The Raspberry Pi Zero W is a mini-path computer. Now with WiFi and Bluetooth. Installation and configuration
Before you continue: the chance I messed up is very high, I don't blame picon or pi, both are excellent tools and I just want to make them work.
I have sincerely followed the desciptions from the picon of the zero website including software installation and hardware setup. There was no point in time where I actually used picon zero. This was the first installation. I don't know yet if picon is zero, ever processed and if so, still works ... I have one spare zero pi, but don't want to risk it before I know what really went wrong it is necessary. Can anyone help?
Here are a few details:
status components
OS: jessie lite (updated), worked just fine over a month
SD Card: plugged it after the incident into another pi zero and it works just fine
Power supply pi 0: 5V adapter, same as I used over the last weeks
setting Picon zero
- Picon zero set to external power supply:
- motor power jumper set to VIN
- 5V jumper set to RPI (not relevant according to picon zero papers)
- external power supplied by LEGO® Power Functions AAA Batteriebox (88000) with 6x1.2V AAA batteries)
- Lego Batteriebox outgoing line 1 (? V / +) connects to VIN, and GND / 0V / -
connects to GND
Motor set on picon zero
motor A) pins: LEGO® Power Functions M-Motor (8883) No other pins are connected.
raspberry Pi Zero State Now
no green LED, Wi-Fi dongle tidy up easy, no network connection. I'm guessing it's bricked. Edit: Yes, it's definitely brick-faced. The Raspberry Pi Zero does not come with a polyfuse, so there is no overnight healing to be expected, and in my case, it didn't either. I connected it to a power source two days later and it didn't jump to life, it just got very hot. I'm using it as "fit" now, kind of ;-)
Applause
Photos: 
1 replies
I got it working and want to describe in my answer: 1. What I wouldn't do again was probably the cause of death, and 2. Describe the installation in more detail for anyone who wants to try it on their own.
First, I wouldn't be again ...
- includes jumper wires and while external DC power is on
- drives the motor directly to RPI, even for testing different try jumper combinations<�лития>, while the DC power goes
I would be, would be ...
- prepares everything with a book, connects it and starts creaky and after checking the I2C device show (on raspie type i2cdetect-y 1) than I would start the external power supply.
- checks with a voltmeter if the external power supply is working as expected check<�лития> to the correct color coding and hack my lego wires accordingly (just to not spoil this simple task)
- monitors the RPI temperature using an onboard sensor. / opt / vc / bin / vcgencmd measure_temp, turn off everything if it pierces and hope it's not too late
Secondly, here is the best installation documentation (I hope). Keep in mind, this is a € 5 Pi, compared to a € 200 Lego brick. If I break another one, then I'll learn a lot in the process, and it won't break the bank.
First the cable is "hacker penetration". I just used some jumper wires. The Lego connector is laid out as follows, from left to right see photo: ground, unfounded positive, unfounded denial, 9 V.
Color coding
- GND: green less positive
- : the black less negative
- : white (POS and negative can switch for direction changes)
- 9V: red
Here's a working setup: You see
Pi powers the powerbank and is controlled over ssh via Wi-Fi and DynDNS.
Compact version of the Raspberry Pi single board computer.
On a board the size of a USB flash drive, a sandwich from a Broadcom BCM2835 chip with 512 MB of RAM, I / O ports, a slot for a microSD card and multimedia interfaces fit.
Platform video review
Installation and configuration
Examples of work
Board elements
BCM2835 chip
The heart of the Raspberry Pi Zero computer is the Broadcom BCM2835 chip, which is made using SoC (System-on-a-Chip) technology. The crystal includes a CPU ARM1176JZ-F processor overclocked to 1 GHz and a graphics dual-core GPU VideoCore IV coprocessor with a frequency of 250 MHz.
Elpida B4432BBPA-10-F 512 MB RAM is located on top of the BCM2835 chip using PoP technology (Package-on-Package).
Mini-HDMI port
Peripheral connector
Micro-USB port for connecting multimedia devices with a standard USB connector.
For communication, you need an OTG adapter USB (F) - USB Micro (M). Use a USB hub to connect multiple devices.
Power connector
MicroSD slot
Composite video output
Analog video signal output in the form of two solder pins. The signal is used to connect to "warm tube TVs" via an RCA connector, or simply "tulip".
