46 SERVO 09.2015
littleBits DIY PCB
Hidden deep inside SparkFun Electronics' free Eagle Parts
Master Library ( github.com/sparkfun/SparkFun-Eagle-Libraries) are
two curiously named schematic and printed circuit board (PCB)
patterns: littleBits Male and littleBits Female. These parts are the
input and output portions found on all of the littleBits modules. By
incorporating these two parts into your SMD/SMT or through-hole
schematic diagrams, you can create your own custom littleBits
In order to test this creation process, I designed a littleBits
module for the Atmel ATtiny85 AVR eight-bit microcontroller. My
"tinylittleBits" creation attempts to follow along with the littleBits
Arduino module (w6). As shown in the schematic (Figure A, for
tinylittleBits), this is a very simple layout that combines several SMD
components with three through-hole headers: two of the headers
enable access to six of the ATtiny85 analog pins, while the 2x3 ICSP
header is used for programming the microcontroller.
There is one big caveat with this tinylittleBits design, however.
Rather than using the officially accepted or approved transient
voltage suppressor (TVS) diodes found on the littleBits Arduino
module, I made a substitute. Yeah, I'm not sure these two part
substitutions are suitable for the design, yet. In point of fact, rather
than using the ON Semiconductor ESD9B5-0ST5G diodes, I'm using
Bourns SMAJ5.0CA diodes.
Why? The reason that I made the substitution is due to the
package size of the ON Semiconductor diodes. They are SOD-923!
That is way too small of a package size for me to solder. Conversely,
my Bourns substitutes are a whopping huge DO-214AC package size
that I'm better equipped to solder.
Luckily, my substitutes are bidirectional TVS diodes with ESD
protection just like the littleBits ones, but there is a slight
capacitance difference between the components.
Also, sharp-eyed robot builders will notice in Figure A that the
part package diagram for the Bourns SMAJ5.0CA diodes in my
tinylittleBits schematic is not bidirectional. I wasn't able to quickly
locate an Eagle bidirectional diode part in a DO-214AC package.
Therefore, I used a generic Schottky diode outline instead. You can
see this footprint error on the diode between the signal pin (SIG) and
the ground pin (GND) of both littleBits male and littleBits female parts.
You can compare the diode differences by studying the
littleBits Arduino module schematic and board diagrams.
These files are located in the Wire directory of the Open
Source Hardware littleBits Eagle files respository
The proof of this substitution will be in the fabricating of a
tinylittleBits PCB like the one in Figure B. I'm holding off on this step
until I can locate a better substitute diode. If a better TVS diode can't
be found, then I'll test my design with a small three-PCB test run.
When complete, the PCB will need two gray bitSnap
connectors. These connectors attach to those SparkFun littleBits
male and littleBits female parts that were included in the schematic
design and subsequently added to the PCB. You can find 12 bitSnap
connectors inside the littleBits Hardware Development Kit (HDK), or
you can purchase 12 bitSnaps separately.
When you test and finalize your littleBits PCB design, tell the
good folks at littleBits about your efforts. If they accept your design
for production, you just might get a little bit of fame and a little bit of
Figure A. My tinylittleBits schematic diagram.
Figure B. The tinylittleBits PCB made with Eagle.
Figure 7. A working V6 engine moves with the Coffin Cruiser. Figure 8. A Robosapien watches over the Coffin Cruiser.