Oscillators and Multivibrators, Principles and Applications for SS 3

Class: SS 3
Term: First Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Radio, TV and Electronics
Curriculum Theme: Electronics Circuits
Previous Lesson: Positive and Negative Feedback, Concepts and Differences
Topic: OSCILLATORS AND MULTI-VIBRATORS.
Subject Matter: Principles of oscillators; Types of oscillators- hartly, colpit, tuned collector, etc; Types of multivibrator (IC555), astable, bistable, mono- stable; Construction of a typical oscillator circuit

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define an oscillator and a multivibrator.
• Explain the basic principles of oscillation.
• Identify different types of oscillators such as Hartley, Colpitts, and tuned collector oscillators.
• Differentiate between astable, bistable, and monostable multivibrators, especially those using the IC555.

Affective Domain

• Appreciate the importance of oscillators and multivibrators in electronic systems.
• Show interest in constructing and testing electronic circuits.

Psychomotor Domain

• Construct a simple oscillator circuit.
• Construct a simple multivibrator circuit (e.g., using IC555).
• Use a Cathode Ray Oscilloscope (CRO) to determine the output waveforms of constructed circuits.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• Electronics Principles and Applications by Charles A. Schuler
• The HeadTeacher Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Schematic diagrams of various oscillators and multivibrators.
• Electronic components (resistors, capacitors, inductors, transistors, IC555, LEDs).
• Breadboards and connecting wires.
• DC power supply.
• Cathode Ray Oscilloscope (CRO).
• Multimeter.
• Soldering iron and solder (for permanent construction, if time permits).

Rationale for the Lesson

This lesson is important because oscillators and multivibrators are fundamental building blocks in almost all electronic devices, from clocks and timers to radio transmitters and computers. Understanding their principles and operation provides students with essential knowledge for advanced studies in electronics and practical skills for circuit design and troubleshooting.

Prerequisite/Previous Knowledge

Students should have a basic understanding of passive and active electronic components (resistors, capacitors, inductors, transistors), basic circuit laws (Ohm’s law), and the concept of AC and DC signals.

Lesson Content/Board Summary

OSCILLATORS AND MULTI-VIBRATORS

Principles of Oscillators

An oscillator is an electronic circuit that produces a repetitive, oscillating electronic signal, often a sine wave or a square wave, without any input signal. It converts DC power from a power supply into an AC signal at a specific frequency.

The basic principle of oscillation relies on positive feedback and a frequency-determining network. A small signal at the desired frequency is amplified and fed back to the input in phase (positive feedback), causing the signal to grow until it reaches a stable amplitude determined by the circuit’s non-linear characteristics (e.g., saturation of active devices).

Types of Oscillators

Oscillators are broadly classified based on their frequency-determining components or feedback networks.

1. Hartley Oscillator:

• Uses a tapped inductor (two inductors in series with a common tap) and a capacitor in its tank circuit to determine the frequency.
• Known for its wide frequency range and ease of tuning.

• Uses a tapped capacitor (two capacitors in series with a common tap) and an inductor in its tank circuit.
• Provides good frequency stability and is suitable for high-frequency applications.

• Employs a parallel LC (inductor-capacitor) tank circuit in the collector of a transistor.
• The tank circuit determines the oscillation frequency, and feedback is usually provided by a transformer or a separate winding coupled to the collector coil.

Types of Multivibrators

Multivibrators are electronic circuits used to implement two-state systems like flip-flops, timers, and oscillators. They are characterised by two stable or quasi-stable states and can switch between them.

The IC555 timer is a versatile integrated circuit widely used to implement various multivibrator types.

1. Astable Multivibrator (Free-Running Multivibrator):

• Has no stable states; it continuously switches between two quasi-stable states without external triggering.
• Generates a continuous square wave or rectangular pulse train.
• Used in clock generators, pulse generators, and tone generators.
• Example: IC555 configured to produce a continuous output waveform.

• Has two stable states and remains in one state until an external trigger causes it to switch to the other.
• Requires two trigger pulses to complete one cycle (SET and RESET).
• Used in memory elements, counters, and frequency dividers.

• Has one stable state and one quasi-stable state.
• When triggered, it temporarily switches to the quasi-stable state for a predetermined period before returning to its stable state.
• Used in pulse stretching, timing circuits, and missing pulse detectors.
• Example: IC555 configured to produce a single pulse of a specific duration after a trigger.

Construction of a Typical Oscillator Circuit

A typical oscillator circuit, such as a Hartley or Colpitts oscillator, requires:

1. An active device (e.g., transistor or operational amplifier) for amplification.
2. A frequency-determining network (e.g., LC tank circuit or RC network).
3. A positive feedback path to sustain oscillations.
4. A DC power supply to bias the active device.

For construction on a breadboard:

1. Identify all components from the schematic diagram.
2. Connect the active device (e.g., NPN transistor) to the power supply and ground.
3. Assemble the frequency-determining network (e.g., inductors and capacitors for Hartley/Colpitts).
4. Connect the feedback path from the output back to the input.
5. Ensure proper biasing of the active device.
6. Connect the output to a Cathode Ray Oscilloscope (CRO) to observe the waveform.

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation, Practical Activity.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Activating Prior Knowledge

Teacher’s Activity: The teacher asks students what they understand by “oscillation” in general terms and if they know any devices that produce repetitive signals. The teacher then introduces the topic of oscillators and multivibrators as essential electronic circuits.

