Generating PWM Waves Using an Astable Multivibrator

ELEC 310 – Electronics Laboratory, Koç University
Fall 2025 – Lab Project

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Team

Name	Role
Ahmet Taha Ekim	Circuit Design, Simulation & Implementation
Hasan Can Güneş	Circuit Design, Simulation & Implementation

Contact TA: Ekin Özgönül – eozgonul18@ku.edu.tr

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Overview

This is our term project for the ELEC 310 Electronics Lab course at Koç University. The goal was to design, simulate, and physically build a circuit that generates a Pulse Width Modulated (PWM) signal using a classic astable multivibrator topology, and then use that signal to control the speed of a DC motor.

The whole thing is built entirely out of discrete components — no ICs, no microcontrollers, no op-amps. Just transistors, resistors, capacitors, diodes, and LEDs on a breadboard.

We went through three design iterations:

1. A basic astable multivibrator to get the oscillation going and make sure we understood the fundamentals.
2. A BJT-based PWM motor driver that takes the multivibrator output and drives a power transistor (2N3055) to control a DC motor.
3. A MOSFET-based PWM motor driver that replaces the power BJT with an IRF510 N-channel MOSFET for better switching performance.

All three circuits were first simulated in LTSpice and then built on a breadboard for the demo.

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How It Works

The Astable Multivibrator

The astable multivibrator is a free-running oscillator that continuously switches between two unstable states — it never settles. It's one of the most well-known transistor oscillator circuits out there.

The basic idea is this: two NPN transistors (2N3904 in our case) are cross-coupled through capacitors. When Q1 is ON (saturated), Q2 is OFF (cut-off), and vice versa. The capacitors charge and discharge through the base resistors, and once a capacitor charges enough to bring the opposite transistor's base above ~0.7V, the circuit flips to the other state. This keeps going back and forth indefinitely, producing a square wave at each collector.

The oscillation frequency is determined by:

$$f = \frac{1}{T} = \frac{1}{0.693 \times (R_2 \times C_1 + R_3 \times C_2)}$$

For a symmetric multivibrator (where R2 = R3 and C1 = C2), this simplifies to:

$$f = \frac{1}{1.386 \times R \times C}$$

The LEDs connected to each collector give a nice visual indication of the switching — they blink alternately.

Making It a PWM Generator

To turn this into a PWM generator, we need to be able to adjust the duty cycle — i.e., how long the output stays HIGH vs. LOW in each period. In the basic version with equal R and C values on both sides, you get a ~50% duty cycle. That's not very useful for motor speed control.

The trick is to make the timing resistors asymmetric. By using different values for R2 and R3 (or by using potentiometers), we can independently control how long each half of the cycle takes. A larger R2 means C1 takes longer to charge, so Q2 stays OFF longer — increasing the duty cycle on one side.

In our PWM versions, we replaced the fixed base resistors with a potentiometer arrangement so you can smoothly vary the duty cycle from roughly 20% to 80% by just turning a knob. This directly controls the average voltage delivered to the motor and hence its speed.

Motor Driver Stage

The astable multivibrator by itself can't deliver enough current to drive a motor. So we added an output stage:

• BJT version: The PWM signal from the multivibrator is fed (through a biasing network with R6, Q3, Q4) to a 2N3055 power NPN transistor (Q5) configured as a switch. When the PWM signal is HIGH, Q5 saturates and current flows through the motor. When LOW, Q5 cuts off. A flyback diode (1N914) across the motor protects Q5 from inductive voltage spikes when it turns off.

• MOSFET version: Same concept, but the output stage uses an IRF510 N-channel MOSFET (M1) instead. The MOSFET has the advantage of a very high input impedance (voltage-driven gate), so it doesn't load the oscillator circuit as much. A gate pull-down resistor ensures the MOSFET turns off cleanly, and a flyback diode protects against back-EMF from the motor.

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Circuit Designs

1. Basic Astable Multivibrator (Astable_Multivibrator.asc)

The simplest version — just the oscillator with two LEDs.

Component	Value	Purpose
Q1, Q2	2N3904	NPN switching transistors
R1, R4	1 kΩ	Collector resistors (LED current limiting)
R2, R3	47 kΩ	Base/timing resistors
C1, C2	10 µF	Timing capacitors
D1, D2	QTLP690C	LEDs for visual output
V1	9V	DC power supply

Estimated frequency: f = 1 / (1.386 × 47k × 10µ) ≈ 1.53 Hz (slow enough to see the LEDs blink)

2. PWM Motor Driver – BJT Version (Astable_Multivibrator_pwm_bjt.asc)

Astable multivibrator with adjustable duty cycle driving a 2N3055 power transistor for motor control.