Camera connector (CSI)
Voltage regulator
PAM2306AYPKE two-channel switching voltage regulator with 3.3 V and 1.8 V outputs.Maximum current of each channel is 1 A.
Pinout
The Raspberry Pi Zero has two rows of 20 pins each in the form of tinned holes.
Power pins
5V: The output is supplied with a voltage of 5 V when the board is connected via USB.
3V3: Pin from a voltage regulator with an output of 3.3 volts and a maximum current of 1 A. The regulator provides power to the processor and other elements of the board.
GND: Ground findings.
I / O ports
Unlike platforms with a logic voltage of 5 V, the voltage of the logic levels of the Raspberry Pi is 3.3 V. Outputs for the logic 1 supply 3.3 V, and in input mode they expect to receive no more than 3.3 V. Higher voltage can damage the single board. ...
Be careful when connecting peripherals: make sure that they can function correctly in this voltage range.
Board Pin Name: physical numbering of contacts on the comb.
BCM Pin Name: pin numbering of the Broadcom processor.
WiringPi Pin Name: pin numbering for the Wiring Pi package.
Digital inputs / outputs: There are 26 GPIO I / O pins on the board.
Logic level of one - 3.3 V, zero - 0 V. Maximum output current - 16 mA. In the world of Raspberry Pi, there are three pin numberings:
PWM0: pin 12 (BCM) or 18 (BCM)
PWM1 pin 13 (BCM) or 19 (BCM)
PWM: Two PWM channels with two streams in each.
The pi-zero meson moving at a speed of 0.8 s (c is the speed of light in vacuum) in the laboratory frame of reference decays into two photons γ 1 and γ 2. In the meson's own reference frame, the photon γ 1 was emitted forward, and the photon γ 2 - backward relative to the direction of flight of the meson. The photon velocity γ 1 in the laboratory frame is equal to ...
Answer options: 1) - 0.2 ∙ s; 2) +0.8 s; 3) - 1.0 ∙ s; 4) + 1.0 ∙ s.
If in its own frame of reference moving with speedv \u003d 0.8 · s, a photon was emitted in the direction opposite to the direction of motion of the system, then its velocity in this frame of reference is equal to u "x \u003d - c. The speed of a photon in the laboratory frame of reference is also equal to u x \u003d - c. This can be proved using the relativistic law of addition of velocities:
u х \u003d (u "х +v )/(1 + v · U "x / s 2).
Let's substitute numerical values \u200b\u200binto this formula:
u х \u003d (- s + 0.8 s) / (1 - 0.8 s / s 2) \u003d - 0.2 s / 0.2 \u003d - s.
Answer: option3.
Test 1 - 37
A rigid body from a state of rest begins to rotate around an axis Z with angular acceleration, the projection of which changes over time, as shown in the graph.
The angular velocity of rotation of the body will reach its maximum value at the moment of time equal to ...
Answer options:
1) 2 s; 2) 10 s;
3) 5 s; 4) 3 sec.
By definition, the angular acceleration is equal to the time derivative of the angular velocity: ε \u003d ... Hence: dω \u003d ε dt. Then the integral is equal to ω \u003d ... Graphically, the integral is numerically equal to the area of \u200b\u200bthe figure bounded by the graph of the function ε z (t), by two ordinates t \u003d t 1 and t \u003d t 2 and by the abscissa axis t. The area of \u200b\u200bthe figure can be calculated as the number of cells, limited by the graph of the function, multiplied by the division price of one cell. In our case, the division price is 1 rad / s. Moreover, the area of \u200b\u200bthe figure above the abscissa axis t should be taken with the "+" sign, and below - with the "-" sign. Thus, at the moment of time t \u003d 2 s, the area of \u200b\u200bthe figure is equal to two cells. Consequently, the angular velocity at this moment will be equal to ω (2) \u003d 2 rad / s .. At the moment of time t \u003d 3 s, the angular velocity reaches the value ω (3) \u003d 2 + 1 \u003d 3 rad / s.
At time t \u003d 5 s, the angular velocity can be found as the difference between two areas: ω (5) \u003d 3-4 \u003d -1 rad / s.
At time t \u003d 10 s, the angular velocity is equal to:
ω (10) \u003d - 1-10 \u003d - 11 rad / s.
Thus, the angular velocity will reach its maximum magnitude at the time t \u003d 10 s.