Pupils’ Activity: Students respond to questions and listen attentively to the introduction.

Learning Point: Introduction to oscillation

Step 2: Principles of Oscillators

Time: 7 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher defines an oscillator and explains its basic principles, focusing on positive feedback and the role of the frequency-determining network. The teacher uses a simple block diagram or schematic to illustrate the concept.

Pupils’ Activity: Students listen, ask questions, and observe the diagrams.

Learning Point: Oscillator definition and principles

Step 3: Types of Oscillators

Time: 7 minutes

Teaching Skill: Explanation/Categorisation

Teacher’s Activity: The teacher discusses different types of oscillators, specifically Hartley, Colpitts, and Tuned Collector oscillators. The teacher highlights their key components and how they determine frequency, using schematic diagrams for each type.

Pupils’ Activity: Students observe the schematic diagrams and take notes on the characteristics of each oscillator type.

Learning Point: Oscillator types and characteristics

Step 4: Types of Multivibrators

Time: 7 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher defines a multivibrator and explains the three main types: astable, bistable, and monostable. The teacher particularly focuses on the IC555 timer’s application in these circuits and illustrates their output waveforms.

Pupils’ Activity: Students compare the characteristics of different multivibrator types and observe the IC555 application.

Learning Point: Multivibrator types (IC555)

Step 5: Demonstration of Circuit Construction

Time: 8 minutes

Teaching Skill: Demonstration/Practical Application

Teacher’s Activity: The teacher demonstrates the construction of a simple astable multivibrator circuit using an IC555 on a breadboard. The teacher connects the circuit to a power supply and uses a CRO to show the output waveform, explaining each step.

Pupils’ Activity: Students observe the teacher’s demonstration carefully, paying attention to component placement and connections, and the CRO output.

Learning Point: Circuit construction demonstration

Step 6: Practical Activity

Time: 6 minutes

Teaching Skill: Guided Practice/Observation

Teacher’s Activity: The teacher provides components and guides students (in groups or pairs) to build a simple oscillator or multivibrator circuit on their breadboards. The teacher assists them in connecting the CRO to check their circuit outputs.

Pupils’ Activity: Students work in groups/pairs to construct the circuits and observe their outputs on the CRO, identifying the waveforms.

Learning Point: Hands-on circuit building

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is an oscillator?
2. Mention two types of oscillators.
3. Distinguish between an astable and a bistable multivibrator.
4. How would you use a CRO to check the output of an oscillator?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding check

Step 8: Note-Taking

Time: 4 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes into their notebooks, ensuring they focus on the definitions, types, and principles discussed.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording key information

Step 9: Conclusion

Time: 1 minute

Teaching Skill: Reinforcement

Teacher’s Activity: The teacher briefly summarises the importance of oscillators and multivibrators in modern electronics and encourages students to explore more complex circuits.

Pupils’ Activity: Students listen and reflect on the lesson.

Learning Point: Lesson summary

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: Answer the following questions in your notebook:

1. Draw and label the schematic diagram of a Hartley oscillator.
2. Explain the role of positive feedback in an oscillator circuit.
3. List three applications for an astable multivibrator.
4. Research and describe one other type of oscillator not discussed in class.

Lesson Keywords

• Oscillator – An electronic circuit that produces a repetitive electronic signal.
• Multivibrator – An electronic circuit used to implement two-state systems, generating square waves or pulses.
• Astable – A type of multivibrator with no stable states, producing continuous oscillations.
• Bistable – A type of multivibrator with two stable states, requiring external triggers to switch.
• Monostable – A type of multivibrator with one stable state, producing a single pulse when triggered.
• IC555 – A versatile integrated circuit commonly used for timer and multivibrator applications.
• Hartley Oscillator – An oscillator using a tapped inductor in its frequency-determining network.
• Colpitts Oscillator – An oscillator using a tapped capacitor in its frequency-determining network.
• Tuned Collector Oscillator – An oscillator with an LC tank circuit in the collector of a transistor.
• CRO – Cathode Ray Oscilloscope, an instrument used to display and analyse electronic waveforms.

Differentiation

For weaker learners: Provide pre-assembled parts of the circuits or simplified diagrams. Focus on identifying the main components and observing the output on the CRO rather than full construction. Offer more direct guidance during the practical activity.

For faster learners: Challenge them to modify the components (e.g., change capacitor values) in their constructed circuits and observe how the output frequency or pulse width changes on the CRO. Encourage them to research and explain the working of a Schmitt trigger as another type of multivibrator.

Suggested Lesson Videos

For further understanding, search on YouTube for:

• “Oscillator principles and types”
• “IC555 astable multivibrator circuit”
• “Hartley and Colpitts oscillator explanation”
• “How to use a Cathode Ray Oscilloscope”

Teacher Guide for Using This Lesson Plan

Before the lesson, ensure all necessary components for constructing simple oscillator and multivibrator circuits (e.g., IC555, resistors, capacitors, LEDs, breadboards, connecting wires, DC power supply, and a CRO) are prepared and in working condition. Begin by engaging students with real-world examples of where these circuits are used. When explaining the principles, use clear analogies for positive feedback. During the demonstration, clearly articulate each connection and component’s role. For the practical activity, group students effectively and provide close supervision, especially regarding power connections and CRO usage. Emphasise safety precautions when handling electronic components and power supplies. Allow students to copy the Board Summary notes after the practical activities and evaluation to consolidate their learning. Encourage questions throughout the lesson and provide constructive feedback during the practical session.