RPI-MDI

Raspberry Pi Servo Card

Features

• Raspberry Pi daughtercard
  • Fits on "old" Model B or "new" Model B Plus board
• Three axis Servo PWM outputs (Servos not included)
• Two PWM Controlled LEDs
• I2C controlled 16-channel PWM controller PCA9685
• 5 channels used as above (Three Servo outputs, 2 LED outputs)
• PWM controller has internal built-in clock generator - no refresh required
• Even though the Raspberry Pi has 3.3V I/O, the PWM outputs run at 5V - compatible with typical servos
• Three switch inputs with on-board pull-ups which can be used as GPIO lines
• 3X of 10 uF SMT capacitors on rear of board for PWM/Servo noise reduction
• Right angle connectors for Low Profile
• Recessed connectors for side connectors for smallest form factor
• Very small size 1.4" x 1"
• Jumper to use 5V from Raspberry Pi (if your power supply has sufficient capacity) or pads for external power or additional capacitance if needed
• Python PWM - Adafruit driver

PCA9685 Block Diagram

Connectors

• Marking is nearest pin 1

• Pin 1 is a square pin

J1 - Servo Output 1 - PWM_LED0 pin on IC

• J1-1 = GND
• J1-2 = VCC (+5V)
• J1-3 = PWM output line

J2 - Servo Output 2 - PWM_LED6 pin on IC

• J2-1 = GND
• J2-2 = VCC (+5V)
• J2-3 = PWM output line

J3 - Servo Output 3 - PWM_LED7 pin on IC

• J3-1 = GND
• J3-2 = VCC (+5V)
• J3-3 = PWM output line

J4 - Switch Input 1 - GPIO_25

• J4-1 = GPIO_25
• J4-2 = GND

J5 - Switch Input 2 - GPIO_4

• J5-1 = GPIO_4
• J5-2 = GND

J6 - Switch Input 3 - GPIO_22

• J6-1 = GPIO_22
• J6-2 = GND

Power/capacitance connector

• This is a three pin L shaped connector located in the following corner of the board.

1. Ground
2. Servo +5V
3. +5V-Raspberry Pi

Power Notes

If your Raspberry Pi +5V power supply has enough current capacity to drive the Pi and your servos then jumper Pins 2 to 3 (the board is shipped in this configuration by default). This will power the Servos from your Raspberry Pi. If when you run servos your Pi crashes it may be from the current draw of the Servos and you will need to consider one of the following options.

If your Raspberry Pi +5V power supply has enough current capacity to drive the Pi and your servos but the voltage drops and your Pi drops out then you may be able to add a capacitor between pin 2 (+ side of your cap) and pin 1 (Ground side of your cap). If your Raspberry Pi +5V power supply does not have enough current capacity to drive the Pi and your servos then remove the jumper between pins 2 to 3. Add external power between pin 2 (+ side of cap) and pin 1 (Ground).

Indicators

• D1 - LED connected to PWM-LED9
• D2 - LED connected to PWM-LED8

Servo Current Draw

Engineering Equations

• q = I * T
• q = C * V
• I * T = C * V
• and
• C = (I * T) / V
• q is the charge (in Coulombs) - divided out in formulas
• I is current (in Amps)
• t is time (in Seconds)
• C is capacitance (in Farads)
• V is the voltage droop (in Volts)

Using the typical values:

• I = 0.5A
• V = 0.25V (allowable voltage drop)
• t = 0.02 secs (50 Hz)

Solving for C:

• C = 0.5 * .02 / 0.25 = 2,500 uF per channel

External Links

• Peak Servo Current Tests - Measured Servo Current Draws

Schematic

• RPI-MDI Schematic

Drivers/Example Code

• Github repo - Driver code

Assembly Sheet

• RPI-MDI Assembly Sheet