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Servo PWM PiZero addon
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The Servo PWM Pi Zero is a 16-channel, 12-bit PWM controller for the Raspberry Pi, suitable for driving LEDs and radio control servos. The board is based around PCA9685 PWM I2C LED controller IC from NXT and can drive each of the 16 outputs with 12 bit (4096 steps) duty cycle from 0% to 100%.
The output frequency is programmable from a typical 40Hz to 1000Hz. Each output driver is programmed to be either open-drain with a 22 mA current sink capability at 5 V or totem pole with a 22 mA sink, 10 mA source capability at 5 V.
220R current limiting resistors are used on each channel allowing you to connect servos or LEDs directly to the outputs.
Note: The Servo Pi is only compatible with analogue RC servos, digital servos will not work.
Introduction:
The PCA9685 contains 5 I2C address selection pins which can be configured using solder bridges on the Servo PWM Pi Zero PCB. Using different I2C addresses on your Servo Pi allows you to stack several boards on a single Raspberry Pi, see the data-sheet for more details on I2C address selection.
The active LOW Output Enable input pin (OE) on the PCA9685 allows asynchronous control of the LED outputs and can be used to set all the outputs to a defined I2C-bus programmable logic state. The OE can also be used to externally ‘pulse width modulate’ the outputs, which is useful when multiple devices need to be dimmed or blinked together using software control.
The OE pin can be connected to pin 7 (GPIO 4) on the Raspberry Pi GPIO port by bridging the solder pads on the Servo Pi marked OE.
Connecting RC Servos
The Servo Pi is only compatible with analogue RC servos, digital servos will not work.
While the Servo Pi uses a 12 bit (4096 step) controller due to the way RC servos are designed only approximately 200 steps will be available for controlling the servo.
You can find an example python library for controlling an RC servo athttps://github.com/abelectronicsuk/ABElectronics_Python_Libraries/tree/master/ServoPi
Powering the Servo PWM Pi Zero
For safety reasons the Servo PWM Pi Zero is not connected to the 5V power rail on the Raspberry Pi.
Due to the high currents used on radio control servos and other PWM devices it is recommended that you use an external 5V power supply to power the Servo Pi. External power can be provided through the 5V and GND pads on the Servo PWM Pi Zero. If you use two power supplies, one for the Raspberry Pi and one for the Servo Pi please ensure that the ground wires on the two supplies are joined and do not have any potential difference between them. Any potential difference between the grounds could damage the Raspberry Pi and yourself.
If you are using the Servo PWM Pi Zero for low current applications, below 300mA, you can power the Servo PWM Pi Zero through the 5V pins on the Raspberry Pi GPIO port by bridging the “PWR Link” pads with a blob of solder.
Bridging the “PWR Link” pads will also allow you to power your Raspberry Pi from the Servo Pi power pins using an external 5V power supply but please note this will bypass the internal fuse on the Raspberry Pi leaving it vulnerable to damage from short circuits on any of the GPIO pins.
Datasheets
Servo Pi Zero Address Selection
Compatibility Chart
| Raspberry Pi Model A | No |
| Raspberry Pi Model B | No |
| Raspberry Pi 1 Model A+ | Yes |
| Raspberry Pi 1 Model B+ | Yes |
| Raspberry Pi 2 Model B | Yes |
| Raspberry Pi 3 Model B | Yes |
| Raspberry Pi Zero | Yes |
| ODroid C1 | Yes |
| ODroid C2 | Yes |
Input Ratings & Specifications
Maximum input voltage: 5.5V when PWR Link is not bridged.
Maximum current on each PWM pin 22mA
Schematic
Mechanical Drawings
Click image to enlarge
Sample Code
We have Arduino, Windows 10 IOT, Python 2 and Python 3 libraries available for this expansion board. You can download all of the libraries from github at:
https://github.com/abelectronicsuk/
Download the Arduino librariesfrom: https://github.com/abelectronicsuk/ABElectronics_Arduino_Libraries
To download the Python 2 libraries to your Raspberry Pi type in terminal:
git clone https://github.com/abelectronicsuk/ABElectronics_Python_Libraries.git
To download the Python 3 libraries to your Raspberry Pi type in terminal:
git clone https://github.com/abelectronicsuk/ABElectronics_Python3_Libraries.git
Download the Windows 10 IOT libraries from:https://github.com/abelectronicsuk/ABElectronics_Win10IOT_Libraries/
PCB Header Assembly Jig
Download and print our PCB Header Assembly Jig to hold your circuit board when soldering the header pins.
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