Dual LED Flasher Circuit Using 2N2907 Transistors

💡 Introduction

Looking for a simple and fun electronics project? In this post, we’ll explore a Dual LED Flasher Circuit using two 2N2907 PNP transistors. This classic circuit is perfect for beginners who want to understand how transistors, capacitors, and resistors can work together to create a blinking light effect—without using a microcontroller!

🧩 Components Used

• 2 x 2N2907 PNP Transistors
• 2 x Electrolytic Capacitors (e.g., 10µF to 100µF)
• 2 x LEDs
• 2 x 220Ω Resistors (for current limiting to LEDs)
• 2 x 100kΩ Resistors (base bias resistors)
• SPST Switch
• Power Supply (e.g., 5V–12V DC)

🔌 Circuit Explanation

This is a simple astable multivibrator circuit—a self-oscillating circuit that turns two LEDs on and off alternately. Here’s how it works:

🔄 Working Principle

1. Power On: When the switch (SW1) is turned on, voltage is supplied to the circuit.
2. Initial Conduction: One of the transistors (Q1 or Q2) turns on slightly faster due to component tolerances. Let's assume Q1 turns on first.
3. Capacitor Action:

• • When Q1 turns on, the collector voltage drops (more current flows through D1, lighting it up).
  • C2 (connected between the collector of Q1 and the base of Q2) charges and sends a negative pulse to Q2’s base, keeping Q2 off.

1. Charge & Discharge Cycle:

• • As C2 charges via R4, its influence on Q2's base decreases until Q2 starts turning on.
  • When Q2 turns on, C1 does the same to Q1, turning it off and lighting up D2 instead of D1.

1. Alternating Blink: This cycle repeats endlessly—each transistor turns on and off alternately, flashing the LEDs.

🔄 Timing Control

The flashing speed depends on:

• Capacitor values (C1, C2): Larger values = slower blinking.
• Resistor values (R3, R4): Higher resistance = slower charging = slower blink.

We can estimate the blinking period per LED using:

⚠️ Notes:

• Make sure to use polarized electrolytic capacitors and connect them correctly.
• You can experiment with different resistor and capacitor values to change the blink rate.
• Ensure supply voltage matches your LED and transistor ratings (e.g., 5V–12V DC).

✅ Applications:

• LED decorations
• Turn signals
• Toy lighting circuits
• Microcontroller-free timing circuits

📷 Visual Reference

Here’s the schematic we used for the explanation:

💬 Conclusion

This dual LED flasher using 2N2907 PNP transistors is a fantastic beginner project to understand astable multivibrators and transistor switching. It's simple, requires minimal components, and is a great way to dive into practical electronics.

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