Getting Started

This guide walks you through setting up a PCA9685 PWM controller to drive RC servos with BB.Servo.PCA9685.

Hardware Requirements

• Raspberry Pi (or any board with I2C support)
• PCA9685 16-channel PWM controller board
• RC servos (standard hobby servos with 3-wire connectors)
• 5V power supply for the servos
• Jumper wires

Understanding the PCA9685

The PCA9685 is a 16-channel, 12-bit PWM controller that communicates via I2C. Key features:

• 16 independent PWM channels - Control up to 16 servos per board
• I2C interface - Only uses 2 GPIO pins regardless of servo count
• Chainable - Connect multiple boards for more channels (up to 62 boards)
• Hardware PWM - Precise timing without CPU load

Board Pinout

PCA9685 Board
┌─────────────────────────────────────────┐
│  V+  VCC  SDA  SCL  GND  OE            │  ← Control side
├─────────────────────────────────────────┤
│  PWM0  PWM1  PWM2  ... PWM15           │  ← Servo outputs
│  (each has 3 pins: V+, GND, PWM)       │
└─────────────────────────────────────────┘

Wiring

I2C Connection

Servo Power

Important: Servos draw significant current. Do NOT power servos from the Pi's 5V pin. Use an external 5V supply.

Servo Connection

Connect each servo to its channel (0-15):

Wiring Diagram

                    External 5V Supply
                         │
                    ┌────┴────┐
                    │  + -    │
                    └────┬────┘
                         │
Raspberry Pi         PCA9685              Servos
───────────         ────────              ──────
3.3V ──────────────► VCC
GND ───────────────► GND ◄──── GND ◄───── All servo GND
SDA (GPIO 2) ──────► SDA                  (brown wire)
SCL (GPIO 3) ──────► SCL
                     V+ ◄───── +5V ◄───── All servo power
                                          (red wire)
                     PWM0 ────────────────► Servo 0 signal
                     PWM1 ────────────────► Servo 1 signal
                     ...
                     PWM15 ───────────────► Servo 15 signal

Software Setup

1. Enable I2C on Raspberry Pi

# Using raspi-config
sudo raspi-config
# Navigate to: Interface Options → I2C → Enable

# Or edit config directly
echo "dtparam=i2c_arm=on" | sudo tee -a /boot/config.txt
sudo reboot

Verify I2C is enabled:

ls /dev/i2c*
# Should show: /dev/i2c-1

2. Install I2C Tools (Optional but Recommended)

sudo apt-get update
sudo apt-get install i2c-tools

Scan for the PCA9685:

i2cdetect -y 1

You should see the device at address 0x40 (default):

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: 40 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: 70 -- -- -- -- -- -- --

3. Add Dependencies

Add bb_servo_pca9685 to your mix.exs:

def deps do
  [
    {:bb, "~> 0.2"},
    {:bb_servo_pca9685, "~> 0.1"}
  ]
end

Then fetch dependencies:

mix deps.get

Verify Your Setup

Create a simple test to verify everything works:

# In IEx on your Raspberry Pi
iex> {:ok, device} = PCA9685.acquire(bus: "i2c-1", address: 0x40)
{:ok, #PID<0.123.0>}

iex> PCA9685.Device.pulse_width(device, 0, 1500)
:ok

This should move the servo on channel 0 to its centre position. If you see :ok, your setup is working correctly.

Release the device when done:

iex> PCA9685.release(device)
:ok

Multiple PCA9685 Boards

To use more than 16 servos, connect multiple PCA9685 boards to the same I2C bus with different addresses. Change the address using the solder jumpers on the board:

Troubleshooting

"No such file or directory" for /dev/i2c-1

I2C is not enabled:

sudo raspi-config
# Interface Options → I2C → Enable
sudo reboot

Device not found at expected address

1. Check wiring connections
2. Verify VCC is connected to 3.3V (not 5V)
3. Run i2cdetect -y 1 to scan for devices
4. Check address jumpers on the board

Servo doesn't move

1. Check servo power supply is connected
2. Verify the channel number (0-15)
3. Try a different channel
4. Check servo wiring (signal to PWM row)

Servo jitters or moves erratically

1. Use a separate power supply for servos (not the Pi's 5V)
2. Add capacitors across the servo power rails
3. Ensure ground is shared between Pi, PCA9685, and power supply

I2C communication errors

1. Keep I2C wires short (under 50cm)
2. Check for loose connections
3. Try reducing I2C speed if using long wires

Next Steps

Now that your hardware is set up, proceed to Basic Usage to learn how to integrate servos into your BB robot.