The circuit diagram for a 4 bit TTL counter, a type of state machine

A circuit diagram (also known as an electrical diagram, elementary diagram, or electronic schematic) is a simplified conventional pictorial representation of an electrical circuit. Transistor–transistor logic ( TTL) is a class of Digital circuits built from Bipolar junction transistors (BJT and Resistors It is called A schematic is a diagram that represents the elements of a System using abstract graphic Symbols rather than realistic pictures An electrical network is an interconnection of Electrical elements such as Resistors Inductors Capacitors Transmission lines Voltage It shows the components of the circuit as simplified standard symbols, and the power and signal connections between the devices. Electric power is defined as the rate at which Electrical energy is transferred by an Electric circuit. In the fields of communications, Signal processing, and in Electrical engineering more generally a signal is any time-varying or spatial-varying quantity Arrangement of the components interconnections on the diagram does not correspond to their physical locations in the finished device.

Unlike a block diagram or layout diagram, a circuit diagram shows the actual wire connections being used. Block diagram is a Diagram of a System, in which the principal parts or functions are represented by blocks connected by lines that show the relationships of The diagram does not show the physical arrangement of components. A drawing meant to depict what the circuit actually looks like is called "artwork" or "layout". Integrated circuit layout, also known IC layout, IC mask layout, or mask design, is the representation of an Integrated circuit in terms of planar

Circuit diagrams are used for the design (circuit design), construction (such as PCB layout), and maintenance of electrical and electronic equipment. The process of circuit design can cover systems ranging from complex electronic systems all the way down to the individual Transistors within an Integrated circuit. A printed circuit board, or PCB, is used to mechanically support and electrically connect Electronic components using conductive pathways or traces

## Legends

Common circuit diagram symbols (with US Resistor Symbol)

On a circuit diagram, the symbols for components are labelled with a descriptor (or reference designator) matching that on the list of parts. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | A reference designator unambiguously identifies a component in an electrical schematic ( Circuit diagram) or on a printed circuit board (PCB. For example, C1 is the first capacitor, L1 is the first inductor, Q1 is the first transistor, and R1 is the first resistor (note that it isn't written R1, L1,…). A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors An inductor is a passive electrical component designed to provide Inductance in a circuit In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | The letters that precede the numbers were chosen in the early days of the electrical industry, even before the vacuum tube (thermionic valve), so "Q" was the only one available for semiconductor devices in the mid-twentieth century. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Often the value or type designation of the component is given on the diagram beside the part, but detailed specifications would go on the parts list.

## Symbols

Circuit diagram symbols have differed from country to country and have changed over time, but are now to a large extent internationally standardized. Simple components often had symbols intended to represent some feature of the physical construction of the device. For example, the symbol for a resistor shown here dates back to the days when that component was made from a long piece of wire wrapped in such a manner as to not produce inductance, which would have made it a coil. A coil is a series of loops A coiled coil is a structure where the coil itself is in turn also looping These wirewound resistors are now used only in high-power applications, smaller resistors being cast from carbon composition (a mixture of carbon and filler) or fabricated as an insulating tube or chip coated with a metal film. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 The internationally standardized symbol for a resistor is therefore now simplified to an oblong, sometimes with the value in ohms written inside, instead of the zig-zag symbol. The ohm (symbol Ω) is the SI unit of Electrical impedance or in the Direct current case Electrical resistance, A less common symbol is simply a series of peaks on one side of the line representing the conductor, rather than back-and-forth as shown here.

### Standards

There are several national and international standards for graphical symbols in circuit diagrams, in particular:

• ANSI standard Y32 (also known as IEEE Std 315)

IEC 60617 originally consisted of 13 parts, from resistors and capacitors to logic symbols and even a generalised drawing standard of connections and bus line widths. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes British Standards are produced by BSI British Standards, a division of BSI Group that is incorporated under a Royal Charter and is formally designated It is now published as a subscription online database IEC 60617-DB [1].

