Overview

The ULN2003A (and 16-pin DIP ULN2003) is a 7-channel NPN Darlington transistor array IC manufactured by Texas Instruments, STMicroelectronics, and ON Semiconductor. Ubiquitously included in Arduino and Raspberry Pi starter kits (often pre-assembled on a small breakout board with 4 LED indicator lights to drive 5V 28BYJ-48 unipolar stepper motors), it acts as a high-current low-side switch driver for relays, solenoids, DC motors, and high-voltage displays.

Capable of sinking up to $500\text{ mA}$ per channel ($600\text{ mA}$ peak) at up to $50\text{V}$ DC, the ULN2003A integrates $2.7\ \text{k}\Omega$ series base resistors on each input channel (enabling direct 5V TTL and 3.3V CMOS microcontroller connection) alongside common free-wheeling flyback clamp diodes on pin 9 (COM).

Quick reference

Pinout (DIP-16 Package)

             ┌───┴───┐
        IN 1 ─┤ 1  16 ├─ OUT 1
        IN 2 ─┤ 2  15 ├─ OUT 2
        IN 3 ─┤ 3  14 ├─ OUT 3
        IN 4 ─┤ 4  13 ├─ OUT 4
        IN 5 ─┤ 5  12 ├─ OUT 5
        IN 6 ─┤ 6  11 ├─ OUT 6
        IN 7 ─┤ 7  10 ├─ OUT 7
         GND ─┤ 8   9 ├─ COM (Common Flyback Diode Catch)
             └───────┘

Specifications

Internal Channel Schematics & Inductive Clamp Logic

Each of the 7 channels consists of a two-stage NPN Darlington pair:

  IN (Pin 1) ─── [2.7kΩ] ───┬─── [Collector Base] ─── Open-Collector OUT (Pin 16)
                             │                                 │
                            [NPN 1]                            │
                             │                            [10kΩ Resistor]
                             ├─── [Base NPN 2]                 │
                             │                                 │
                            [NPN 2] ───────────────────────────┴─── [COM Pin 9 Flyback Catch]
                             │
                            GND (Pin 8)

Wiring (28BYJ-48 Stepper Motor Driver Board)

Example (Arduino Stepper Library Code)

#include <Stepper.h>

// 2048 steps per revolution for 28BYJ-48 5V stepper
const int stepsPerRevolution = 2048;

// Initialize Stepper library on pins 8, 10, 9, 11 (Note 10 and 9 sequence)
Stepper myStepper(stepsPerRevolution, 8, 10, 9, 11);

void setup() {
  myStepper.setSpeed(10); // 10 RPM
  Serial.begin(9600);
  Serial.println("28BYJ-48 Stepper Motor Test with ULN2003");
}

void loop() {
  Serial.println("Clockwise rotation...");
  myStepper.step(stepsPerRevolution);
  delay(1000);

  Serial.println("Counter-clockwise rotation...");
  myStepper.step(-stepsPerRevolution);
  delay(1000);
}

Common mistakes

• Forgetting to connect Pin 9 (COM) to $+V_{LOAD}$ when driving relays/solenoids: Pin 9 connects the internal flyback diodes across output channels. Leaving Pin 9 floating allows inductive back-EMF spikes to destroy the output Darlington transistors.
• Exceeding total IC dissipation limit: While each individual channel can sink $500\text{ mA}$, driving all 7 channels simultaneously at 500mA exceeds the $1.5\text{W}$ total DIP-16 package dissipation limit. Limit continuous total current to $\le 1.5\text{A}$ across all active channels.

Notes

• ULN2003A vs ULN2803A vs L293D: ULN2003A has 7 channels (5V TTL compatible); ULN2803A has 8 channels; L293D has 4 half-H-bridge push-pull outputs for bi-directional motor control.