Introduction:

This Raspberry Pi project shield is an I/O expansion board to Raspberry Pi 1a+, 1b+, 2 , 3 , Zero and Zero W single board computers. It is designed to have features for projects connecting multiple sensors and servos easily. The features include providing 18 connectors for RC servos, for four i2c slave connectors, re-routed SPI connector and RX/TX UART pins. Extra 5V and 3.3V output power pins will further ease the case of supplying power to other external electronics. Originally RPI can only directly connects to 1 single i2c slave. With this project shield, people can directly connect 4 slaves. In many applications, it takes more i2c sensors or actuators.

Integrating a 40W (5V, 8A) power regulator allows it to drive high load electronics. The regulator takes input of 3.7V lithium batteries and then stepping up to output 5V and 3.3V to connected electronics. With two XH2.54 battery connectors it can embrace very large capacity power in separated battery packs.  The second battery connector can be used to chain up 2nd boards which makes single battery pack to power up multiple boards.  A switch controls the input power on/off. The external on/off switch connector gives convenience to application.

The built-in charging circuit can charge the batteries without disconnecting battery for recharging scenario. Charging port is standard micro USB socket which people  can do charging with any 5V USB power source.  Without battery connecting, this port is also the power source to power up Raspberry Pi. In addition, external pins for connecting different power socket type are also provided.   

With such many good features and characteristics, this project shield is a comprehensive solution for different projects from robotics to IOT applications using Raspberry Pi.

 

Board Layout

 

PWM and Servo Control:

PWM (Pulse Width Modulation) is technique used to control LED dimming, DC motor speed and RC servo movement. Raspberry Pi native hardware support to PWM is very limited.  Widely and commonly approach  is to use GPIO to generate PWM signal. This approach is known as soft PWM.  Supporting connection of  large number of servos is the main feature of this project shield. There are 18 connectors for servos.  Each signal pin is routed to GPIO pins on Raspberry Pi 40-pin header for soft PWM. Couple of soft PWM solutions existing in Raspberry Pi.  Below are most effective implementation.

 

Connector Description:

 

 Above figure describes the connector function. Servo connector signal pin and GPIO mapping is as below

 

Servo Connector GPIO Number 40-pin Header Pin Number
P4 4 7
P5 5 29
P6 6 31 
P7 7 26 
P12 12 32
P13 13 33 
P16 16 36
P17 17 11
P18 18 12 
P19 19 35 
P20 20 38 
P21 21 40 
P22 22 15 
p23 23 16 
P24 24 18 
P25 25 22 
p26 26 37 
P27 27 13 

 

A Robotic Project:

Raspberry Pi powered 3D printed 8 DOF quadruped robots were implemented using this project shield, see www.instructables.com for details.