Test 1 - 38
A ball, attached to a spring and set on a horizontal guide, vibrates harmonically.
The graph shows the dependence of the projection of the elastic force of the spring F x on the positive direction of the x axis on the coordinate of the ball.
The work of the elastic force when the ball is displaced from position 0 to position B is ...
Answer options:
1) - 4 10 - 2 J; 2) 4 10 - 2 J;
3) 8 10 - 2 J; 4) 0 J
The work of force when the body is displaced from position x 1 to position x 2 is equal to the integral: A \u003d dx. Graphically, the integral is numerically equal to the area of \u200b\u200bthe figure bounded by the graph of the function F x (x), by two ordinates x \u003d x 1 and x \u003d x 2 and by the abscissa axis x. The area of \u200b\u200bthe figure can be calculated as the number of cells, limited by the graph of the function, multiplied by the division price of one cell. Moreover, the area of \u200b\u200bthe figure above the x-axis should be taken with the “+” sign, and below - with the “-” sign.
In our case, the elastic force according to Hooke's law is: F x \u003d - kx, k is the elastic coefficient, x 1 \u003d 0, x 2 \u003d 40 mm, the division price of one cell is 10 N٠mm \u003d 10 - 2 J. Thus, the work of the elastic force when the ball is displaced from the position0 into positionAT is equal to: A \u003d - 4٠10 - 2 J.
You are interested in buying a Raspberry Pi, but once you start looking, you have a problem: why are there so many models?
While all Raspberry Pi can perform the same tasks, there are specific tasks that are more suited to specific projects. For example, the Raspberry Pi Zero has a lower form factor than others and is therefore often used in projects with limited space.
Here's what you need to know about each Raspberry Pi model and which project they are best suited for.
3 kinds of Pi
As you probably know, there are several major Raspberry Pi models available. At the time of this writing, there are eight options available to buy, but they are divided into three types. 
Model A: first released in 2013, the second revision (A +) appeared in 2014.
Model B: Appearing in April 2012 as the very first Raspberry Pi, the "+" model followed in July 2014. A few months later, in February 2015, the Raspberry Pi 2 launched. This was facilitated by v1.2 of the Raspberry Pi 2 in October 2016. Meanwhile, the Raspberry Pi 3 arrived in February 2016.
Each of these raspberries use board B, not the smaller board A.
ZeroFinally, there is the Raspberry Pi Zero, a smaller version of an already tiny computer. It was first released in November 2015 for just $ 5, and was replaced in May 2016 by the 1.3 board. The third board, Zero W hit shelves in February 2017, and Zero WH was introduced in early 2018.
While they may seem very similar, these boards only have a few identical components. For example, they all use the same graphics setting: Broadcom VideoCore IV, OpenGL ES 2.0, MPEG-2 and VC-1 (licensed) and HDMI support with 1080p30 H.264 / MPEG-4 AVC high-profile decoder and encoder.
Raspberry Pi 1 Model A +
Measuring 65mm x 56.5mm x 10mm and weighing 23 grams, Model A + is a revision of the discontinued Model A. Smaller than its predecessor, it uses the ARMv6Z architecture (32-bit) with Broadcom BCM2835 System-on-a - Chip (SoC). Includes a 700MHz single-core ARM1176JZF-S processor, 512MB of RAM and the same graphics found on every Pi model. 
The device has one USB port and a 15-pin MIPI camera serial interface (CSI) connector. It can be used with the raspberry camera module and its NoIR variant. A MIPI display interface is provided for LCD panels, while a 3.5mm TRRS jack controls composite video and audio output. The board also has a standard HDMI-out. Unlike the original Model A, the A + is equipped with a microSD card slot.
There is also a 40-pin array - 28 of them are for GPIOs and the rest are for I2C, UART, and SPI. They are designed to connect various types of equipment.
Best use... The dimensions of the A + make it particularly suitable for on-camera motion detection. Consider it also for a robot brain, a NAS controller, or even in the center of a high altitude balloon.
Raspberry Pi 1 Model B +
Using the same architecture and SoC as the A +, the Raspberry Pi B + model is larger than 85.6mm × 56.5mm, providing space for additional connectivity. This increases its weight to 45 grams. 
