BC547 Transistor Guide: Pinout, Specifications & Arduino Projects

The BC547 stands as one of the most widely used transistors in electronics, found in everything from simple LED circuits to complex amplifiers. This tiny three-legged component bridges the gap between low-power microcontrollers and high-current loads, making it essential for Arduino projects. Whether you're switching LEDs, driving relays, or building amplifier circuits, mastering the BC547 opens doors to projects that require more current or voltage than microcontrollers can provide directly.

What is BC547 and Why It's Essential

The BC547 is a bipolar junction transistor (BJT) of the NPN type, meaning it controls current flow from collector to emitter when base current is applied. Think of it as an electronic switch or amplifier controlled by small signals—applying tiny base current (microamps to milliamps) controls much larger collector current (up to 100 milliamps).

This current amplification capability makes BC547 invaluable for Arduino projects. Arduino pins can safely output up to 20 mA, insufficient for motors, relays, or high-power LEDs. The BC547 bridges this gap, allowing low-current microcontroller outputs to control high-current loads. It's the interface between your Arduino's brain and the real world's brawn.

The BC547's popularity stems from several factors: extremely low cost (often under $0.10), widespread availability, reliable performance, and beginner-friendly characteristics. It doesn't require heat sinks for typical applications, operates on standard voltages, and forgives minor calculation errors that would destroy more sensitive components. Understanding BC547 teaches fundamental transistor principles applicable to thousands of other transistors.

BC547 Pin Configuration and Identification

The BC547 comes in a TO-92 plastic package, featuring three leads (pins) arranged in a row. Correct pin identification is crucial—reversing connections damages the transistor or circuit.

Pin Configuration (facing flat side, pins pointing down):

  Flat side (front)

   _______________

  /               \

 |    BC547        |

  \_______________/

      |   |   |

      C   B   E

  Collector Base Emitter

Collector (C): Left pin when viewing the flat side. Connects to the positive side of the load you're controlling. Current flows from collector to emitter when the transistor is on.

Base (B): Middle pin. Receives control signal from Arduino or other control circuit. A small current here (typically 1-5 mA) controls a much larger collector-emitter current.

Emitter (E): Right pin when viewed from the flat side. Connects to ground in most common circuits (common-emitter configuration). Current exits through the emitter.

Identification Tips

Look for the flat and curved sides—the flat side faces you for standard CBE (Collector-Base-Emitter) orientation from left to right. Some transistors have printing indicating the base pin or part number. When in doubt, consult the datasheet or use a multimeter's transistor-testing function (hFE mode) to identify the pins correctly.

Many beginners accidentally wire BC547 backward. Unlike resistors, transistors have strict polarity. Reversed connections prevent operation and can cause permanent damage during power application.

BC547 Technical Specifications

Understanding specifications helps select appropriate BC547 variants and design safe circuits.

Maximum Ratings:

• Collector-Emitter Voltage (VCEO): 45V maximum

• Collector Current (IC): 100 mA maximum continuous

• Power Dissipation (Ptot): 500 mW at 25°C ambient

• Operating Temperature: -55°C to +150°C junction temperature

Electrical Characteristics:

• DC Current Gain (hFE): Varies by variant (110 to 800)

• Collector-Emitter Saturation Voltage: 0.2V typical

• Base-Emitter Voltage: 0.6-0.7V when conducting

BC547 Variants by Gain:

The BC547 family includes three gain-sorted variants:

• BC547A: hFE = 110 to 220

• BC547B: hFE = 200 to 450

• BC547C: hFE = 420 to 800

Higher gain means less base current needed to control the collector current. BC547B represents the standard choice, balancing gain with predictability. BC547C's higher gain is better for amplifying weak signals, while BC547A works fine when the base current isn't limited.

The formula relating base current (IB) to collector current (IC): IC = hFE × IB

For example, with hFE=200 (BC547B), 0.5 mA base current controls 100 mA collector current: IC = 200 × 0.0005 = 0.1A = 100 mA

How BC547 Works: Switch vs. Amplifier Mode

The BC547 operates in three distinct regions depending on base-emitter voltage and current.

Cutoff Region (Off State): When base-emitter voltage stays below approximately 0.6V, the transistor remains off. No significant current flows from collector to emitter, acting as an open switch.

Active Region (Amplifier Mode): When the base-emitter voltage reaches 0.6-0.7V, the transistor enters the active region, where the collector current is proportional to the base current via the gain (hFE). Analog circuits use this region for amplification.

Saturation Region (On State): Providing sufficient base current entirely turns on the transistor, minimizing collector-emitter voltage (typically 0.2V). The transistor acts as a closed switch, allowing maximum current flow. Digital switching applications operate in saturation.

Saturation Design Formula

Calculate base current, ensuring saturation even with the minimum specified gain:

IB(saturation) = IC / (hFE_min × 0.5)

The 0.5 factor provides a margin ensuring saturation across temperature and component variations.

BC547 vs BC548 vs BC549: Choosing the Right One

BC547: General-purpose choice for switching applications, LED drivers, relay control, and digital circuits. The higher voltage rating (45V) provides an extra safety margin.

BC548: Better for audio applications due to lower noise characteristics. Slightly lower voltage rating (30V vs 45V) but sufficient for most Arduino projects.

BC549: Optimized for low-noise audio preamplifiers and signal processing where noise minimization is critical.

For Arduino projects involving switching (LEDs, relays, motors via drivers), BC547 works perfectly. Audio amplification projects benefit from the lower noise specifications of BC548 or BC549.

