Questions about custom Raspberry Pi build.

[cjp87]

I wasn't really sure where to put this, so I figured that "general hardware" might be applicable.

I just ordered a Raspberry Pi, and one of my ideas for a build was to build a miniature (and I mean miniature) MAME cabinet. The fine folks working in Raspbian distro compiling have gotten quite a bit of video game/arcade emulation working, and I think a super small and battery-operated single-player cabinet would be a fun and interesting way to dive straight into tinkering with the little computer.

I'll put this up front: I have little to no experience in in-depth hardware builds. I have built several desktop computers, but that doesn't involve anything so complex as trying to ensure a battery can power both the computer and the display, or even how to hook the display up to the computer itself.

And those are probably my two biggest questions: What will I have to do to get a display working with the Pi, and what will I have to do to get a rechargeable battery pack to power the system.

For the display, I was thinking something like a replacement netbook display. Ebay has an Acer 8.9" LCD @ 1024x600 for a reasonable $40, but that's panel only with no controller. I've looked for controllers, but I don't really know what I'm looking for. The Pi can do HDMI and composite out for video.

http://www.ebay.com/itm/New-8-9-WSV...-for-Acer-Aspire-One-ZG5-Matte-/180946739296?

For the battery, I'm at a bit of a loss. I've found some cell phone external batteries and the like on Amazon and eBay, and they seem to offer okay capacities, but since the whole purpose of this is for me to learn something about hardware while making something fun, I wouldn't be opposed to trying to make a battery pack with rechargeable cells, so long as that's not cost-prohibitive. It would need to 1) power the Pi + Display for several hours and 2) output power via micro USB (which is how the Pi receives power).

I feel like I could figure most things out outside of those two pieces. As I said before, I'm pretty inexperienced here, but I'm very willing to learn.

Thanks!

 

[splat_ed]

Well for the USB charging, a quick google provides a large number of products. Usually they're 2-4AA batteries for charging a mobile phone over USB.

As to how long they could power a Pi and monitor, I can't say... but I'm curious as well so I'm going to look around. I'm waiting for my Pi to arrive (hopefully this month)

 

[mfenn]

Look for Lithium Polymer batteries designed for RC planes, cars, and boats. They're essentially the same thing as a cell phone battery, but they come with leads onto which you can solder/crimp any connector that is convenient. You can also get plenty of ready to use chargers.

As for powering the internal devices, most lithium batteries are going to output a multiple of 3.7V, so you will need to buy (or build) a step-down transformer to get the 5V that the Pi wants as input. With luck, you'll be able to find a panel that expects an input voltage reasonably close to what the battery delivers (close enough such that you can tweak it with a couple of resistors).

 

[cjp87]

Thanks for the tips, I've got another question.

So here is one of the displays I'm considering. I found the technical manual, but I'm not sure what it's telling me regarding battery requirements. It looks like it would still operate at 5V, is that accurate? What would I need to do to hook this and the Pi up to the same battery source?

http://www.avidia.com.tw/docs/product/HSD089IFW1-A00.pdf

Edit: Also, how does this battery look? I went with your LiPo idea, and 10,000mAh sounds like a pretty good capacity, especially if the Pi apparently draws 500mA.

http://www.all-battery.com/74volt-10000mah20cli-polylipobatterypack31168.aspx

 

[mfenn]

Yeah, the Pi itself is going to be the least of your power draw. The backlight on the display is going to be the real power hog. The panel you found does accept +5V as input, so you would be able to operate the Pi and the display on the same voltage plane.

You can buy a simple DC-DC converter to convert the 7.4V (or whatever) of the battery to the 5V needed by the other components. This little TI looks pretty cool since it can output up to 6A of 5.5V (adjust to 5V with a resistor) from any input in the range 4.5V to 14V.

My main concern about that panel (or any laptop panel really) is that it uses LVDS signaling. I really know nothing about how to do such a thing, but check out this thread on the Raspberry Pi forums.

 

[cjp87]

I tried to find that TI part, but couldn't find it for sale anywhere. I found this little thing, though:

http://www.ebay.com/itm/1pcs-DC-DC-...6-Power-Supply-Output-1-23V-30V-/251066005460

And it looks like it should allow the voltage I need, but I don't see anything about amperage output, though the tech sheet on the monitor suggests it doesn't require much.

Is there a tutorial somewhere on how to use these converters? Like I said, I'm pretty new to wiring, but this is the some of the kindof stuff I'm hoping to learn from this endeavor.

 

[mfenn]

What you found might work, but its not obvious to me how you adjust the output voltage to match what you want. In theory, you're just going to wire the battery up to the input side of the converter and the Display and Pi (in parallel since you want constant voltage, different currents).

 

[cjp87]

Hmm. I'll have to try and look into it more. I did find a cop-out alternative that doesn't get me the electronics experience but would probably give me a more elegant solution. It's a 10,000mAh battery pack for mobile device charging that has two 5v/2A USB charging ports and a standard wall charger for itself. Could use a standard USB-micro USB for the Pi and a USB-whatever the panel requires for that.

http://www.amazon.com/10000mAh-Exter...dp/B005NGKR54/

I think I'd rather go the DIY route, but given prices of a LiPo battery pack, it may just be smarter to go with the lazy way out.

 

[mfenn]

That thing does look like exactly what you want. It's gonna have the same components as what we've been discussing, but already integrated into a nice package.