How a breadboard is internally connected:

Follow this link to discover the dimensions of the arduino boards.
This will help you build your own shields.

http://blog.protoneer.co.nz/arduino-uno-and-mega-dimensions/

As I notice a lot of people get confused when they try to build their own schematic. The complexicity in the power input is one of the major problems for the usb power input.So i decide to design a more friendly schematic for those who want to use a 12 power supply only directly.So have a look at this.

Fill in the form and push "Calculate" to get an updated schematic.
link here<----

The B Series logic gates are constructed with P and N channelenhancement mode devices in a single monolithic structure(Complementary MOS). Their primary use is where low powerdissipation and/or high noise immunity is desired.

datasheet here<------

As you've already realized we love make the things simple...
This is a way i prefer to protect some circuits and varius loads from a reversed battery...

We all make mistakes so a protection is n

This is an N-Channel enhancement mode silicon gate power field effect transistor designed for applications such as switching regulators, switching converters, motor drivers, relay drivers and drivers for high power bipolar switching transistors requiring high speed and low gate drive power. This type can be operated directly from integrated circuits

datsheet here<-----------

This is  a simple circuit based on the famous 555 timer...
With this circuit you can control any load you want but consider first the switching component that the 555 will control....
The n-channel mosfet buz11 can handle 30A and up to 50volts...
You can operate the device connected on the switching component from 5% to 95%.

White LED Driver With Digital and PWM Brightness Control
-->datasheet here<--

This circuit is capable of regulating 12V motors and DC light bulbs. No coolrib required for Q1 or D3 if the current does not exceed 2A. If it does, a sufficient coolrib for both the Schottky Diode (D3) and MosFET Q1 is required.
The regulation is obtained via PWM or Pulse Width Modulation. The output pin 3 of the 555 provides square-wave with an adjustable duty-cycle. What that basically means is that the pulse width changes the speed of the motor. The output from the 555 feeds the mosfet via current limiting resistor R2 of 47 ohms. Because the MosFET Q3 only "switches" and is not behaving like a pot, its energy-waste level is negliable, and also provides the motor more coupling at low rpm. The maximum current Q1 can provide (safely) is about 10A.

link to the circuit page<-------