ok so i decided to show off my stuffs. a couple days ago i got my raspberry pi. i then ****ed around with linux till i was sick of it. so instead i put my time and energy ito a couple other things. first of which every computer needs a case. since the pi ships caseless you usually need to provide for this yourself. one thing i have in abundence is legos. technics all, with some mindstorms thrown in for good measure. so it seemed like a choice building material for the case.
another problem i had was the lack of an appropriate power supply. somehow i was under the impression that i had a microusb adapter somewhere. turns out i did not. i had a bunch of usb adapters but these were for the larger though slightly similar mini b (i think) configuration. why they didnt just throw on a standard 5v barrel jack instead is beyond me. theyre cheaper. but i found out that you can use pins 2 and 6 on the gpio header. this bipasses a protection fuse, but oh well. safety 3*10^666rd. so i have a box full of linear regulators. some of them can do 100mw, some can do 500, but i used a big ol' to-220 package lm7805 which can feed an amp and a half with only some minor heating issues.
i had a supply running on a breadboard for the first couple days, which ultimately proved to be unreliable due to cats. well the next obvious step in prototyping is to break out some strip board and cobble a more semi-permanent solution. of course i skipped that step right into actual board design. its a fairly basic circuit after all. i didnt want another wall of text post like i posted so heres some pics instead:
starting off a horrible pic for demonic effect. gradient rendering and 666 in binary etched into the desk next to it. idk why i kept this one other than that it looks like it came out of hell, but here you go.
why i picked yellow was beyond me, i could have built it in red. black was not an option because i dont have any of those cool panels in black. the composite and audio ports have good access, the ethernet jack is a little bit recessed but is still accessible. the usb ports are almost flush with the exterior but its easy to plug in most normal sized usb plugs.
this pic shows the card slot, and impossible to find unless your a phone whore micro usb plug that i will probibly never use. hdmi plug could be broken out better. thing is i dont have a single hdmi cable at all, so i couldnt test it to see if it connects up fine. you can see the edge of my power board here. it has a power switch a 12v barrel jack. the 2 pins next to it are for connecting a battery pack. the two pins on the other side of the switch are power output. i was going to connect a cable from here to the pi, but decided to connect it internally. these pins can provide power to a usb hub if need be. i wanted to make it a female plug (i tend to favor female for sources and male for things that draw power, but **** gets mixed up), but didnt feel like cutting some breakaway headers down with my dremel.
opening it up to show the pi inside. it just sits on the bottom of the panels. it rests on the usb ports and the bottom of the card reader. i wanted to put some rubber feet in there but i couldnt find any in my junk boxes. i usually save stuff like that just for such an occasion. you can also see the power board here.
thats the top view. the design of this board puts most of the copper to work as a heat sink. the big fin sticking up in the back is also for heat dissipation. this was actually a peice of board trimming that still had a good layer of copper on it, so i soldered it to the heat sink portion of the board to dump a little extra heat. its a little crooked and warped and the soldering aint too pretty, but it was meant to get rid of heat, not look pretty.
the heat sink on the regulator is bolted to the board with some arctic silver 5 under it to help transfer heat better. even the screw and bolt have ac5 under them. to better transfer heat to the other side of the pcb. the big capacitor is crooked because i had to make a last minute substitution. i was going to use a shorter, fatter 1000 uf capacitor, but it was only rated for 6.5 volts. it takes at least 7 to get a steady 5v out the other end. so this was no good. i desoldered this 470uf cap from a mid 80s stereo board (the barrel jack was also taken from there) the switch was also a salvage item, though i cant remember from what. needless to say the pin spacing was not the same, and i had to solder new pins to the cap because the manufactuerer of the board i took it from clipped the leads too short, ****ers.
the only protection is a schottky diode which prevents reverse polarity. this thing probibly adds half a volt of drop and so is probably not the best choice. this kinda keeps the minimum input voltage at about 7.5 volts. which is a tenth of a volt higher than a nominal voltage for a 2 cell in series lithium battery pack. it works though. i could have used a pfet (according to
afroman you can use one to save reverse polarity damage without a huge voltage drop or a fuse), but im a cheap bastard.
the layout is pretty basic lm7805 setup. the 100nf caps take out high frequency noise, big caps handle low frequency stuffs. so i get a nice filtered voltage. it doesnt need to be too clean because the pi has its own filters and regulators. though it does use the 5v line in a couple places on some of the chips. heres the other side:
all the actual wiring is on this side, the other side is all heat sink. you may notice the kitty in the corner, i cut that with an exacto knife while i was fixing all the holes with a sharpie before etching. i was actually supprised it came out of the chemical bath resembling its original form quite well. decided while populating the board to solder the leads directly to the traces on the board. rather than use the output connector. it looked better will help prevent power connection errors. the lead was clipped off of a front panel header in a long discarded computer case. i always save **** like this because im too cheap to buy modular housings, crimp pins and a crimper.
heres the internals without the power board installed. everything just floats around but the structure in the middle hovering a couple millimeters over the processor keeps the power board from touching the pi. and the heat fin keeps it from sliding out.
and with the power board in place. the board connects right to the gpio port underneath it.
i figure there is enough space for another small board. i might do a breakout for the spi/i2c interfaces, as you can support a large number of external bits of hardware with those and some programming. you can take a dual channel adc and a port expander and do a joystick entirely on the i2c bus. but yea, this is enough to get me started with basic stuffs. il see what else i can do with pi later on.