Component	Value	Purpose
Q1, Q2	2N3904	Multivibrator transistors
Q3, Q4	2N3904	Intermediate driver/buffer stage
Q5	2N3055	Power transistor (motor driver)
R1, R4	1 kΩ	Collector resistors
R2, R3	50 kΩ	Timing resistors (potentiometers)
R5	4.7 kΩ	Bias resistor for center tap
R6	75 kΩ	Base resistor for Q3
R7	100 Ω	Emitter resistor for Q4
R8	2.2 kΩ	Collector resistor for R8
R10	75 kΩ	Diode biasing resistor
C1, C2	100 nF	Timing capacitors
D1, D2	QTLP690C	LEDs
D3, D4	1N914	Flyback / protection diodes
DC_MOT	50 Ω	DC motor (modeled as resistor)
V1	12V	DC power supply

Estimated frequency: f = 1 / (1.386 × 50k × 100n) ≈ 144 Hz (fast enough for smooth motor control)

3. PWM Motor Driver – MOSFET Version (Astable_Multivibrator_pwm_mosfet.asc)

Same oscillator concept but with an IRF510 MOSFET driving the load.

Component	Value	Purpose
Q1, Q2	2N3904	Multivibrator transistors
M1	IRF510	N-channel power MOSFET (motor driver)
R1, R4	1 kΩ	Collector resistors
R2	20 kΩ	Timing resistor (one side)
R3	80 kΩ	Timing resistor (other side)
R5	4.7 kΩ	Center tap bias resistor
R6	50 Ω	Motor load resistor
R7	100 kΩ	Gate drive resistor
R8	1 MΩ	Gate pull-down resistor
C1, C2	10 µF	Timing capacitors
D1, D2	QTLP690C	LEDs
D3	Diode	Flyback protection
V1	12V	DC power supply

Note: R2 ≠ R3 here (20k vs 80k), so the duty cycle is intentionally asymmetric (~80%) to demonstrate speed control without a potentiometer.

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Repository Structure

ELEC310-Project/
├── README.md                                  ← You are here
├── LTSpice/
│   ├── Astable_Multivibrator.asc              ← Basic oscillator simulation
│   ├── Astable_Multivibrator_pwm_bjt.asc      ← PWM + BJT motor driver simulation
│   └── Astable_Multivibrator_pwm_mosfet.asc   ← PWM + MOSFET motor driver simulation
├── ELEC310-PROJECT.docx                       ← Project report
├── Elec310_Fall25_Project_Document (1).pdf    ← Course project manual

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How to Run the Simulations

1. Install LTSpice — it's free from Analog Devices. Available for Windows (and runs fine on macOS via Crossover/Wine or the native Mac version).
2. Open any of the .asc files from the LTSpice/ folder.
3. Click Run (the running man icon) to start the transient simulation.
4. Probe the collector nodes of Q1/Q2 to see the oscillator waveforms, or the output node to see the PWM signal driving the motor.

For the basic multivibrator, the simulation runs for 3 seconds so you can observe several cycles of the blinking LEDs. The BJT PWM version runs for 50ms to show several PWM cycles at the higher frequency.

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Parts We Used

Here's the actual bill of materials we sourced for the breadboard build:

• 2× 2N3904 NPN transistors
• 1× 2N3055 NPN power transistor (for the BJT version) or 1× IRF510 N-channel MOSFET (for the MOSFET version)
• Assorted resistors (1kΩ, 4.7kΩ, 2.2kΩ, 47kΩ, 75kΩ, 100Ω)
• 2× 50kΩ potentiometers (for adjustable duty cycle)
• Capacitors (100nF ceramic, 10µF electrolytic)
• 2× LEDs
• 2× 1N914 signal diodes
• Small DC motor
• 9V / 12V power supply or battery
• Breadboard and jumper wires

Most of these parts can be found at electronics shops in Karaköy or from online retailers like direnc.net, Robotistan, or Özdisan.

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Design Notes & Lessons Learned

• Start with the basic astable multivibrator. Get that working first before adding the motor driver stage. It saves a lot of debugging time.
• Potentiometers are your friend. Even if you calculated exact resistor values, real-world component tolerances and transistor variations mean you'll need to tweak things. Having pots in the timing network lets you dial in the frequency and duty cycle on the fly.
• The MOSFET version is cleaner for switching. Since the IRF510 gate is voltage-driven, it doesn't draw current from the oscillator. The BJT version needed extra buffering stages (Q3, Q4) to avoid loading down the multivibrator.
• Always include flyback diodes across inductive loads. We learned this the hard way — without D3, the back-EMF from the motor when Q5 turns off can destroy the transistor.
• Simulate before you build. LTSpice saved us from a lot of wasted effort. Several component values that looked fine on paper didn't work in practice, and the simulation caught those issues early.
• Capacitor values matter a lot. Switching from 10µF to 100nF in the BJT version raised the frequency from ~1.5 Hz to ~144 Hz. Make sure you're in the right ballpark for your application.

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References

• Sedra & Smith, Microelectronic Circuits – Chapter on multivibrators and oscillator circuits.
• LTSpice Documentation
• Homemade Circuits – Transistor Projects
• 2N3904 Datasheet – ON Semiconductor
• 2N3055 Datasheet – ON Semiconductor
• IRF510 Datasheet – Vishay Siliconix

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License

This project was done for educational purposes as part of the ELEC 310 course at Koç University. Feel free to use it as a reference, but please give credit if you base your work on ours.