The PCB is the electronic backbone for an interactive product. It connects vital pieces to each other. The Stripe PCB has to perform several functions: power the Carambola, connect Carambola to the LED strip, provide a USB port for future extensions and house connectors for programming and troubleshooting.
The Carambola development board
The guys behind Carambola, 8devices, sell a full-featured development board for Carambola. It does all the above, which is why the development board was used in the Stripe prototype #2. It takes an input voltage of 9V or higher and turns that into 3.3V for the Carambola and 5V for the USB port and other peripherals. It has an old-fashioned serial port, two wired network ports, a USB port and a small prototyping area. Perfect for the development phase, but for Stripe it does too much.
From 230V (or 115V) to 5V
The RGB LED strip runs at 5V. The development board takes a higher voltage (9V-20V) and turns that into 3.3V and 5V using two switching regulators. When the LED strip runs at high brightness, the 5V switching regulator has to provide so much current that it starts buzzing audibly. This can be solved by choosing the correct components and filters for the regulator. However, I chose to skip it completely and go for a 5V input instead. Many appliances in the home and office run on 5V, so adapters are easy to get. It also makes the PCB design smaller, simpler and cheaper.
From 5V to 3.3V
To go from 5V to 3.3V it’s tempting to go with a linear regulator. A cheap and tested IC, two capacitors and you’re done:
The linear regulator takes 5V, outputs 3.3V and expels the rest as heat. The maximum efficiency of this circuit results from a simple calculation: (3.3V / 5V) × 100% = 66%. Meh. Time to look around for a more efficient switch-mode DC-DC converter. Although they are harder to design and cost more, a switch-mode DC-DC converter can have efficiencies that easily reach over 90%. My switch-mode converter of choice is the relatively new and inexpensive NCP3170 from ON Semiconductors. Thanks to the trend of semiconductor manufacturers making life easier and easier for electronics designers, all that is needed is a few capacitors, resistors and an inductor:
Turning that into a schematic
Why all this talking about voltage regulators? Well, because apart from that there is not else much to the Stripe v0.1 PCB. Beside the schematic as depicted above, there are some pin headers, a USB port and two rows of female headers for the Carambola module:
Design, order, solder, test
So, what’s next? Pour it into a nice layout:
Send the files off too Seeed Studio’s Fusion PCB service and wait for four weeks for the boards to arrive (yes, that long unfortunately…)
Fire up the soldering iron & hot air gun and be happy:
On to version 0.2
Although the v0.1 PCB works as designed, some footprints don’t match exactly and the placement of some components is not optimal. Also, in the four weeks between ordering and receiving the PCBs a lot has happened in the design of Stripe. I’ve already identified dozens of new features and changes for revision 0.2 of the PCB, so expect more on this topic soon!