Monday, December 31, 2012

Mounting the Pi's

By far the easiest part of the project to bring a dozen or so Raspberry Pi's online was finding a place for them to stay in one location. Rather than going with a Lego approach, I decided to obtain an acrylic sheet (plexiglass) from the Home Depot (SKU: 241610) and mount the Pi's on there.

There were a few reasons for that choice:
1) Plexiglass is cheap and easy to obtain
2) With a simple drill (Dremel-type), it is easy to drill the right size holes in it
3) It doesn't break easily
4) It looks nice

The disadvantage is that standard plexiglass is rather flexible. I expect that, once we are fully completed, I'll need to reinforce it somehow.

The Pi's come with two pre-drilled holes in the circuit board that can be used to mount them  perfectly. I decided to use Radioshack stand-offs (#276-195). They come in packages of four, and include two screws per standoff, which translates to two RPi's per bag. Without crowding the plexiglass board, 14 Pi's easily fit on a 18"x24" board.

Since my goal is to network the Pi's and to power them as well (duh), I decided to mount the top row of RPi's with the SD card facing down and the bottom row of RPi's with the SD card facing up. The means that

a) power is on the inside of the board
b) SD cards are on the inside of the board
c) Network cables are on the edge of the board.

Labeling is an important thing to keep in mind, since the RPi's all look the same. After mounting the first set, the board looks like this:

Note that my idea was that I was going to power the boards via a USB hub. According to spec, each USB port must be able to provide 500mA of current, which I believe to be enough. It turned out very quickly that this was not the case, and I'll elaborate on that in a later post. Suffice it to say, the USB hub is no longer part of the design.

Anyhow; after also mounting the bottom row, adding network cables and a few switches, the whole thing looks like this:

Note the two zip-ties that keep the cable mess together on the board. When we come closer to finishing up, I'll make sure that the network cables are all of the right length, the same color, etc. The eye needs to be pleased also, after all.

In a next post, I'll cover getting power to the board.

Next: Power requirements of the Raspberry Pi Model B

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