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https://github.com/cscott/xorduino

XOrduino -- an arduino leonardo/scratch sensor board mash up
https://github.com/cscott/xorduino

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XOrduino -- an arduino leonardo/scratch sensor board mash up

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README 

        
This is an Open Hardware peripheral for the OLPC XO-1/1.5/1.75/4.



The files are created using Eagle 6.2.0 on Linux and the SparkFun footprint

libraries.



Strong inspiration drawn from the following Open Hardware designs (thanks for

sharing!):

* The Arduino Leonardo (arduino-compatible ATmega32U4 design)

* SparkFun's ATmega32U4 breakout 

  for figuring out which parts of the Leonardo design are actually

  mandatory

* SparkFun's Scratch Sensor Board-compatible PicoBoard

    



Some notes:



0. This board integrates Arduino, Scratch Sensor Board, and turtle robot

   functionality.  There are many optional parts: if you want to build

   just the Arduino, you may; if you want to populate the robot functionality

   but not the sensor board, that's fine, too.  If you populate the

   peripherals but not the ATmega32u4 processor, this makes a splendid

   "shield" for the XO Stick -- one way to amortize the cost of

   the robot functionality is to have just one XOrduino-bot shield per

   classroom, while still giving the kids their own XO Stick "brains"

   which they can mount on top of it.



    The "bot" features of the board include a current-limited motor driver,

   two bump switches at the front of the board, and two optical sensors

   for measuring motor rotation, allowing the user to write code like

   "rotate left 90 degrees" instead of "rotate left 2 seconds", which

   experimentally kids find much harder to understand.  The XOrduino

   board mounts on a Tamiya Twin Motor Gearbox (part #70097), and there's

   a mounting hole for a ~25mm long standoff to serve as a front skid.



    The motors are set up to draw 1A max, which matches the power source

   ability of the XO's USB ports.  The USB standard only allows 500mA

   draw, which would require adjusting F1 and RN7.



1. Many of the Leonardo design's components are optional in a

   lowest-possible-cost design, for a number of different reasons:

   a) USB port protection which is already included on the XO

      motherboard,

   b) ESD and protection diodes (helpful but not required),

   c) bypass caps and ferrites for less noise on analog signals

      (helpful but not required).

   Wherever possible I've included the pads anyway, so that less

   budget-conscious users can populate these components.  At times

   this has required jumper traces which you need to cut if you

   decide to populate the optional components.



2. I eliminated the multi-way 3.3V/5V internal/external power supply

   functionality of the Arduino to save cost.  The XOrduino is 5V

   only, powered by the USB port.  The Arduino "3.3V" pin on the

   shield connector is left disconnected.  I've replaced this with

   a lower cost step-up regulator, allowing "bot" users to power

   the XOrduino with a 3-cell (4.5v) battery pack.  Again, all

   these components are optional: the XOrduino will run fine

   powered by its USB tether.



3. The first version of this board used a large number of through-hole

   parts.  This revision is less afraid of surface mount parts (since

   the ATmega32U4 is only available in an SMT package anyway), which

   save board space and lower BOM cost.  I'm attempting to make the

   XOrduino local-assembly-friendly, so I surface mount footprints are

   as large as possible: all discrete components are 0805 or larger.

   Like the XO Stick, this has been routed as a two layer board to

   lower reproduction costs, although if you fabricate with a 4-layer

   board you will get more robust power and ground planes.



    One possibly-controversial change in this version is to use

   surface mount resistor arrays to reduce part count and BOM cost;

   they may turn out to be too small to hand assemble.  We'll see.



4. I made this Scratch Sensor Board compatible.  Even though many of

   the scratch sensors are redudant on the XO (it has a microphone

   already, and the XO-1.75 includes an ambient light sensor), they

   gain new relevance in the "turtle bot" mode -- they make it easy to

   build a standalone robot which is "afraid of light", "stops when

   you clap", etc.  The scratch community has a large number of

   interesting examples based around the Scratch Sensor

   Board/Picoboard, many of which can use the 4 additional channels

   provided by the XOrduino ("color etch-a-sketch", for example, with

   knobs for X, Y, color, and line width).  I used 3.5mm jacks instead

   of the Picoboard-standard 2.5mm jacks in order to be compatible

   with probes built for the XO; I hope that was a good choice.



    See https://github.com/osbock/ScratchSensors for an arduino sketch

   to support the Scratch Sensor board functionality; it might be nicer

   to integrate this into the bootloader so that it's "always there".



5. There's a USB plug integrated with the PCB with break-off tabs to

   increase the thickness to the USB standard 1.6mm.  There are pads

   for a through-hole USB mini-B connector and a more-standard surface

   mount USB micro-B connector, but they are rather expensive (~$1).



    When the discrete connector is populated, it leaves the USB

   signals and power/gnd exposed on big metal tabs, which could be

   non-ideal.  Certainly you shouldn't plug in to more than one jack

   at a time.



  -- C. Scott Ananian, 2012-06-09, revised 2012-07-30, 2012-08-08.