Different symbols may be used depending on the discipline using the drawing; for example, lighting and power symbols used as part of architectural drawings may be different from symbols for devices used in electronics.

Schematic wire junctions:
1.  Old style: (a) connection, (b) no connection.
2.  Early CAD style: (a) connection, (b) no connection.
3.  New style: (a) connection, (b) no connection.

The linkages between leads were once simple crossings of lines; one wire insulated from and "jumping over" another was indicated by it making a little semicircle over the other line. With the arrival of computerized drafting, a connection of two intersecting wires was shown by a crossing with a dot or "blob", and a crossover of insulated wires by a simple crossing without a dot. However, there was a danger of confusing these two representations if the dot was drawn too small or omitted. Modern practice is to avoid using the "crossover with dot" symbol, and to draw the wires meeting at two points instead of one. It is also common to use a hybrid style, showing connections as a cross with a dot while insulated crossings use the semicircle.

## European and Australian codes

The following codes which vary slightly from the American codes are in common use in European and Australian standard electrical circuit diagrams. These codes are used for the "reference designators" printed on PCBs (which match the corresponding ones written on the corresponding schematic).

• A: Assemblies
• B: Transducers (photo cells, inductive proximity, thermocouple, flame detection)
• C: Capacitors
• D: Storage devices
• E: Miscellaneous
• F: Fuses
• G: Generator, battery pack
• H: Indicators, lamps (not for illumination), signalling devices
• K: Relays, contactors
• L: Inductors and filters
• M: Motors
• N: Analogue devices
• P: Measuring/test equipment
• Q: Circuit breakers, isolators, re-closers
• R: Resistors, brake resistors
• S: Switches, push buttons, emergency stops and limit switches
• T: Transformers
• U: Power converters, variable speed drives, soft starters, DC power supplies
• V: Semiconductors
• W: Wires, conductors, power, neutral and earthing busses
• X: Terminal strips, terminations, joins
• Y: Solenoids, electrical actuators
• Z: Filters

Detailed rules for reference designations are provided in the International standard IEC 61346.

## Organization of drawings

It is a usual although not universal convention that schematic drawings are organized on the page from left to right and top to bottom in the same sequence as the flow of the main signal or power path. For example, a schematic for a radio receiver might start with the antenna input at the left of the page and end with the loudspeaker at the right. Positive power supply connections for each stage would be shown towards the top of the page, with grounds, negative supplies, or other return paths towards the bottom. Schematic drawings intended for maintenance may have the principle signal paths highlighted to assist in understanding the signal flow through the circuit. More complex devices have multi-page schematics and must rely on cross-reference symbols to show the flow of signals between the different sheets of the drawing.

Detailed rules for the preparation of circuit diagrams (and other document kinds used in electrotechnology) are provided in the International standard IEC 61082-1.

Relay logic line diagrams (also called ladder logic diagrams) use another common standardized convention for organizing schematic drawings, with a vertical power supply "rail" on the left and another on the right, and components strung between them like the rungs of a ladder. Relay logic is a method of controlling industrial electronic circuits by using Relays and contacts Ladder logic is a philosophy of drawing electrical logic schematics

## Art Work

A Rat's Nest

Once the schematic has been made, it is converted into a layout that can be fabricated onto a Printed Circuit Board (PCB). The layout is usually prepared by the process of schematic capture. The result is what is known as a Rat's Nest. The Rat's Nest is a jumble of wires (lines) criss crossing each other to their destination nodes. These wires are routed either manually or by the use of Electronics Design Automation (EDA) tools. The EDA tools arrange and rearrange the placement of components and finds paths for tracks to connect various nodes. This results into an Art Work.

A generalized design flow would be as: Schematic -> Schematic Capture -> Rat's Nest -> Routing ->Art Work -> PCB Development & etching -> Component Mounting -> Testing.