Power to BlinkyTile constructions
I read Honghong's excellent presentation on building a large BlinkyTile sculpture (http://www.instructables.com/id/Pentagon-LED-Sculpture/), and she provided some very good advice about powering Blinkys.
I have another question that I would appreciate some help with:
If I power a BlinkyTile construction (particularly a large one that draws more than 500mA) with an external power supply through a separate connector (like a barrel jack), do I need to do anything differently with the connector cable between the Lightbuddy and the BlinkyTiles?
Specifically, do I need to modify the connecting cable so that only the ground and data are connected (i.e., cut or remove the 5V line) or would it be OK to leave the 5V line intact?
i don't really understand the circuit well enough to know what happens if you connect power separately and through the Lightbuddy, too. Can someone comment on that?
With the LightBuddy, you don't need to modify the cable. In general, this is something to be concerned about, however the LightBuddy has a diode to prevent voltage leaking from the external power supply back to the computer, and a fuse to prevent shorting the computer power supply to an external power supply.
So, as I understand it, the fuse should limit the current sufficiently that the current through the USB doesn't exceed the what either the LightBuddy or PC can handle, while current/power requirements are met by the external supply? Is this correct? Or is there something else about the circuit that makes the BlinkyTiles preferentially draw current from the external supply? BTW - what is the limit for the LightBuddy - 500mA? I thought I read somewhere that you shouldn't draw more than 500mA from a PC USB. (Please forgive my ignorance; my only formal education is in medicine, I am pretty much self-taught in software and electronics.)
Yep, that sounds basically correct. For normal use (when the power is only coming from USB), there is a fuse on the BlinkyTile controller that prevents it from drawing too much current from the PC (it will draw a bit more than 500mA before it shuts off, which is out of spec but most devices are actually capable of providing around 1A).
There are two diodes between the USB power supply and an external one, and those act to keep the power supplies separated.
The schematic for the board is available here, but it looks like we didn't upload a PDF version (the design file is in Eagle format):
Another question regarding BlinkyTile power:
Is it possible to daisychain BlinkyTile constructions? That is, could three dodecahedrons be connected in series along their edges with either Blinky connectors or jumper wires? I'm guessing that this type of circuit, with a 5v power source, will probably reduce the voltage to around 1.7v for each dodecahedron, and I'm wondering if that would be enough to power the LED's.
Yes, you can chain them- the voltage drop is probably not that big for a single dodecahedron. We've built structures with around 150 tiles on a single (larger) 5v supply without issue. Running a couple of dodecahedrons should be fine, just test the voltage drop with a multimeter if you have one.
I'm interested in powering the Lightbuddy through the pins on the board (rather than through the USB connector). Can I do that? I haven't been able to find a description of the pins. It looks like one of them is labled "3V3". Does that mean it will only safely take a 3V source?
Yes, you can power it from pins on the board. I'd recommend using the two pins just above the USB connector, that are labeled '+' and '-'. Those will accept roughly 4-5V as an input.
The 3.3V connection is an output, and it's not recommended to connect an external power source to that pin.
So, presumably '+' is for 5V input and '-' is for ground? Or should I use the 'GND' pin for ground?
That's correct. You can use either '-' or 'GND' for ground, they're both the same (sorry for the confusing labeling!)