Basic Schematic Symbols

Circuit layouts and schematic diagrams are a simple and effective way of showing pictorially the electrical connections, components and operation of a particular electrical circuit or system. Basic electrical and electronic graphical symbols called Schematic Symbols are commonly used within circuit diagrams, schematics and computer aided drawing packages to identify the position of individual components and elements within a circuit.

Graphical symbols not only identify a components position but the type of electrical element too, whether its resistive, inductive, capacitive, mechanical, etc. Thus in circuit diagrams and schematics, graphical symbols identify and represent electrical and electronic devices and show how they are electrically connected together while drawing lines between them represents the wires or component leads.

A the connecting leads or pins of a component in a schematic diagram can be identified using letters or abreviations. For example, the connecting leads of a bipolar junction transistor, (BJT) are identified as E (emitter), B (base), and C (collector). Arrows are also used within schematic symbols to indicate the direction of convertional current flow around a circuit or through a component, or are used as part of their graphical symbol to show that the components has a variable or adjustable value. For example, a potentiometer or rheostat.

Although electrical components are represented by universally accepted schematic symbols, there are a number of variants and alternative symbols used throughout the world to represent the same electrical component or device. For example, the IEC (International Electrotechnical Commission) have one set of symbols, while the IEEE (Institute of Electrical and Electronics Engineers) have an alternative set of symbols for the same component.

The basic electrical and electronic graphical symbols presented here are the more generally accepted graphical symbols because of their common usage across a range of electrical and electronic fields. The individual graphical symbols below are given along with a brief description and explanation.

Power Supply Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Single Cell	A single DC battery cell of 0.5V
	DC Battery Supply	A collection of single cells forming a DC battery supply
	DC Voltage Source	A constant DC voltage supply of a fixed value
	DC Current Source	A constant DC current supply of a fixed value
	Controlled Voltage Source	A dependent voltage source controlled by an external voltage or current
	Controlled Current Source	A dependent current source controlled by an external voltage or current
	AC Voltage Source	A sinusoidal voltage source or generator

Electrical Grounding Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Earth Ground	Earth ground referencing a common zero potential point
	Chassis Ground	Chassis ground connected to the power supplies earthing pin
	Digital Ground	A common digital logic circuit ground line

Resistor Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Fixed Resistor (IEEE Design)	A fixed value resistor whose resistive value is indicated next to its schematic symbol
	Fixed Resistor (IEC Design)
	Potentiometer (IEEE Design)	Three terminal variable resistance whose resistive value is adjustable from zero to its maximum value
	Potentiometer (IEC Design)
	Rheostat (IEEE Design)	Two terminal fully adjustable rheostat whose resistive value varies from zero to a maximum value
	Rheostat (IEC Design)
	Trimmer Resistor	Small variable resistors for mounting onto pcb’s
	Thermistor (IEEE Design)	Thermal resistor whose resistive value changes with changes in surrounding temperature
	Thermistor (IEC Design)

Capacitor Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Fixed Value Capacitor	A fixed value parallel plate non-polarised AC capacitor whose capacitive value is indicated next to its schematic symbol
	Fixed Value Capacitor
	Polarized Capacitor	A fixed value polarised DC capacitor usually an electrolytic capacitor which must be connected to the supply as indicated
	Variable Capacitor	An adjustable capacitor whose capacitance value can be varied by means of adjustable plates

Inductor and Coil Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Open Inductor	An open inductor, coil or solenoid that generates a magnetic field around itself when energised
	Iron Core Inductor	An inductor formed by winding the coil around a solid laminated iron core indicated by solid lines
	Ferrite Core Inductor	An inductor formed by winding the coil around a non-solid ferrite core indicated by dashed lines

Switch and Contact Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	SPST Toggle Switch	Single-pole single-throw toggle switch used for making (ON) or breaking (OFF) a circuits current
	SPDT Changeover Switch	Single-pole double-throw changeover switch used for changing the direction of current flow from one terminal to another
	Pushbutton Switch (N.O)	Normally open contacts pushbutton switch – push to close, release to open
	Pushbutton Switch (N.C)	Normally closed contacts pushbutton switch – push to open, release to close
	SPST Relay Contacts	Electromechanical relay with internal single-pole single-throw toggle contacts
	SPDT Relay Contacts	Electromechanical relay with internal single-pole double-throw changeover contacts
	DPST Relay Contacts	Electromechanical relay with internal double-pole single-throw toggle contacts
	DPDT Relay Contacts	Electromechanical relay with internal double-pole double-throw changeover contacts
	DIP Switch Assembly	PCB mounted DIP switch with 1-to-10 toggle switches either single-pole, double-pole, rotary or with a common terminal

