or the product-over-sum rule:
\(R_{\text{Total}} = \frac{R_1 \times R_2}{R_1 + R_2}\)
Potential Divider (Voltage Divider) Circuits
A potential divider is a simple circuit consisting of two resistors connected in series across a power supply. It is used to produce an output voltage (\(V_{\text{out}}\)) that is a fraction of the total supply voltage (\(V_{\text{Total}}\)).
The output voltage taken across the bottom resistor (\(R_2\)) is calculated using:
\(V_{\text{out}} = V_2 = \left( \frac{R_2}{R_1 + R_2} \right) \times V_{\text{Total}}\)
Worked Example:
A \(9\text{ V}\) power supply is connected across \(R_1 = 10\text{ k}\Omega\) and \(R_2 = 20\text{ k}\Omega\). Find \(V_{\text{out}}\) across \(R_2\).
\(V_{\text{out}} = \left( \frac{20}{10 + 20} \right) \times 9\text{ V} = \left( \frac{20}{30} \right) \times 9 = \frac{2}{3} \times 9 = 6\text{ V}\).
Variable and Environmental Resistors (Sensors)
Potential divider circuits become powerful sensing circuits when one of the fixed resistors is replaced by an environmental or variable resistor:
1. Potentiometer / Rheostat:
A manually adjustable resistor with a rotary knob or sliding slider. Used for user inputs such as volume dials or manual sensitivity calibration controls.
2. Light-Dependent Resistor (LDR):
A semiconductor sensor whose resistance changes with light levels.
• In the Dark: High resistance (often hundreds of \(\text{k}\Omega\)).
• In the Light: Low resistance (often drops to a few hundred \(\Omega\)).
Memory Aid: Dark = Dense (High Resistance), Light = Low.
3. Thermistor (NTC - Negative Temperature Coefficient):
A temperature-sensitive resistor whose resistance decreases as temperature increases.
• Cold conditions: High resistance.
• Hot conditions: Low resistance.
Memory Aid: Hot = Low Resistance.
Common Mistake: Many students mistakenly assume that more heat or more light causes higher resistance. Remember: both standard LDRs and NTC thermistors have a negative relationship with their environmental trigger (more light/heat = less resistance).
Key Takeaway: Resistors limit current. Potential dividers split voltage based on resistor ratios. LDRs drop resistance when light increases; NTC thermistors drop resistance when temperature increases.
3. Switches (Mechanical and Sensor Input Devices)
Switches are input devices used to make (close), break (open), or redirect electrical connections within a circuit.
Switch Actions and Classifications
1. SPST (Single Pole, Single Throw):
A simple on/off switch with two terminals. It opens or closes a single circuit path.
Applications: Basic light switches, power switches on appliances.
2. SPDT (Single Pole, Double Throw):
Has one input (common) terminal and two output terminals. It routes the current between two alternative paths.
Applications: Two-way hallway lighting, switching a device between 'Battery' and 'Mains' mode.
3. DPDT (Double Pole, Double Throw):
Contains two separate SPDT switches physically linked so they operate at the exact same time with a single movement.
Applications: Reversing the polarity of DC motors to change their direction of rotation (forward/reverse).
Push Switches (Momentary Action)
1. Push-to-Make (PTM) / Normally Open (NO):
The electrical contacts are separated (open) by default. Pressing the button brings the contacts together to complete the circuit. Releasing the button breaks the circuit again.
Applications: Doorbells, keyboard keys, computer reset buttons.
2. Push-to-Break (PTB) / Normally Closed (NC):
The electrical contacts touch (closed) by default. Pressing the button separates the contacts to break the circuit.
Applications: Refrigerator interior door lights (the closed door keeps the button pushed down so the light stays off; opening the door releases the switch and turns the light on), emergency stop circuits.
Specialised Mechanical and Sensor Switches
1. Reed Switch:
Consists of two flexible, magnetisable metal contacts enclosed inside a sealed glass envelope. When an external magnetic field is brought close, the contacts attract each other and touch (close).
Applications: Burglar alarm window/door sensors, bicycle speedometer wheel sensors.
2. Tilt Switch:
Contains a small conductive ball or liquid capsule inside a chamber. When tilted past a specific angle, the conductive material rolls onto the contacts to complete or break the circuit.
Applications: Tip-over safety cut-offs for space heaters and motorbikes, orientation sensors.
3. Microswitch:
A miniature, very sensitive mechanical switch that requires very little physical force or travel to operate. Usually fitted with an extended lever or roller actuator.
Applications: Limit switches on 3D printers and CNC machines (to detect when the print head reaches the end of the axis), microwave door interlocks, printer paper feed sensors.
Key Takeaway: Switches control circuit flow. SPST turns one line on/off; SPDT selects between two paths; DPDT reverses polarity. PTM is open until pressed; PTB is closed until pressed. Reed switches detect magnets, tilt switches detect angle, and microswitches detect precise mechanical travel.
Quick Revision Checklist
Before sitting your exam, make sure you can:
• State and rearrange Ohm's Law (\(V = I \times R\)) and convert \(\text{mA}\) to \(\text{A}\) and \(\text{k}\Omega\) to \(\Omega\).
• Correctly draw an ammeter in series and a voltmeter in parallel.
• Read a 4-band resistor code and identify tolerance values (Gold \(\pm 5\%\), Silver \(\pm 10\%\)).
• Calculate series and parallel resistor totals.
• Calculate the output voltage (\(V_{\text{out}}\)) of a potential divider circuit.
• Explain how an LDR and an NTC Thermistor respond to changing environmental conditions.
• Identify and explain the operation of SPST, SPDT, DPDT, PTM, PTB, Reed, Tilt, and Microswitches.