The step motor 28BYJ-48 is a 5V unipolar stepper with an integrated gear reduction, delivering roughly 1.0 kg·cm (approx. 0.098 N·m) of safe continuous output torque. If your project requires precise, low-speed positioning for under $3 and does not demand high dynamic torque or rapid acceleration, this motor is your default pick. It dominates light-duty automation like motorized blinds, camera sliders, and small robotic arms because it eliminates the need for expensive microstepping chopper drivers while providing massive step resolution.
The 28BYJ-48 Spec Sheet and Wiring Identification
Before wiring this motor, you must understand its internal gear train. The datasheet nominally lists a 1:64 gear ratio, but the actual physical gear stack yields a ratio of 63.68395:1. This means a half-step sequence of 4,096 steps does not complete a perfect 360-degree revolution; it falls short by roughly 0.18 degrees. For single-pass positioning, this is negligible. For continuous multi-turn rotation, this drift accumulates and requires software compensation.
| Parameter | Value | Practical Note |
|---|---|---|
| Rated Voltage | 5V DC | 12V variants exist; check your casing stamp. |
| Phase Resistance | 50 Ω ± 7% | Draws ~100mA per phase at 5V. |
| Step Angle (Motor) | 5.625° / 64 | Yields 64 steps per motor shaft revolution. |
| Gear Reduction | 1:63.68395 | Actual ratio; 4075.77 half-steps per output rev. |
| Pull-in Torque | >34.3 mN·m (motor) | Multiplied by gear ratio at the output shaft. |
| Safe Output Torque | ~1.0 kg·cm (0.098 N·m) | Theoretical max is higher, but plastic gears strip. |
Terminal Identification and Pinout
The motor terminates in a standard 5-pin JST connector. It is a unipolar design, meaning it has two center-tapped coils. The center taps are tied together internally and brought out to the red wire.
- Red (Pin 5): Common / VCC (Connect to 5V positive supply).
- Pink (Pin 4): Coil A+ (Usually driven by IN1 on ULN2003).
- Yellow (Pin 3): Coil B+ (Usually driven by IN2).
- Orange (Pin 2): Coil A- (Usually driven by IN3).
- Blue (Pin 1): Coil B- (Usually driven by IN4).
Motor Type Comparison: Where the 28BYJ-48 Fits
Selecting the right actuator requires matching the torque curve and control complexity to your load. The 28BYJ-48 is frequently compared to NEMA 17 steppers and micro servos, but they serve entirely different mechanical profiles. Steppers and servos are not interchangeable; servos rely on potentiometer feedback for absolute position over a limited arc, while steppers rely on open-loop step counting for continuous relative positioning.
| Motor Type | Output Torque | Torque Curve Profile | Control Needs | Typical Cost |
|---|---|---|---|---|
| 28BYJ-48 (Unipolar Stepper) | ~1.0 kg·cm | High holding torque at zero speed; drops sharply above 15 RPM. | ULN2003 Darlington array; 4 GPIO pins; simple sequencing. | $1.50 - $3.00 |
| NEMA 17 (Bipolar Stepper) | ~4.0 kg·cm (400 mN·m) | Flat torque curve up to 300+ RPM; excellent dynamic response. | Chopper driver (A4988/TMC2209); STEP/DIR pins; current limiting. | $12.00 - $25.00 |
| SG90 (Micro Servo) | ~1.8 kg·cm | Peak torque only at stall; relies on closed-loop feedback. | PWM signal (50Hz); single GPIO pin; absolute angle limits. | $2.00 - $4.00 |
The 28BYJ-48 wins when you need continuous 360-degree rotation with high step resolution at very low speeds, but cannot justify the cost and wiring complexity of a NEMA 17 system. According to Electronics Tutorials, unipolar steppers like this sacrifice top-end speed and dynamic torque for the sake of simplified drive circuitry, making them ideal for battery-powered, low-RPM applications.
Sizing Rule of Thumb and Worked Load Example
When sizing a stepper motor for a mechanical load, the golden rule of thumb is: The motor's rated holding torque must be at least 2x the calculated peak load torque. This safety factor accounts for breakaway friction, inertial loads during acceleration, and the fact that a stepper's pull-in torque (the torque available to start moving from a standstill) is significantly lower than its holding torque.
