BJT AstableMultivibratorPCB

Dual-LED Astable Multivibrator PCB

A compact, through-hole printed circuit board design for a classic discrete dual-LED transistor flasher (astable multivibrator). Designed as an educational electronics kit and soldering practice board.

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📋 Table of Contents

• Overview
• Circuit Operation
• Schematic & Bill of Materials (BOM)
• PCB Specifications
• Design Considerations & Tips
• Assembly Guide
• License

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🔍 Overview

The astable multivibrator is a classic oscillator circuit made using two bipolar junction transistors (BJTs), two capacitors, and cross-coupled resistor networks. Because it lacks a stable state, the circuit continuously switches back and forth between two quasi-stable states, causing two LEDs to flash alternately.

Key Features

• All Through-Hole Technology (THT): Easy to hand-solder for beginners, students, and hobbyists.
• Adjustable Frequency: Blink rate can be tuned by changing capacitor ($C_1, C_3$) or base resistor ($R_1, R_4$) values.
• Wide Supply Voltage Range: Operates cleanly on DC supplies between 5 V and 9 V (standard 9V battery compatible).

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⚡ Circuit Operation

The switching frequency is determined by the RC time constants of the cross-coupled networks:

$$t_1 \approx 0.693 \cdot R_1 \cdot C_3$$ $$t_2 \approx 0.693 \cdot R_4 \cdot C_1$$

The total period $T$ and flash frequency $f$ are calculated as:

$$T = t_1 + t_2 \approx 1.386 \cdot R \cdot C \quad (\text{if } R_1 = R_4 \text{ and } C_1 = C_3)$$ $$f = \frac{1}{T} \approx \frac{1}{1.386 \cdot R \cdot C}$$

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📦 Schematic & Bill of Materials (BOM)

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🛠️ PCB Specifications

• Layers: 2 Layers (Top copper: Red, Bottom copper: Blue)
• Board Thickness: 1.6 mm
• Copper Weight: 1 oz ($35,\mu\text{m}$)
• Minimum Trace Width: 0.5 mm (20 mil) recommended for signals and power
• Minimum Spacing / Clearance: 0.25 mm (10 mil)
• Solder Mask Color: User choice (Green / Matte Black / Blue)
• Silkscreen: Top overlay (White)

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💡 Design Considerations & Tips

1. Polarity Check:
   • Ensure electrolytic capacitors ($C_1, C_3$) have their positive terminal facing the transistor collector node (high side) and negative terminal facing the opposite base node.
   • Align the flat edge / short cathode lead of LEDs ($D_1, D_2$) with the corresponding pad marking on the silkscreen.
2. Component Clearances: Maintain at least 1.5 mm spacing between adjacent component bodies (especially between $C_3$ and $D_1$) to prevent mechanical interference.
3. Trace Routing: Avoid 90-degree trace corners; use 45-degree chamfered bends to maintain clean copper deposition and avoid acid traps during etching.
4. Ground Plane: Pour an unfilled copper zone on the bottom/top layer tied to GND for noise reduction and etching economy.

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🔧 Assembly Guide

1. Resistors ($R_1$–$R_4$): Bend leads cleanly and solder flat against the board first. Clip excess leads flush.
2. Transistors ($Q_3, Q_4$): Match the flat edge of the TO-92 package with the silkscreen outline. Keep lead height uniform.
3. LEDs ($D_1, D_2$): Check cathode (flat side / short leg) orientation before soldering.
4. Capacitors ($C_1, C_3$): Note polarity stripe (negative side) and insert according to pad markings.
5. Power Connection: Solder battery clip or terminal block to supply pads ($V_{CC}$ and $GND$). Power up with 5V–9V DC to verify operation.

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📄 License

This hardware project is open-source and shared under the CERN Open Hardware Licence Version 2 - Permissive (or MIT License for documentation/software).