Four USB ports are provided. Via 5-Port USB Hub - The fifth port is for an Ethernet port (10/100 Mbps). Like Model A +, a 15-pin MIPI camera interface (CSI) connector is attached, as well as a MIPI display interface (DSI) for raw LCD panels (a variety of LCDs can be purchased for LCD monitors).
This is the first Pi to offer a microSDHC slot for high-speed microSD cards.
Best use: you can start a low level server with B +. Something like a wireless print server, for example, or a network monitoring tool.
Raspberry Pi 2 Model B
Repeating the B + form factor, the Raspberry Pi 2 has the same dimensions and weight (85.6 mm x 56.5 mm, 45 grams). However, this time the hardware has been beefed up.
Now built on ARMv8-A architecture (64/32-bit), the Pi 2 is powered by a Broadcom BCM2837 SoC with a 900MHz 64-bit quad-core ARM Cortex-A53 processor and a comparatively massive 1GB of RAM. Other than that, there is no difference from B +, but a faster CPU and RAM significantly improves performance.
Best use: as an inspiring digital to install and run Doom without emulation. It is a versatile device that can even be used as a desktop computer.
Raspberry Pi 3 Model B
Further improving the performance of Model B, the Raspberry Pi 3 - with the same weight and dimensions - is equipped with a 64-bit quad-core ARM Cortex-A53 processor clocked at 1.2 GHz (as opposed to the Pi 2 at 900 MHz). 
A powerful USB resource is also included. The Pi 3 is equipped with WiFi 802.11n and Bluetooth 4.1 on board. This means that up to two USB ports are freed up for other purposes.
Even though it has a standard microSDHC card slot, note that the Raspberry Pi 3 can also be booted from USB.
Best use: almost anything that is not limited in size. Pi 3 is the most powerful version, making it especially suitable for retro gaming emulation or as a Kodi box.
The Raspberry Pi 4 is expected to replace this model, but not expected until at least 2019.
Raspberry Pi Zero models
The third version was released at the end of 2015. Only cost $ 5, the computer was the first one mounted on the front of the magazine! Measuring only 65mm x 30mm x 5mm and weighing 9 grams, the Pi Zero is ideal for any project where space and weight are a premium. It's smaller than previous Raspberry Pi models, but the Zero is just as easy to use. 
The release of the Pi Zero marked a major change in the perception of raspberries. The device was no longer limited by its size. USB ports and GPIOs have been completely compressed or completely removed. Lightweight and half the size of the B +, the Zero is still a powerful little computer. 
The Raspberry Pi Zero has a 32-bit ARMv6Z architecture with a Broadcom BCM2835 SoC found in Model A and Model B +. Likewise, the processor is a 1GHz single-core ARM1176JZF-S, similar to the one found on the original Pi (but clocked at 700MHz). It has 512MB total RAM, and 1.3 revision boards (released since May 2016) also have a MIPI camera interface.
Equipped with a micro USB for power and another for data, the Pi Zero has a mini HDMI output and a microSD slot as expected. Stereo audio can be output via GPIO. Although the GPIO pins are removed, the array - along with Run and TV I / O - remains. This means they can still be used either by soldering or manually using GPIO pins (kits available).
Raspberry Pi Zero W
While the Raspberry Pi Zero was good, limited USB connectivity just got trickier. Extracting a leaf from the Pi 3 book, the Pi Zero W added Bluetooth and wireless connectivity to the previous model. Running a compact project that you need remote access to? Pi Zero W is perfect!
Raspberry Pi Zero WH
Released in early 2018, this version of the Raspberry Pi Zero is identical to the previous version. Identical except for one key difference: it has GPIO pins! This makes it ideal for anyone who hates soldering but needs a Pi Zero due to space constraints.
Best Use: Pi Zero models are ideal for any project where space is a premium. For example, it can be used to run a retro gaming system where the Pi is embedded in the TV. Pi Zero is also popular for many other retro game projects.
Other versions of Raspberry Pi
Apart from the standard models, the Raspberry Foundation has released another version of its SBC. The computational module had three revisions and an IO board. These devices have the same specs as the main Pi, but are intended for IoT developers. However, enthusiasts can buy them as well. Nothing prevents you from completing IoT projects on your standard "consumer" raspberry. 
Which Raspberry Pi do you prefer?
