28BYJ-48 — 5V Unipolar Stepper Motor with ULN2003 Driver Board

Overview

The 28BYJ-48 is a 5V DC 4-phase 5-wire unipolar geared stepper motor. It is almost universally paired with a ULN2003 Darlington Transistor Array driver breakout board in beginner electronics, Arduino, and Raspberry Pi starter kits.

The motor incorporates an internal 1/64 gear reduction gearbox that produces high torque ( $>300\text{ g}\cdot\text{cm}$ ) at slow rotational speeds. The ULN2003 driver board accepts 3.3V or 5V logic inputs (IN1–IN4) to energize the motor coils and includes 4 status LEDs to visualize stepping sequences.

Quick reference


Motor operating voltage	5.0 V DC (nominal)
Coil resistance	$50\text{ }\Omega \pm 7\%$ per phase
Internal step angle	5.625° / 64 = 0.08789° per output shaft step
Gearbox reduction ratio	1:64 (exact ratio: $\frac{64}{1}$ or $63.6839:1$ )
Steps per full revolution	2048 steps (full-step) / 4096 steps (half-step)
Pull-in torque	$> 300\text{ g}\cdot\text{cm}$ (at 100 Hz frequency)
Driver IC	ULN2003A (7-channel Darlington transistor array)
Driver input logic	3.3 V to 5.0 V TTL/CMOS Compatible

Terminals & cable pinout

Motor 5-Pin JST Connector (Plugs into ULN2003 board)

Pin	Wire Color	Coil Connection	Description
1	Blue	Coil 1 (Phase A)	Driven by ULN2003 `OUT1`
2	Pink	Coil 2 (Phase B)	Driven by ULN2003 `OUT2`
3	Yellow	Coil 3 (Phase C)	Driven by ULN2003 `OUT3`
4	Orange	Coil 4 (Phase D)	Driven by ULN2003 `OUT4`
5	Red	Common Positive (`COM`)	Connects to +5V power supply rail

ULN2003 Driver Board Pins

Pin	Name	Type	Description
1–4	`IN1` – `IN4`	Digital Input	Logic inputs from microcontroller (Active-HIGH)
5	`GND`	Power	Common Ground (0 V)
6	`VCC` / `+`	Power	Motor power supply (+5 V to +12 V DC)
Jumper	`ON/OFF`	Power Jumper	Enables power to the ULN2003 chip and indicator LEDs

Electrical & mechanical specifications

Electrical Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Operating Voltage	$V_{CC}$	4.5	5.0	5.5	V	DC
Phase Current	$I_{phase}$	—	160	240	mA	2 coils energized simultaneously
Coil Resistance	$R_{phase}$	46.5	50.0	53.5	Ω	At $25^\circ\text{C}$
Insulated Resistance	$R_{ins}$	10	—	—	MΩ	500 V DC
Max Pull-in Frequency	$f_{pull}$	500	—	—	Hz	No load
Max No-load Frequency	$f_{free}$	900	—	—	Hz	No load

Mechanical Specifications

Parameter	Value	Unit	Notes
Motor Diameter	28.0	mm	Cylindrical housing
Output Shaft Diameter	5.0	mm	D-shaft (flatted)
Gear Reduction	1:64	—	Internal spur gear train
Noise Level	$< 35$	dB	At 120 Hz operating frequency
Temperature Rise	$< 40$	K	At 120 Hz, 5V DC supply

Stepping sequences

4-Step Sequence (Full-Step / High Torque)

Step	`IN1`	`IN2`	`IN3`	`IN4`
1	HIGH	HIGH	LOW	LOW
2	LOW	HIGH	HIGH	LOW
3	LOW	LOW	HIGH	HIGH
4	HIGH	LOW	LOW	HIGH

8-Step Sequence (Half-Step / Smooth Motion — 4096 steps/rev)

Step	`IN1`	`IN2`	`IN3`	`IN4`
1	HIGH	LOW	LOW	LOW
2	HIGH	HIGH	LOW	LOW
3	LOW	HIGH	LOW	LOW
4	LOW	HIGH	HIGH	LOW
5	LOW	LOW	HIGH	LOW
6	LOW	LOW	HIGH	HIGH
7	LOW	LOW	LOW	HIGH
8	HIGH	LOW	LOW	HIGH

Wiring

ULN2003 Driver Board	Microcontroller / Power Supply	Notes
`IN1`	Digital Pin `8` (GPIO output)	Phase A control
`IN2`	Digital Pin `9` (GPIO output)	Phase B control
`IN3`	Digital Pin `10` (GPIO output)	Phase C control
`IN4`	Digital Pin `11` (GPIO output)	Phase D control
`GND`	Common Ground (`GND`)	Connect to MCU GND and Power Supply GND
`VCC` / `+`	External `+5V` Power Supply	Power supply (do not use Arduino 5V pin)

Warning

Inductive Spike & Thermal Warning:

ULN2003 inputs are active HIGH: driving an input HIGH turns ON the internal Darlington transistor, pulling the corresponding motor coil pin to Ground.
De-energize all coils (IN1–IN4 = LOW) when the motor is stopped to prevent continuous current draw ( $240\text{ mA}$ ) and overheating.

Common mistakes

Powering motor directly from microcontroller 5V rail: Energizing 2 coils draws up to 240 mA, which causes supply voltage droop and resets the microcontroller.
Leaving coils energized when stopped: Unlike bipolar stepper drivers with automatic current reduction, keeping inputs HIGH when idle wastes power and heats up the motor.
Stepping too fast at startup: Attempting to start stepping at frequencies above 500 Hz without a frequency acceleration ramp will cause the motor to hum and stall without rotating.

Notes

Exact gear ratio is $\frac{3125}{49} \approx 63.6839:1$ . For precise $360^\circ$ rotation without cumulative positional drift over long periods, use $4075.7728$ steps per revolution in half-step mode.

Interface	4-Wire Parallel (IN1-IN4)
Package	28mm diameter geared motor housing + ULN2003 breakout PCB