Q:

Raspberry Pi wifi wlan0

So I got 2 raspberrys and both are running raspbian. So my wifi adapter is TP-LINK WN725N. 
I configured my wifi like this:
    auto lo

iface lo inet loopback
iface eth0 inet dhcp
allow-hotplug wlan0
iface wlan0 inet dhcp

        wpa-ssid "mynetworkid"
        wpa-psk "mynetworkpassword"

From these posts:
How to set up WiFi with Raspbian "wheezy"
http://learn.adafruit.com/adafruits-raspberry-pi-lesson-3-network-setup/setting-up-wifi-with-occidentalis
Now my problem is, my wifi on my first raspberry pi works but on my second raspberry pi it doesn't work. I noticed that when I booted my second raspberry pi it says:"failed to start wlan0" and "wlan0:no such device"
lsusb:
Device002:Standard Microsystem Corp.
Device001:Linux foundation 2.0 root hub
Device003:Standard Microsystem Corp.
Device004:USB port HUB
Device005:Realtek Semiconductor Corp.
Device006:SiGma Micro HID controller.
Device007:Elan Microelectronics Corp.

ifconfig -a:
It doesn't give the connection I want
What am I doing wrong and how do I fix this?

A:

I got the answer from the raspberry pi official forum. Thanks to MrEngman
There are two versions of this wifi adaptor. V1 has the driver included in most images, V2 requires you to download and install a driver as it is not included in the Linux images.
It appears you probably have V2 and need to download and install the driver. You can determine the wifi version you have if you use command lsusb. This will list the WN725N V2 as
Bus 001 Device 004: ID 0bda:8179 Realtek Semiconductor Corp.

The important info is the ID 0bda:8179.
If it shows this you need to download the driver. If you use a recent version of Raspbian there are precompiled versions available.
Precompiled versions of the driver are available for Raspbian as follows
for 3.6.11+ #538, #541, #545, #551 and #557 use 8188eu-20130830.tar.gz
for 3.6.11+ #524, #528 or #532 use 8188eu-20130815.tar.gz
for 3.6.11+ #371 up to #520 use 8188eu-20130209.tar.gz
Use command uname -a to determine the version you have e.g.
pi@raspberrypi ~ $ uname -a
Linux raspberrypi 3.6.11+ #551 PREEMPT Mon Sep 30 14:42:10 BST 2013 armv6l GNU/Linux
pi@raspberrypi ~ $

Download and install the driver using the commands
wget https://dl.dropboxusercontent.com/u/80256631/8188eu-2013xxyy.tar.gz <--set data code for driver version above
tar -zxvf 8188eu-2013xxyy.tar.gz                                         <--set data code for driver version above
sudo install -p -m 644 8188eu.ko /lib/modules/3.6.11+/kernel/drivers/net/wireless
sudo insmod /lib/modules/3.6.11+/kernel/drivers/net/wireless/8188eu.ko
sudo depmod -a

You can check if the driver is loaded OK with command ifconfig It should show wlan0 something like 
wlan0     Link encap:Ethernet  HWaddr a0:f3:c1:25:7d:28
          inet addr:192.168.16.15  Bcast:192.168.16.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:125431 errors:0 dropped:305 overruns:0 frame:0
          TX packets:5821 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:21001551 (20.0 MiB)  TX bytes:762826 (744.9 KiB)

Again Thanks to: MrEngman on the raspberry pi original forum
Also I had the #474 which needs: 8188eu-20130209.tar.gz

A:

Ran into the same problem a few days back.
Short answer: you need the 8188eu kernel driver for the wireless interface to work.
A bit longer answer based on personal experience: Check the box that came with your product. It should list V2 or V3 somewhere on or near the barcode sticker. That is the revision number. I found out that pre-built drivers posted on the official rPi forums or this repository tend to work very fine with V2, but I did have issues with V3. After building the kernel driver from source(instructions and source can be found on github) my V3 adaptor started working flawlessly.
I also found this blog which features a download for the 8188eu kernel driver allegedly built for V3 but I haven't tested it myself

