TinyBot: Robot Family Development Board

I assembled a couple of tinybots from the latest version of the PCB, and it seems to work great! A few build notes:

1. I am using slices of pencil eraser for wheels. The axles will press right into the eraser without any sort of pilot hole, but seem to grip pretty well.
2. The latest PCB has solder pads for attaching the motors which works pretty well with plenty of flux and heat. Just make sure the motors are either perfectly level or slightly sloped down at the outside ends or else the eraser wheels (which are barely larger than these motors) won't touch the ground.
3. Don't forget to wire the motors in opposite polarity or the tinybot will spin in circles =). Since they are facing opposite directions, they need to rotate in opposite directions!

At some point I would like to build a small army of these with different sensors, but I think for now I will move on to other projects (#pizero!). Here are a couple things I would add / modify for the next version if and when I (or you!) get around to it:

• Smaller battery that nevertheless has sufficient output to drive the tinybot for a reasonable number of minutes.
• On/off switch!
• Alternatively, On/Off/Program switch along with header for connecting to Arduino (running ArduinoISP sketch) for in circuit programming. The programming circuit is pretty simple: http://www.instructables.com/id/ATtiny-Programming-Shield-for-Arduino-1/
• Protection circuit that "debounces" charging voltage when first plugging the tinybot into USB port (I have fried a tinybot's charging circuit due to the rapid "flickering" that occurs as you insert it).

I have updated the PCB design to address the following issues:

- Custom Eagle library I created for the battery holder had holes that were too small.

- Wrong footprint for the mosfets--made it harder than necessary to solder

- I added some solder pads on the under side for potentially soldering the motors (or even motor mounts) directly onto the board (if successful, that would mean no more hot glue!)

Note that these HAVE NOT BEEN TESTED YET...design is at the board house (using dirtypcbs.com again), and should be in my hands in a few weeks (went with the free shipping this time -- Hong Kong Airmail).

Sorry for the lack of updates. <excuses> insert usual excuses here </excuses>. Since my last update I have identified a couple bugs in the board...most notably that the holes are too small for the battery holder, and drilling them larger breaks one of the traces that runs next to it. I will update the gerber files and post them here in the next week or two. Also still thinking about the ideal wheels/grippers for these guys.

PCBs (12 of them!) arrived from dirtypcbs.com, they look great!  A couple of issues to resolve...got the hole sizes wrong for the battery holder so needed to re-drill.  Drilled through a tracing so had to add a jumper.  But after that they work!  Will update eagle files ASAP.  

The main issue now is motor mounts.  I am using the hot glue method at the moment.  But if suitable wheels could be found (see for example ZaidPirwani's comment down below) then we could look at a clip or other method for mounting the motors parallel to the board. 

First batch of PCBs are manufactured and are on their way from the PCB house.  Should be here next week!  All other components are in hand and ready to go.  In the mean time, I've been studying youtube videos on hand soldering SMD components.  Here's my favorite: https://www.youtube.com/watch?v=5uiroWBkdFY

Finally found (on ebay, of all places) a source for cheaper rechargeable lithium ion 2450 size batteries.  At 95 cents each, that is less than 1/2 the cost of prior source: http://goo.gl/HYa7EU, and brings the project cost below the sacred threshold of $10 per tinybot (not including the cost of component shipping from Mouser, although even then the cost would only be a hair more than $10 if you build 10 at a time).

I created a Mouser.com project page to facilitate ordering of the parts for this project: http://goo.gl/kvc0AO. Caveat emptor, I have tried to make sure the parts listed there are correct, but have not yet placed the order myself.

Note that, as mentioned in the details section above, the Mouser project page does not include the items that cannot (or should not, for price reasons) come from Mouser.com.  I.e., the PCB, the battery, and the motors come from elsewhere.  (See updated BOM in the Github repository for details.)

Note also that I am not partial to Mouser...it just so happens neither Newark nor Digikey have the requisite battery holder in stock, and to control shipping costs in makes sense to get everything from one supplier to the extent possible.  (I still use aliexpress.com for the motors since they have them very cheap there and free shipping, and Ebay since that is the cheapest source for the batteries, and the seller ships for free also.)

I would like to find an inexpensive rechargeable coin cell with solder tabs with reasonable discharge capacity. The solder tabs would enable a smaller footprint. 

Ooof.  Caught a couple schematic blunders on the mosfet wiring for the motors.  Corrected the schematic and board, and update github repository.

And...ordered first patch of PCBs from dirtypcbs.com!   10 boards for $14 (although I paid extra for faster shipping).  Should be here in a week or two.