Thanks to the Raspberry foundation, there is an entire computer system available on the market, being mounted on a tiny board only 8.5cm long and just 5.5cm wide.
The idea behind the foundation is a cheap, tiny computer for schools, universities or individuals to enable pupils, students or kids to start programming.
What else but the Raspberry Pi do you need to start this enterprise?
Initially a power supply, because even economic computers consume electric power.
A memory card with an operating system - according to the open mind of the foundation, Linux is a good choice - as you can see this is a very lightweight distribution.
Big and heavy are the components needed to communicate with humans:
A Keyboard...
an HDMI cable, maybe with an DVI adapter...
...and finally a Monitor.
When working with a graphical interface, a mouse will be useful.
A self powered USB-hub is a good choice.
The electric power is normally lead in through a micro USB connector and the 5V power adapter should provide a current of a at least 700mA.
The boot process starts as soon as the Raspberry Pi is connected to the input voltage.
There is no power switch at the board, because that would have lifted the production costs into astronomical heights...
Here the power supply is connected using two pins at the board to be able to detect the current running through the device.
Let's have a closer look at the components of the device while the  computer is booting.
There is an HDMI and a composite socket to connect a monitor, a 3.5mm audio jack, an LAN connector and two USB 2.0 interfaces are available.
The maximum current running through the USB interface should not exceed 100mA.
Because of the fact that even some computer mice consume a higher current, an USB hub with a separate power supply is a good upgrade of the system.
There will be more about the GPIOs in course of this video.
At the back of the board we can see the slot for the SD cards.
Meanwhile the boot process is finished and the Debian Linux has entered runlevel 2 without a graphical interface.
To do some coding you don't need a brightly colored, power wasting user interface and that's one of the pros of the Raspberry Pi.
The maximum current during the boot process was approximately 400mA, which correlates to an electric power consumption of 2W.
Keyboard and mouse are connected to the powered hub, hence their power consumption is not included.
With the help of the keyboard, the graphical mode can be started after the login procedure.
Now the whole computing power of the Raspberry Pi is required.
The ARM11 CPU and the Broadcom GPU are sharing 256MB of RAM, and those chipset is brought to it's limits.
That illustrates that more and more colored, animated 3D and so on Desktops are eating up lots of your valuable computing power.
Nevertheless, the Raspberry Pi can run full HD videos.
Starting the web browser Midori requires some patience.
This is no high speed surfing, but while considering the spartan equipment, it's not so bad.
You might already have noticed that the Raspberry Pi is operating as it's own client.
The webserver Apache 2 with Pearl support is running.
The tiny computer can do all the things done by big computer farms everywhere around the web.
While using the X-Server, the maximum current was just slightly higher than 400mA.
Let's leave the jewel-colored world of consumer persuasion and reenter the command line.
Teaching coding is why the Raspberry Pi was created and to my mind this is best done from scratch.
At the video about the Wondermedia Laptop I have written a very simple benchmark program doing nothing else but counting.
Only 3.7 seconds are passing by until the computer was counting from zero to 100000.
Counting, adding and other arithmetic operations make sense if there is a connection to the world outside of the silicon chip.
There is a very special interface at the board of the Raspberry Pi: 17GPIOs.
GPIO is the acronym for General Purpose Input Output.
Each of the 17 pins can operate as an digital input or output channel.
In output mode, a pin provides a voltage of zero respectively 3.3V, which can be switched by software.
By using power transistors, electric devices can be turned on or off by a single pin.
While switched to input mode, the software returns zero if no voltage is attached to the pin respectively one, if it is connected to 3.3V.
To avoid damage caused by shortcuts respectively by attaching a too high voltage to a pin, they should not be wired directly to an  input or output circuit.
Instead, they should be connected to an extension board, equipped with electric circuits saving the Pi from faulty voltages.
The board shown here is populated with 17 small signal transistors and 17 LEDs indicating the switching status.
14 of the small transistors are driving 7 power H-bridges.
3 Pins can be used in input mode.
Doing physical computing, meaning to control peripherals and so the world outside of bits and bytes is very easy with the Raspberry Pi and it's GPIOs.
Let's discover the maximum switching frequency.
By software the turn on time is set to one millisecond, but the measured value is slightly above that millisecond.
That is because some time is needed to switch the pins from on to off or vice versa.
Those relative error increases, the shorter the turn on time becomes.
By an adjusted value of 0.1 milliseconds we can detect more than 0.17 milliseconds.
At a value of 0.01 milliseconds which is 10 microseconds, we can detect 90 microseconds.
By decreasing the turn on time to just one microsecond, we can see no more decreasing of the measured value.
The maximum switching frequency is about 5kHz, turning the pins on and off lasts for approximately 100 microseconds.
Those relative high frequency can be used to built a 2 channel 8 bit frequency generator by using 16 GPIO pins.
Nearly any output voltage progression with a frequency up to 1kHz can be attached to the output pins.
For example Lissajous figures can be displayed on an oscilloscope.
Like demonstrated at the video about the Wondermedia chipset, stepper motors can be actuated by using H-bridges...
...LEDs can be dimmed by pulse width modulation...
...the revolution speed of electric motors can be driven by using this principle...
...and finally servos can be actuated.
At the previous video, drops falling through a light sensor were counted to measure even small volumes of a fluid.
Here the fluid is pumped.
It is a relatively tiny pump, accordingly the amount of fluid coming out of the nozzle with each pulse is very small.
When replacing the pen by this pump, the plotter turns into an ink jet printer.
The drops are significantly larger than those of commercially available devices, but the working principle is identically.
Line by line is executed, controlled by software and the dots of the loaded bitmap file are transformed into ink drops.
Besides normal ink, other fluids can also be applied to nearly any surface by using such an arrangement.
Up to now this is the most complex way I have spread some commercially available ink on a sheet of paper - but for sure it is not the most uninteresting way.
When replacing the pump by a milling machine, the Raspberry Pi can start to build it's own casing.
The computer was delivered without such a protection.
The acrylic plastic is 4mm thick, hence the milling machine is processing each line more than once.
With every step the cutter is diving deeper into the material.
The poor power output of the stepper motors in correlation with the do-it yourself mechanics doesn't allow a faster treatment of the material.
The result is a lucent case for a lucent computer.
The input pins can be used to count pulses.
By using a magnet and an inductor, a revolution counter can be built.
With a propeller you can use the arrangement as an air speed indicator.
A control system consists of a sensor, measuring an instantaneous value and an actuator, used to manipulate those value.
The software calculates the control value of the actuator.
The revolution speed of a combustion engine is measured with the sensor and a strong servo actuates the throttle lever of the carburetor to adjust a given value.
Starting and stopping of the engine is also no problem to the Raspberry Pi which consumes just 2W of electric power while the combustion engine has a power output of up to 21000W.
A setpoint of 1100 rounds per minute is given and monitored respectively controlled by the computer and it's software.
With the help of the GPIOs, the results of simple, but also very complex computing work can be turned into the physical world.
I have built a camera equipped robot, which can be controlled via Internet with my Raspberry Pi, so you can drive around my RoboSpatium  - try it out!
Thanks to the folks of the Raspberry Foundation and all those hardworking people of the community, which have at long last handed a real computer to mankind once again.
During the last decades the toys with their brightly colored, power consuming you are not allowed to do something interfaces have dominated the markets.
So if you are tired of looking at closed windows or if you want more than just a little bite of the fruit, you should try to handle Linux and the Raspberry Pi - its profitable for you!