Undoubtedly, all of these raspberries are especially good for specific tasks. But their flexibility means they are interchangeable. So, for example, you can run Kodi on the Pi Zero W, or send a balloon into orbit with the Raspberry Pi 3.
What model of Raspberry Pi do you prefer? Is there an option that you always come back to, or are you lucky enough to have them all? And what do you expect from the Raspberry Pi 4 when it eventually goes on sale?
(the price varies greatly from seller to seller)
What is it?
Perhaps many people know about the Raspberry Pi series of mini-computers.
Just in case, let me remind you that these are single-board computers based on the ARM architecture that run a fairly large selection of OS (the most popular choice is different Linux distributions).
From the special, you just need to add a Micro SD card with the OS.
I offer a small overview of one of the models in this series.
Interesting? We continue the conversation.
Why is this?
Raspberry Pi is distinguished by its low price, compact size and rather modest power consumption.Initially, these mini-computers were aimed at teaching in schools, but they have also become very popular among DIY hobbyists.
The dimensions allow them to be embedded almost anywhere.
What are they?
Back in 2012, two ARM-based Raspberry Pi models with a 700MHz processor and 256 / 512MB of memory were initially announced.It was very good for a start especially considering the price of about $ 25- $ 35.
After that, there were several reincarnations and now the Raspberry Pi series is released in the form of the following options:
- Raspberry Pi 3 and 3+:
full board with multiple USB ports, "adult" HDMI and wired network port - Raspberry Pi Zero W:
It has a smaller size due to the lack of a network port, reduced HDMI output, etc. - Compute Module and a few other less popular options
Raspberry Pi Zero W
This model is very well suited for embedding in some of your crafts, where you need a full-fledged Linux, and the capabilities of platforms based on the same OpenWRT are not always enough.The letter "W" means that wireless interfaces have been added to the previous Zero model - WiFi 802.11n and Bluetooth.
In general, the Raspberry Pi Zero series consists of three models (not all are available):
- Raspberry Pi Zero v1.2
- Raspberry Pi Zero v1.3 (added a camera connector)
- Raspberry Pi Zero W v1.1 (added WiFi / Bluetooth)
From top to bottom: Pi Zero 1.2, Pi Zero 1.3, Pi Zero W:
Without "W" the price is announced at $ 5, and for "W" it is already $ 10.
Sure, the $ 10 price tag for "W" was announced to be cool, but not everyone has the opportunity to buy at that price, unfortunately.
There are offline stores where you can buy for $ 10, but few people have it close by.
I link to the seller who has the lowest price on eBay right now. But it makes sense to look if someone offers more interesting options, especially when you consider that the delivery address can be anywhere.
Appearance
Top view next to Micro SD card for comparison:
Here you can clearly see the Elpida B4432BBPA-10-F memory chip (this is 512MB of memory). There is a processor underneath a sandwich.
You can see a place for a 40-pin connector for connecting peripherals.
To the right of the processor memory is a small rectangular chip - this is our WiFi / Bluetooth. Next to it there is a place for an external antenna connector. The internal antenna is wired on the board and is visible nearby.
The rear view is not particularly happy with the abundance of components: 
Here you can see several control points to which additional peripherals can be connected.
This is what the port side looks like: 
From left to right: Mini HDMI output, Micro USB port for peripherals, Micro USB port for power.
In the background on the left you can see the Micro SD slot for a flash card. To our right, the CSI connector for the camera is in vain.
Technical details
- Raspberry Pi Zero W
- Initial release date: February 2017
- Broadcom BCM2835 32bit CPU, 1GHz, 1 core
- 512MB SDRAM memory
- WiFi / Bluetooth: BCM43438, 802.11n 2.4GHz, supports Bluetooth Low Energy (BLE)
- 40-pin connector for connecting additional equipment
- 1 x USB 2.0 (Micro USB)
- Mini HDMI output
- CSI port for camera
- Micro SD slot for storage and boot
- Micro USB port for power
- Dimensions: 65mm * 30mm * 5mm
- Products webpage:
Some information that Linux gives out on Zero W:
Our Linux:
# uname -a Linux alarmpi 4.14.34-1-ARCH # 1 SMP Mon Apr 16 19:16:02 UTC 2018 armv6l GNU / Linux
CPU:
# lscpu Architecture: armv6l Byte Order: Little Endian CPU (s): 1 On-line CPU (s) list: 0 Thread (s) per core: 1 Core (s) per socket: 1 Socket (s): 1 Vendor ID : ARM Model: 7 Model name: ARM1176 Stepping: r0p7 CPU max MHz: 1000.0000 CPU min MHz: 700.0000 BogoMIPS: 997.08 Flags: half thumb fastmult vfp edsp java tls
# cat / proc / cpuinfo processor: 0 model name: ARMv6-compatible processor rev 7 (v6l) BogoMIPS: 697.95 Features: half thumb fastmult vfp edsp java tls CPU implementer: 0x41 CPU architecture: 7 CPU variant: 0x0 CPU part: 0xb76 CPU revision: 7 Hardware: BCM2835 Revision: 9000c1 Serial: 00000000bbe78b00
Single core processor. Armv6l architecture (must be considered when choosing an OS).
