jamtholee
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 8:28 am

Hi,

For a DT GCSE Project, I was thinking of designing and building a case for a Raspberry Pi Board B.

I'll post some CAD designs later, but I want to ask a few questions:

1) Can anyone provide accurate dimensions of the board and ports?

2) If I get a 40mm fan and connect the 5v to the 5v on the GPIO headers and the Ground to the Ground on the GPIO; will that work?  (40mm fan runs on 5v)

Thanks very much.

joshhodgson
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 4:50 pm

1. From the Wiki: "The Raspberry Pi measures 85.60mm x 53.98mm x 17mm, with a little overlap for the SD card and connectors which project over the edges."

If it's a GCSE project, though, it might be worth measuring it yourself. Prove your skills and bla bla bla (it was the same last year when I did GCSE Photography). You could do it with a set of Vernier Callipers, which are accurate to at least 0.1mm (and definitely exist somewhere in the school, probably in Physics), or digital ones, which are often accurate to 0.01mm.

2. Theoretically, yes, that would work, but you'd only have 300mA coming through it, based on the 1A usb input to the Pi (and if that's the actual max current drawn). If you were to use a powered USB hub inside the case, that would get you at least 500mA+ for the fan. My guess is a 40mm fan would be safe on that current.

jamtholee
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 5:08 pm

Josh Hodgson said:


2. Theoretically, yes, that would work, but you'd only have 300mA coming through it, based on the 1A usb input to the Pi ... If you were to use a powered USB hub inside the case, that would get you at least 500mA+ for the fan. My guess is a 40mm fan would be safe on that current.


Thanks very much!  I was just researching possible fans and the current rating seems to be between 80mA and 180mA (I've reduced the fans to 20-15mm fans - yep - that's how small the RPi is!).  Is that going to be a problem if there's 300mA supplied?

joshhodgson
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 5:26 pm

Nope, devices only draw what they need (when it comes to 5v DC, at least)

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frying_fish
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 6:49 pm

My only comment:

Why bother with the fan at all? From what I've heard so far, they stay reasonably cool under load, and can operate at moderate temperatures anyway. You would be better off just having some ventilation holes and not having a fan. At the very least it would be quieter to not have a fan. Fans can also just be dust suckers, and will just pull in a lot of dust into the system making it more of a pain to keep it clean and not overheating as much.

Fans are noisy and annoying, I would hope this is capable to run without need of a fan, since I really want to make my kit quieter (especially if I'm going to leave it on 24/7).

jamtholee
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 7:08 pm

Actually, that's quite a good point.  The only reason for it, is to spice up my project (and get extra marks for innovation and blah blah...).  Does anyone have any ideas of what to put in the case (using the GPIO 5v)?

Besides, as promised; some of my (elementary) CAD skills:


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frying_fish
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 7:21 pm

If it is there to provide extra parts for the marks then sure thing go for it. Technically it shouldn't need it, but no harm in designing space for it.

joshhodgson
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 9:22 pm

jamtholee said:


Does anyone have any ideas of what to put in the case (using the GPIO 5v)?


A light-up logo/name tag/just lights in general (which you can switch off, of course)

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rurwin
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Re: GCSE Project: RPi Case

Sat Apr 28, 2012 9:45 pm

jamtholee said:


1) Can anyone provide accurate dimensions of the board and ports?


The "Beta Boards mechanical data" thread pinned at the top of this forum gives measurements as accurately as is possible. The board dimensions seem to be +/- 1mm.

A small fan should be able to run off the GPIO header without problems. The standard rule of thumb is to draw air into the case using the fan. That ensures over-pressure in the case which aids cooling. It also lets you put a dust filter over the fan if you want to be bothered with that. You will need to solve the problem of how to plug the fan onto the GPIO header. You will probably need two connectors each connecting to a single pin.

As regards other features, light-pipes would be a good idea to get the on-board LEDs showing on the outside of the case.

jamtholee
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 9:23 am

Thanks everyone!

I was wondering, can the GPIO headers be programmed from within the OS?  Like can I limit the voltage or current going through the fans if I wanted to.

Also, how do the GPIO headers work?  How do I program them or what can I do with them?  (Can I add temperature sensor)?

Thanks, and apologies for my "newbieness" - I'm still learning...

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nick.mccloud
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 10:21 am

jamtholee said:


I was wondering, can the GPIO headers be programmed from within the OS?


Definitely!


Like can I limit the voltage or current going through the fans if I wanted to.


If you want to programatically vary the speed of the fans this can be arranged with an array of resistors. Not sure about voltages though as the IO pins run at 3.3v


Also, how do the GPIO headers work?  How do I program them or what can I do with them?


The simplest pins to use you can toggle on/off and you can do it from the command line or a shell script or some python. See http://elinux.org/RPi_Low-leve......28GPIO.29 for some programming examples.


(Can I add temperature sensor)?


Yes but not so easy - one of the interface boards that are being developed would be easier here.

The golden rule with projects like this is to do the core part of the project first - the case design - and then add the bells & whistles. I'd start with some slowly flashing LEDs on the GPIO first and then build up from there.

jamtholee
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 1:27 pm

I've just thought of some "crazy" idea:  Is it possible to "join" two Raspberry Pies to make them process quicker or really, for the heck of it?

Some sort of "RAID" array maybe?  And if so, how? (maybe I can factor it in into my design...)

Dapa
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 1:51 pm

It is said to be possible to link two or more PIs together and increase the processing power. There's open source software available that could be complied and will allow code to run across many devices. Not that many people have two or more PIs at the moment!

jamtholee
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 2:01 pm

Thanks, but how are they linked?

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nick.mccloud
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 2:36 pm

jamtholee said:


Thanks, but how are they linked?


Via the network connection. Search the forum for Bramble – the official name for a cluster of RPi's. Be prepared for some seriously technical stuff!

larsth
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 3:34 pm


nmcc said:

jamtholee said:

Like can I limit the voltage or current going through the fans if I wanted to.

If you want to programatically vary the speed of the fans this can be arranged with an array of resistors. Not sure about voltages though as the IO pins run at 3.3v

Also, how do the GPIO headers work?  How do I program them or what can I do with them?


(forum got quotes wrong for some unknown reason):

Max 5mA from any GPIO pin, so you will need a transistor.

In my opinion arrays of resistors is a bad idea. Instead use PWM together with a transistor, and a diode. The diode protects the transistor from the high reverse voltage from the coils in the fan when the transistor disconnects the power.

if you (for fun) want to change the direction in which the fan blows, you will need to use a so called H-bridge circuit.

PWM:

http://en.wikipedia.org/wiki/F.....modulation

http://en.wikipedia.org/wiki/P.....modulation



http://www.barrgroup.com/Embed.....Modulation

H-bridge circuit:

http://www.robotroom.com/Bipol.....ridge.html

http://www.mcmanis.com/chuck/r...../h-bridge/



http://en.wikipedia.org/wiki/H_bridge

In H-bridge circuits it is import to include a delay when you change the direction of the current through the fan, otherwise you may create short circuits for a short amount of time. The delay should be so long what the fan had stopped before you change direction, and switch on the current again.

You can of course connect a PWM circuit to a H-bridge circuit, so you can control both speed and direction.

jamtholee
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Re: GCSE Project: RPi Case

Sun Apr 29, 2012 5:13 pm

Wow... Thanks!  That's a lot of info, but I'll have a look at it.  It looks far too much for a GCSE project, but it might be something to do in the future!

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