Servo - September 2015

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 module PCB.

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 ( github.com/littlebitselectronics/eagle-files).

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 fortune.