Broadcom BCM2835 has one USB port, on which nothing hangs:
# lsusb Bus 001 Device 001: ID 1d6b: 0002 Linux Foundation 2.0 root hub
A little about performance
Of course, the Raspberry Pi Zero W is not designed for high performance, but it is enough for many purposes.The easiest Micro SD speed test. A SanDisk Ultra 64GB Class 10 Micro SDXC card was used in this test.
Write first, then read. The size of the file is 4GB, which would surely not fit into the cache.
# sync && dd if \u003d / dev / zero bs \u003d 1024k of \u003d test.bin conv \u003d fdatasync count \u003d 4096 4096 + 0 records in 4096 + 0 records out 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 363.781 s, 11.8 MB / s # dd if \u003d test.bin of \u003d / dev / null bs \u003d 1024k 4096 + 0 records in 4096 + 0 records out 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 212.825 s, 20.2 MB / s
CPU utilization for AES and SHA1 / 2. I chose these algorithms because they are often used for VPNs (one of the uses for such minicomputers):
# openssl speed aes-256-cbc aes-128-cbc sha1 sha256 OpenSSL 1.1.0h 27 Mar 2018 built on: reproducible build, date unspecified options: bn (64,32) rc4 (char) des (long) aes (partial) idea (int) blowfish (ptr) compiler: gcc -DDSO_DLFCN -DHAVE_DLFCN_H -DNDEBUG -DOPENSSL_THREADS -DOPENSSL_NO_STATIC_ENGINE -DOPENSSL_PIC -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DAES_ASM -DBSAES_ASM -DGHASH_ASM -DECP_NISTZ256_ASM -DPOLY1305_ASM -DOPENSSLDIR \u003d "\\" / etc / ssl \\ "" -DENGINESDIR \u003d "\\" / usr / lib / engines-1.1 \\ "" -Wa, - noexecstack -D_FORTIFY_SOURCE \u003d 2 -march \u003d armv6 -mfloat-abi \u003d hard -mfpu \u003d vfp -O2 - pipe -fstack-protector-strong -fno-plt -Wl, -O1, - sort-common, - as-needed, -z, relro, -z, now The "numbers" are in 1000s of bytes per second processed ... type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes 16384 bytes sha1 7877.55k 18909.88k 33737.12k 42122.02k 45244.42k 45693.77k aes-128 cbc 20002.91k 22338.63k 23031.27k 23202.33k 23225.2k aes 22975.96 c. 17481.05k 17648.07k 17655.27k 17501.84k sha256 5741.76k 13324.56k 23334.70k 28692.89k 30852.88k 30981.65k
If you need to run some other specific tests, please check in the comments.
Housing
In order not to get up twice, I will also add a mini-review of the simple and popular Pi Zero case.I bought here:
"Corpus" is a very big word for this, but for many purposes it is quite enough. This consists of two acrylic plates (top-bottom), four screws, and four acrylic washers.
There is a cutout for a 40-pin connector on the front and back of the board.
The kit looks like this: 
And this is already assembled: 
Application
It is difficult to say something unambiguous here, since there are a lot of applications.It can be a simple computer for surfing the Internet (a little squeaky if you use graphics, but really) to various gadgets and fairly advanced crafts.
Also very popular for automation, camera, mini server, game console, music, robotics, VPN client, media center, games, php / python / nodejs / etc / c ++ development etc.
I was personally going to do FPV with a camera on them for my crawling, running, walking robots. But this is a topic for a separate post.