Switch boxes

   We can treat a switch as a box that hold some type of technology that when activated opens or closes a connection or a circuit. The switch is "n.o" or "no" meaning "normal open" when is not activated and the circuit is open. In contrast the switch is normally closed ("n.c." or "nc") when is not activated and the circuit is closed powering something without our intervention.

In this example the switch is high side.

   Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil (or activated) in one of three ways:
    Normally-open (NO)
    Normally-closed (NC)
    Double-throw (DT or Change-over)

Contact terminology defines simple switch topologies.
     SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
    SPDT – Single Pole Double Throw. A common terminal connects to either of two others.
    DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil.
    DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil.

   The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw switch.

In conclusion we can describe a switch by his location in circuit (high side/low side) relative to load, by his default state (NO or NC), number of poles (contacts) and the number of throws (how the poles can be connected when the switch is activated).

Low side or high side control

 Low side or high side topologies are common for loads controlled from both AC or DC power supplies, but they make more sense if there is a unidirectional flow of electric charge (DC)

Implementations are related to different switching technologies available: bipolar (PNP, NPN), MOS (P - channel / N - channel), relay...

Usually low side implementations are simpler.

Combining low side and high side switches:
Half bridge or Half-H

The half bridge is a combination of high side and low side switches. Used in some switched-mode power supplies that use synchronous rectifiers and in switching amplifiers. A double switch for the same load can be used as a fail proof technique.

Full bridge or H bridge or Full-H

 An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction composed by pairs of low side high side switches. An H bridge is built with four switches (solid-state or mechanical). When the opposing switches S1 and S4 are closed and S2 and S3 are open a positive voltage will be applied across the load. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the load.
Common uses of the H bridge are DC motor control and inverters.
Multi phase control
Topologies that combine more than one power supply and more than one load.

ON OFF Switching

A switching device is any mechanical, electrical, magnetic, pneumatic or hydraulic device designed to open or close an electrical circuit (permit or interrupt the flow of electrons in a circuit).

Switches are essentially binary devices: they are either completely on ("closed") or completely off ("open").

For power applications, when closed, switches are required to carry a certain amount of continuous current without overheating, and in the open position they must provide enough insulation to isolate the circuit electrically.
Device capabilities are represented by some general parameters like power (max current, max voltage) and max speed (switching frequency) and also control flow: unidirectional or bi-directional.
The power capability propriety can be enhanced using parallel pairing or Darlington pairing.

Switch examples:

• Electromagnetic Relay 
• BJT - Bipolar junction transistor
• MOSFET - Metal–oxide–semiconductor Field-effect transistor
• TRIAC - Triode for Alternating Current 
• SCR - Thyristor, Silicon-controlled rectifier
• Solid State Relays
• UJT - Unijunction transistor
• GTO - Gate Turn-Off thyristor
• MCT - Metal–oxide–semiconductor Controlled Thyristor
• JFET - Junction gate Field-effect transistor 
• IGBT - insulated-gate bipolar transistor
• H bridge (Full-H or Half-H) (solid-state or mechanical)

LEGO vs Puzzle

Lego bricks can be assembled and connected in many ways, anything constructed can then be taken apart again, and the pieces used to make other objects.

Lego pieces of all varieties constitute a universal system. Despite variation in the design and purpose of individual pieces over the years, each remains compatible in some way with existing pieces.

A puzzle is a problem or enigma that tests the ingenuity of the solver. In a basic puzzle, one is intended to put together pieces in a logical way in order to come up with the desired solution.

   So puzzle is a challenge with an expected solution while Lego is a creative game first imagine something then try to build it - the solution is not unique.

Abstract layers and modular design.

An abstraction layer (or abstraction level, or a layer of abstraction) is a way of hiding the implementation details of a particular set of functionality.

The simplification provided by a good abstraction layer allows for easy reuse.

Modularity is the degree to which a system's components may be separated and recombined.
wiki - Modular design
Modular design, or "modularity in design", is an approach that subdivides a system into smaller parts (modules) that can be independently created and then used in different systems to drive multiple functionalities. A modular system can be characterized by the following:

1. Functional partitioning into discrete scalable, reusable modules consisting of isolated, self-contained functional elements
2. Rigorous use of well-defined modular interfaces, including object-oriented descriptions of module functionality
3. Ease of change to achieve technology transparency and, to the extent possible, make use of industry standards for key interfaces.

Besides reduction in cost (due to lesser customization, and less learning time), and flexibility in design, modularity offers other benefits such as augmentation (adding new solution by merely plugging in a new module), and exclusion.

In industrial design, modularity refers to an engineering technique that builds larger systems by combining smaller subsystems.

Anything constructed can then be taken apart again, and the pieces used to make other objects.