I've build this quite a while ago, but I think it's definitely worth sharing this with the internet!

There is a SD-Card with Wifi on the market, which can be easily rooted, making it pretty "insecure", but also pretty interesting for me.

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It by default supplies a webinterface and shows you it's files (you should have access to), but with root access the possibilities (in theory) are endless on this.

So I thought: Let's make this thing mobile!

I got some random SD-Card reader from my local electronics store (the card-reader that comes with the card didn't work for this/for me sadly):

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This thing (should) connect to your PC via USB and also gets power via USB. Connection (meaning data) doesn't matter in this case.
So I had a little thought to connect a battery to the power USB-contacts and this way give the card-reader and thus the SD-Card power.

You can use these
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to connect a 9V battery, but they also work very good for interconnecting parts in electronics.
Combine one of these with a cut up USB-Extension cable and some soldering and you get this connection:

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USB requires constant 5V so we can't use a 9V battery, or about anything else that's not a 5V battery. And even a 5V battery will slowly run out of juice and then deliver less than 5V making the device we power unstable.
So we'll also need a linear transformator. The L78505 is just perfect for that, as it transforms the current from 8-40V to 5V (I don't recommend going to high with the current though, as heat loss is quite high in the L78505).
Soldered all together we get another part:

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Plonk this on a battery (be careful with the direction of the L78505) and
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we get 5V from 6*1,5V=9V. 6 AA batteries should last quite a time as well!

So now we have a battery-driven 5V power source that outputs power to an USB-Plug:

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Plug in the Card-Reader:
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And have a look at our surrounding WLAN-Networks (I called the SD-Card "FBI Van", more to that later :D ):
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What is powered via USB as well?

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The Raspberry Pi is!

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I also found a car plug that outputs 5V for my handsfree speakerset:

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Now you know why my SD-Card is called "FBI Van" :)