ULN2003 IC Pinout, Working Principle, and Circuit

All About IC ULN2003

The ULN2003 IC is a high-voltage, high-current Darlington transistor array that is widely used in relay control, stepper motor driving, LED matrix displays, and solenoid operations. It acts as a power interface between low-power microcontrollers and high-power loads, making it an essential component in Arduino, Raspberry Pi, ESP32, and STM32-based projects.

This article provides a comprehensive guide on ULN2003 IC pinout, working principle, circuit design, and practical applications, including stepper motor control, relay switching, and LED driving.

What is ULN2003 IC?

The ULN2003 is a seven-channel Darlington transistor array designed to drive high-current loads using low-power logic signals. It is frequently used for relay activation, stepper motor control, LED display driving, and industrial automation systems.

One of its key advantages is the built-in freewheeling diodes, which provide protection against back EMF (electromotive force) when switching inductive loads such as relays, motors, and solenoids.

ULN2003 Key Features:

• Number of channels: 7 Darlington transistor pairs
• Input voltage compatibility: TTL and CMOS logic
• Maximum output current: 500mA per channel
• Maximum output voltage: 50V
• Built-in protection diodes: Yes (for back EMF suppression)
• Common package types: DIP-16, SOIC-16

The ULN2003A is a popular variant of the ULN2003 series and is widely used in motor driver modules and relay control circuits.

ULN2003 IC Pinout and Configuration

Understanding the ULN2003 pin configuration is crucial for proper circuit design. The 16-pin ULN2003 IC consists of 7 input pins, 7 output pins, a common cathode diode pin, and a ground pin.

ULN2003 Pinout:

Pin Number	Name	Description
1-7	IN 1-7	Inputs for the seven Darlington pairs. Controls corresponding outputs.
8	GND	Ground connection.
9	COMMON FREE WHEELING DIODES	Connects to freewheeling diodes for inductive load protection.
10-16	OUT 7-1	Outputs of the Darlington pairs. Drives external loads.

ULN2003 Pin Functionality:

Inputs (Pins 1-7): he ULN2003 accepts TTL (5V) and CMOS (3.3V) logic signals to control outputs.
Outputs (Pins 10-16): The outputs act as open-collector switches to drive high-voltage, high-current loads (e.g., relays, motors, LEDs).
Ground (Pin 8): It connects to the system ground.
Common (Pin 9): It’s used for freewheeling diodes that protect against back EMF from inductive loads (like motors and relays).

How Does ULN2003 Work? (Working Principle)

The ULN2003 working principle is based on the Darlington transistor configuration, which amplifies weak input signals to control high-power loads.

How ULN2003 Works Step-by-Step:

1. A low-power input signal (from a microcontroller like Arduino) is applied to one of the input pins.
2. The input signal activates a Darlington transistor pair, which significantly amplifies the current.
3. The corresponding output pin switches ON, allowing current to flow through the connected load.
4. The built-in freewheeling diodes protect against voltage spikes when switching inductive loads (e.g., motors, solenoids).
5. When the input signal goes LOW, the transistor switches OFF, cutting power to the load.

Advantages of Using ULN2003:

Supports high current and voltage loads (up to 50V, 500mA per channel)

Works with microcontrollers (Arduino, ESP32, STM32, Raspberry Pi)

Built-in protection diodes prevent damage from back EMF

Ideal for relay driver, stepper motor driver, LED driver applications

ULN2003 Schematic and Circuit Design

The ULN2003 IC is widely used in relay circuits, stepper motor controllers, and LED drivers.

ULN2003 Schematic

Basic ULN2003 Circuit for Relay Control

To drive a relay with an Arduino, you can use the following circuit:

Components Required:

• ULN2003 IC
• 5V relay
• Arduino board (e.g., Arduino Uno)
• Diode (1N4007 for protection)
• Power supply (5V-12V, depending on the relay voltage)

Circuit Connection:

1. Connect the input pin (e.g., IN1) to an Arduino digital output pin (D3).
2. Connect the output pin (OUT1) to the relay coil.
3. Connect the COM pin (Pin 9) to the power supply (Vcc).
4. Place a diode across the relay coil (cathode to Vcc, anode to OUT1).
5. Connect ULN2003 GND (Pin 8) to Arduino GND.
6. Use a power supply (5V or 12V) to power the relay.

When the Arduino sends a HIGH signal to the ULN2003 input, the relay is activated. The diode prevents back EMF from damaging the circuit.

ULN2003 Applications and Practical Uses

The ULN2003 transistor array is used in a wide range of applications, including:

Stepper Motor Control: Drives unipolar stepper motors (e.g., 28BYJ-48) in robotics and automation.

Relay Driver: Controls multiple relays in home automation and industrial switching.

LED Driver: Powers high-voltage LED arrays in electronic display boards.

Solenoid and Buzzer Control: Used in door locks, alarms, and automation projects.

Logic Buffering and Voltage Level Shifting: Interfaces between low-voltage logic and high-power circuits.

ULN2003 and Arduino Projects

The ULN2003 is a Darlington transistor array commonly used with Arduino to control stepper motors, relays, LEDs, and solenoids. It can handle up to 500mA per channel, making it ideal for high-power applications.

Popular Projects:

• Stepper Motor Control – Drives 28BYJ-48 stepper motors for robotics and automation.
• Relay Switching – Controls high-voltage devices like lights and fans.
• LED Effects – Powers LED chasers and dynamic lighting.
• Solenoid Activation – Operates door locks and vending machines.
• Buzzer Control – Drives alarms and sound notifications.

With its simplicity and versatility, ULN2003 is a key component for various Arduino-based electronics projects.

ULN2003 Equivalent and Alternatives

If ULN2003 is unavailable, consider these equivalent ICs:

Model	Transistor Type	Channels	Output Current (per channel)	Max Output Voltage	Supply Voltage	Package Option
ULN2003	Darlington Transistor	7	500 mA	50 V	Not required	DIP-16
TPL7407LA	N-channel MOSFET	7	600 mA	40 V	8.5 V min	SOIC-16
TBD62003AFWG	N-channel MOSFET	7	500 mA	50 V	Not required	HSOP-16
ULN2803	Darlington Transistor	8	500 mA	50 V	Not required	DIP-18
TLP621	Phototransistor Array	4	50 mA	35 V	4.75 V min	DIP-4
MOC5031	Optocoupler	4	50 mA	35 V	4.75 V min	DIP-4

Each alternative has specific advantages depending on the application.

ULN2003 Package Types

The ULN2003 IC is available in multiple package types:

DIP-16

DIP-16 (Through-Hole) – Best for prototyping and breadboard circuits.

SOIC-16

SOIC-16 (Surface Mount) – Used in compact PCB designs.

Conclusion

The ULN2003 IC is an essential Darlington transistor array used in relay control, stepper motor driving, LED arrays, and automation projects. Its ability to handle high-current loads while interfacing with low-power logic circuits makes it a go-to choice for Arduino and Raspberry Pi projects.

By understanding the ULN2003 pinout, working principle, and circuit design, you can effectively integrate this IC into your electronics projects. Whether you’re building a home automation system, controlling motors, or working with industrial electronics, ULN2003 remains a reliable and cost-effective solution.