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ULN2003 + 28BYJ-48 Assembly Guide

This guide covers assembling the budget-friendly configuration using the ULN2003 driver with 28BYJ-48 stepper motor.

Prerequisites

Before starting, ensure you have: - ✅ Gathered all required components - ✅ Basic soldering skills - ✅ Multimeter for testing - ✅ PlatformIO installed for firmware upload

Wiring Diagram Reference

ULN2003 Wiring Diagram

Step-by-Step Assembly

Step 1: Prepare the ESP32-C3 Board

  1. Inspect the board for any obvious damage
  2. Test basic functionality: 
    # Connect via USB and verify in device manager/terminal
# Board should enumerate as USB device
  3. Note pin locations - Compare with your specific board layout

Pin Compatibility

Different ESP32-C3 boards may have different pin layouts. Verify your board matches the expected GPIO assignments.

Step 2: Prepare the ULN2003 Driver

  1. Inspect the ULN2003 board:
  2. Check for cold solder joints
  3. Verify all components are properly seated

  4. Look for the LED indicators (A, B, C, D)

  5. Test the driver (optional but recommended):

  6. Connect 5V and GND
  7. Briefly touch IN1-IN4 to 5V - corresponding LEDs should light

Step 3: Motor Driver Connections

Connect the ESP32-C3 to ULN2003 following this pinout:

ESP32-C3 Pin ULN2003 Pin Wire Color Function
GPIO2 IN1 Blue Motor phase A
GPIO3 IN2 Green Motor phase B
GPIO4 IN3 Yellow Motor phase C
GPIO10 IN4 Orange Motor phase D
5V VCC Red Power supply
GND GND Black Ground

Wiring Tips

  • Use different colored wires for easy identification
  • Keep control wires away from power wires
  • Use solid core wire for breadboard connections
  • Use stranded wire for final assembly

Step 4: Motor Connection

  1. Connect the 28BYJ-48 motor:
  2. The motor comes with a 5-pin JST connector
  3. Connect directly to the ULN2003 motor output

  4. Ensure connector is fully seated

  5. Verify motor wiring:

  6. The wire colors should match (usually Red center, then Blue, Pink, Yellow, Orange)
  7. If colors don't match, refer to your motor's datasheet

Step 5: Display Connection (I2C)

Connect the 0.42" OLED display:

ESP32-C3 Pin OLED Pin Function
GPIO5 SDA I2C Data
GPIO6 SCL I2C Clock
3.3V VCC Power (3.3V only!)
GND GND Ground

Voltage Warning

The OLED display requires 3.3V, not 5V. Connecting 5V will damage the display permanently.

Step 6: Encoder Connection (Optional)

If using the AS5600 magnetic encoder:

ESP32-C3 Pin AS5600 Pin Function
GPIO5 SDA I2C Data (shared with OLED)
GPIO6 SCL I2C Clock (shared with OLED)
3.3V VCC Power (3.3V only!)
GND GND Ground

Magnet placement: - Position the diametric magnet 0.5-3mm above the AS5600 chip - Ensure the magnet rotates with the filter wheel - Center the magnet over the chip for best accuracy

Step 7: Power Supply Options

Option A: USB Power (Simple)

  • Power everything through the ESP32-C3's USB connection
  • Limitation: May not provide enough current for continuous operation
  • Best for: Testing and light-duty use
  • Use a 5V/2A wall adapter or bench supply
  • Connect to ESP32-C3's VIN pin and ULN2003's VCC
  • Best for: Reliable operation and continuous use

Step 8: Initial Testing

  1. Visual inspection:
  2. Check all connections against the diagram
  3. Ensure no short circuits

  4. Verify proper wire routing

  5. Power-on test: 

    # Connect power and verify:
# - ESP32-C3 LED indicators
# - OLED display shows content
# - No smoke or unusual heat

  6. Basic functionality test: 

    # Upload firmware and test basic commands:
#GP          # Should return current position
#STATUS      # Should show system status
#MP2         # Should move motor (if calibrated)

Step 9: Mechanical Assembly

  1. Mount the motor:
  2. Secure the 28BYJ-48 to your filter wheel assembly
  3. Ensure proper alignment with the filter wheel axis

  4. Use flexible coupling if needed to prevent binding

  5. Mount the electronics:

  6. Install ESP32-C3 and ULN2003 in enclosure
  7. Route wires cleanly to avoid interference

  8. Ensure adequate ventilation

  9. Install the encoder (if used):

  10. Mount AS5600 board so the magnet passes over the sensor
  11. Verify 0.5-3mm air gap
  12. Test rotation through full range

Step 10: Software Configuration

  1. Configure the firmware: 

    // In src/config.h, ensure this line is uncommented:
#define MOTOR_DRIVER_ULN2003

// Verify pin assignments match your wiring:
#define MOTOR_PIN1 2    // Connected to ULN2003 IN1
#define MOTOR_PIN2 3    // Connected to ULN2003 IN2
#define MOTOR_PIN3 4    // Connected to ULN2003 IN3
#define MOTOR_PIN4 10   // Connected to ULN2003 IN4

  2. Build and upload: 

    pio run -t upload

  3. Initial calibration: 

    # Connect to serial monitor (115200 baud)
#CAL         # Calibrate current position as home
#MP2         # Test movement to position 2
#GP          # Verify position

Troubleshooting

Motor doesn't move

  • Check power supply - ULN2003 needs adequate 5V current
  • Verify wiring - Ensure all IN1-IN4 connections are correct
  • Test driver - LEDs on ULN2003 should light during movement
  • Check motor - Verify motor connector is fully seated

Erratic movement

  • Loose connections - Check all wire connections
  • Power supply noise - Use decoupling capacitors
  • Interference - Route control wires away from power wires

Display issues

  • Blank display - Check 3.3V power and I2C connections
  • Partial display - Verify I2C address (usually 0x3C)
  • No response - Ensure SDA/SCL aren't swapped

Encoder problems

  • No angle reading - Check magnet position and 3.3V power
  • Erratic readings - Verify magnet distance (0.5-3mm)
  • I2C conflicts - Ensure AS5600 and OLED have different addresses

Performance Optimization

Motor Tuning

// In src/config.h, adjust these values:
#define MOTOR_SPEED 300.0           // Steps per second
#define MAX_MOTOR_SPEED 500.0       // Maximum speed
#define MOTOR_ACCELERATION 200.0     // Acceleration

Movement Optimization

  • Lower speeds reduce noise and improve reliability
  • Higher acceleration provides faster movements
  • Unidirectional mode prevents backlash issues

Next Steps

Once assembly is complete:

  1. Initial Setup - Configure filter names and positions using serial commands
  2. Calibration - Perform precision calibration with #CAL and #REVCAL commands
  3. ASCOM Integration - Connect to astronomy software using the serial protocol

Additional Resources