Arduino Projects Using BC547

Project 1: High-Power LED Control

Arduino pins output 20 mA maximum. High-power LEDs require 100-350 mA or more, necessitating BC547 driver circuits.

Circuit Design:

Arduino Pin 9 → 1kΩ Resistor → Base (BC547)

                                Collector → LED → Current-Limiting Resistor → VCC (5-12V)

                                Emitter → GND

Code Example:

cpp

const int ledPin = 9;  // PWM pin for brightness control

void setup() {

  pinMode(ledPin, OUTPUT);

}

void loop() {

  // Fade LED in and out

  for (int brightness = 0; brightness <= 255; brightness++) {

    analogWrite(ledPin, brightness);

    delay(5);

  }

  

  for (int brightness = 255; brightness >= 0; brightness--) {

    analogWrite(ledPin, brightness);

    delay(5);

  }

}

```

**Base Resistor Calculation:**

Assuming 1W LED at 350 mA, Arduino outputs 5V, transistor VBE = 0.7V, minimum hFE = 200:

```

IB(sat) = IC / (hFE × 0.5) = 0.35 / (200 × 0.5) = 3.5 mA

RB = (Vout - VBE) / IB = (5 - 0.7) / 0.0035 = 1.2 kΩ

```

Use a one kΩ resistor (standard value, provides a safety margin).

### Project 2: Relay Driver Circuit

Relays control AC appliances or high-voltage DC loads but require 60-90 mA, more than Arduino provides. BC547 switches relay coils safely.

**Circuit Configuration:**

```

Arduino Pin 7 → 4.7kΩ Resistor → Base (BC547)

                                 Collector → Relay Coil → VCC (5V)

                                 Emitter → GND

Flyback Diode (1N4007): Cathode to VCC, Anode to Collector

CRITICAL: The flyback diode protects the transistor from voltage spikes when the relay coil de-energizes. Never omit this diode—inductive kickback destroys transistors instantly.

Code Example:

cpp

const int relayPin = 7;

void setup() {

  pinMode(relayPin, OUTPUT);

}

void loop() {

  digitalWrite(relayPin, HIGH);  // Relay on

  delay(5000);

  

  digitalWrite(relayPin, LOW);   // Relay off

  delay(5000);

}

```

**Base Resistor Calculation:**

5V relay coil typically draws 70-80 mA. Using BC547 with hFE = 200 minimum:

```

IB(sat) = 0.08 / (200 × 0.5) = 0.8 mA

RB = (5 - 0.7) / 0.0008 = 5.4 kΩ

```

Use a 4.7 kΩ resistor (a standard value that provides sufficient base current).

### Project 3: Small DC Motor Control

While dedicated motor drivers suit most applications, the BC547 can directly control small DC motors up to 100 mA.

**Circuit:**

```

Arduino Pin 6 (PWM) → Base Resistor → Base (BC547)

                                      Collector → Motor (+) → VCC

                                      Emitter → GND

Flyback Diode (1N4007): Cathode to VCC, Anode to Collector

Code:

cpp

const int motorPin = 6;  // PWM pin

void setup() {

  pinMode(motorPin, OUTPUT);

}

void loop() {

  // Ramp up motor speed

  for (int speed = 0; speed <= 255; speed++) {

    analogWrite(motorPin, speed);

    delay(10);

  }

  delay(1000);

  

  // Stop motor

  analogWrite(motorPin, 0);

  delay(2000);

}

Add a flyback diode across the motor terminals to protect against inductive kickback.

Project 4: Multiple LED Array Control

Control 10-15 LEDs with BC547, drawing power from an external supply rather than the Arduino's limited current capacity.

Design: Wire the LEDs in parallel with current-limiting resistors, connecting all anodes to the BC547 collector. Arduino controls the base through a resistor. This powers the entire array from an external 5-12V supply, while the Arduino provides only a tiny base current (1-3 mA).

Common Mistakes and Troubleshooting

Transistor Not Switching On:

• Verify correct pinout (CBE when flat side faces you)

• Check that the base resistor isn't too large

• Ensure sufficient voltage at the Arduino output pin

• Test the Arduino pin with the LED and the resistor first

Transistor Gets Hot:

• The collector current may exceed the 100 mA rating

• Check for short circuits in the load

• Verify transistor is saturating (VCE should be ~0.2V when on)

• Consider a higher-current transistor like TIP120

Load Doesn't Turn Completely Off:

• Base pin may be floating (add 10kΩ pull-down resistor from base to ground)

• Check for leakage current in the circuit

• The transistor may be damaged

Transistor Failed/Damaged:

• Missing flyback diode on inductive load (relay, motor)

• Exceeded voltage or current ratings

• Reversed base-emitter voltage

• Overheating due to insufficient base current

Conclusion

The BC547 transistor represents an essential building block in electronics, bridging the gap between low-power microcontrollers and higher-power loads. Its simplicity, reliability, and low cost make it ideal for learning fundamental transistor concepts while building practical projects.

Start with simple LED switching circuits. Graduate to relay control. Experiment with motor speed control. Each project builds understanding of current flow, saturation, and proper base resistor calculation—skills transferable across all transistor applications.

Ready to start switching? Build the LED control circuit first, calculating and installing the appropriate base resistor, test with different Arduino PWM values, observing how the transistor switches cleanly on and off. Within hours, you'll confidently design transistor switching circuits for any application. The BC547's three pins unlock countless possibilities from simple switches to complex control systems, limited only by your creativity and understanding.