Worked Load Example: Motorized Spool
Suppose you are building an automated fishing line spooler or a small winch. You need to lift a 30-gram mass using a 3D-printed spool with a 10mm (1 cm) radius.
- Calculate the Force: Mass (0.03 kg) × Gravity (9.81 m/s²) = 0.294 Newtons.
- Calculate the Load Torque: Force (0.294 N) × Radius (0.01 m) = 0.00294 N·m (or 0.3 kg·cm).
- Apply the Safety Factor: 0.3 kg·cm × 2.0 = 0.6 kg·cm required.
The 28BYJ-48 provides a safe continuous output torque of roughly 1.0 kg·cm. Since 1.0 kg·cm > 0.6 kg·cm, the step motor 28BYJ-48 is correctly sized for this load. If your calculation had exceeded 1.0 kg·cm, you would need to either increase the gear reduction externally or step up to a NEMA 17.
Driver Selection and Failure Signatures
Because the 28BYJ-48 is a unipolar motor, it demands a driver that can sink current from the center-tapped coils to ground sequentially. The universal standard for this is the ULN2003 Darlington transistor array. This board takes 5V logic from your microcontroller (Arduino, ESP32, Pi Pico) and switches the motor's higher current phases. Do not attempt to drive this motor directly from microcontroller GPIO pins; the ~100mA per phase will instantly fry your MCU's output registers.
Recognizing Failure Signatures
Stepper motors fail in highly specific, diagnosable ways. If your 28BYJ-48 is misbehaving, match the symptom to the fix:
- Humming without moving (Stall): The load exceeds the pull-in torque, or your software acceleration ramp is too aggressive. Fix: Reduce the starting RPM in your code, or implement a linear acceleration profile rather than instantly commanding top speed.
- Overheating (Too hot to touch): You are either driving a 5V motor with a 12V supply, or you are leaving the coils energized at 100% duty cycle while stationary. The ULN2003 lacks current chopping; holding the motor stationary burns continuous power as heat. Fix: Disable the driver pins in software when the motor reaches its target position.
- Clicking or grinding (Mechanical Strip): The internal plastic gears are stripping due to shock loads or operating near the absolute stall torque limit. Fix: Add a software current-limiting delay or upgrade to a metal-geared variant if available.
The Bipolar Modification
If you need microstepping for ultra-smooth motion, you can convert the 28BYJ-48 to a bipolar motor. By opening the casing and snipping the PCB trace that connects the red common wire to the coil center taps, the motor becomes a standard bipolar stepper. You can then drive it with modern chopper drivers like the TI DRV8825 or the silent TMC2209, unlocking microstepping and higher usable torque at the cost of a more complex wiring setup.
Decision Tree: Should You Use the 28BYJ-48?
Use this decision matrix to finalize your actuator selection. Follow the logic paths based on your mechanical and electrical constraints.
| Condition | If True... | If False... |
|---|---|---|
| Is the required continuous torque less than 1.0 kg·cm (0.098 N·m)? | Proceed to next question. | Stop. Use a NEMA 17 with an A4988 driver. |
| Is the maximum operating speed below 20 RPM? | Proceed to next question. | Stop. Use a NEMA 17 or a brushed DC motor with an encoder. |
| Do you need absolute position feedback without a homing switch? | Stop. Use a closed-loop servo (e.g., SG90 or SCS-15). | Proceed to final pick. |
| Is the budget strictly under $5 per axis? | Confirm Pick. | Consider NEMA 11 for slightly more torque. |
Final Verdict and Default Picks
For low-speed, low-torque, open-loop positioning tasks like automated pet feeders, lightweight camera pans, or HVAC vent louvers, the hardware choice is absolute.
Default Pick: The 5V 28BYJ-48 step motor paired with a ULN2003 driver board. It provides the highest step resolution per dollar on the market and requires only four standard GPIO pins to operate. Ensure your power supply can deliver 500mA at 5V, and always implement a software acceleration ramp to prevent stalling on startup.
Upgrade Pick: If your load calculations exceed 1.0 kg·cm, or you require speeds above 50 RPM, abandon the 28BYJ-48 immediately. Upgrade to a NEMA 17 (17HS4401) driven by a TMC2209 UART driver. The TMC2209 provides stealthChop silence and StallGuard sensorless homing, solving the torque and noise limitations inherent to the 28BYJ-48's plastic gear train.
