Resistors are components that control the amount of current flowing in a circuit. They have two terminals: current flows in one end and out the other.
Fixed Resistors
Made of metal wire (alloys) or carbon.
Carbon resistors have high resistance and a high melting point.
Variable Resistors
These allow you to change the resistance in a circuit, which changes the current.
How it works: A sliding contact moves over a resistive track. The more track the current flows through, the higher the resistance.
Used in volume controls for radios or stereos.
Variable Resistors
Insidea variable resistor
Circuit with a variable resistor.
Light-Dependent Resistors (LDRs)
Resistance changes with light intensity.
In the dark: High resistance (over 1 MΩ).
In bright light: Low resistance (around 400 Ω).
Used in automatic security lights and brightness controls for displays.
Image of an LDR and ciruit symbol
Thermistors
Resistance changes with temperature.
NTC Thermistors: Resistance decreases as temperature increases (e.g., from 2 kΩ at room temperature to 20 Ω at 100 °C).
Used in temperature sensors.
Image of a thermistor.
Graph showing resistance vs. temperature. (graph above shows Resistance decreases as temperature increases)
Relays
A relay is an electrically operated switch.
It uses an electromagnet to control a second circuit.
How it works:
A small current flows through the electromagnet (first circuit), creating a magnetic field.
The magnetic field pulls a switch in the second circuit, closing it.
This allows a larger current to flow in the second circuit, powering devices like motors or lamps.
Diagram of a relay in a circuit.
Sensing Circuits
Relays can be used in circuits that detect changes in temperature or light.
Example: Temperature Alarm Circuit
Purpose: Turns on a lamp when the temperature rises (e.g., in an industrial freezer).
How it works:
At low temperatures, the thermistor has high resistance. The current in the first circuit is small, so the relay remains open. No current flows in the second circuit.
When the temperature rises, the thermistor’s resistance decreases. The current in the first circuit increases, closing the relay switch.
This allows current to flow in the second circuit, lighting the lamp.
Diagram of the temperature alarm circuit
Key Points to Remember
Resistors control current in a circuit.
Fixed resistors have a constant resistance.
Variable resistors allow you to change resistance.
LDRs change resistance with light.
Thermistors change resistance with temperature.
Relays use a small current to control a larger current in another circuit.
Sensing circuits use components like thermistors and LDRs to detect changes in the environment.
Diodes
A diode is a component that allows electric current to flow in one direction only. It acts like a one-way valve for electricity.
How a Diode Works
Circuit Symbol: The arrow in the symbol shows the direction conventional current can flow. The bar blocks current in the opposite direction.
Analogy: Think of a diode as a waterfall. Current can flow “downhill” (in the direction of the arrow) but cannot flow “uphill” (opposite direction).
Light-Emitting Diodes (LEDs)
LEDs are special diodes that emit light when current flows through them.
Circuit Symbol: Similar to a diode, but with arrows to show light emission.
Uses:
Indicator lights (e.g., on TVs, stereos, or chargers).
Traffic lights (LEDs are bright, energy-efficient, and long-lasting).
Modern lighting (LED bulbs use less power and last longer than traditional bulbs).
Key Points to Remember
A diode allows current to flow in one direction only.
The arrow in the diode symbol shows the direction of conventional current.
LEDs are diodes that emit light and are used in many electronic devices and lighting systems.