Semiconductor Diode Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Semiconductor Diode	Semiconductor pn-junction diode used for rectification and high current applications
	Zener Diode	Zener diode used in its reverse voltage breakdown region for voltage limiting and regulation applications
	Schottky Diode	Schottky diode consisting of an n-type semiconductor and metal electrode junction for low voltage applications

Transistor Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	NPN Bipolar Transistor	Characterised as being a lightly doped p-type base region between two n-type emitter and collector regions with the arrow indicating direction of conventional current flow out.
	PNP Bipolar Transistor	Characterised as being a lightly doped n-type base region between two p-type emitter and collector regions. Arrow indicates direction of conventional current flow in.
	Darlington Pair Transistor	Two bipolar transistor npn or pnp connected in a series common collector configuration to increase current gain
	N-JFET Transistor	N-channel junction field effect transistor having an n-type semiconductive channel between source and drain with the arrow indicating direction of conventional current flow
	P-JFET Transistor	P-channel junction field effect transistor having a p-type semiconductive channel between source and drain with the arrow indicating direction of conventional current flow
	N-MOSFET Transistor	N-channel metal-oxide semiconductor field effect transistor with an insulated gate terminal which can be operated in depletion or enhancement mode
	P-MOSFET Transistor	P-channel metal-oxide semiconductor field effect transistor with an insulated gate terminal which can be operated in depletion or enhancement mode

Photodevice Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	Light Emitting Diode (LED)	A semiconductor diode which emits coloured light from its junction when forward biased
	7-segment Display	A 7-segment display used common cathode (CC) or common anode (CA) for displaying single numbers and letters
	Photodiode	A semiconductor device which allows current to flow when exposed to incident light energy
	Solar Cell	P–N junction photovoltaic cell transducer which converts light intentsity directly into electrical energy
	Photoresistor	Light dependent resistor (LDR) which changes its resistive value with changes in light intensity
	Indicator Lamp or Light Bulb	A filament lamp, indicator or other which emits visible light when a current flows through it
	Opto-isolator or Optocoupler	An Opto-isolator or Optocoupler which uses photo-sensitive devices to isolate its input and output connections

Digital Logic Schematic Symbols

Schematic Symbol	Symbol Identification	Description of Symbol
	NOT Gate	Logic gate with only one input and one output and outputs a logic 1 (HIGH) when input is 0 (LOW) and outputs a 0 when input is 1 (Inverter)
	AND Gate	Logic gate with two or more inputs which outputs a logic 1 (HIGH) when ALL of its inputs are at logic 1 (HIGH)
	NAND Gate	Logic gate with two or more inputs that outputs a logic 0 (LOW) when ALL of its inputs are HIGH at logic 1 (Equivalent to NOT + AND)
	OR Gate	Logic gate with two or more inputs which outputs a logic 1 (HIGH) when ANY (or both) of its inputs are at logic 1 (HIGH)
	NOR Gate	Logic gate with two or more inputs that outputs a logic 0 (LOW) when ANY (or both) of its inputs are HIGH at logic 1 (Equivalent to NOT + OR)
	XOR Gate	Exclusive-OR gate with two inputs that outputs a logic 1 (HIGH) whenever its two inputs are DIFFERENT
	XNOR Gate	Exclusive-NOR gate with two inputs that outputs a logic 1 (HIGH) whenever its two inputs are the SAME (NOT + XOR)
	SR Flip-Flop	Set-Reset Flip-flop is a bistable device used to store one bit of data on its two complementary outputs
	JK Flip-Flop	JK (Jack Kilby) Flip-flop has the letter J for Set and the letter K for Reset (Clear) with internal feedback
	D-type Flip-Flop	D (Delay or Data) Flip-flop is a single input flip-flop which toggles between its two complementary outputs
	Data Latch	Data latch stores one data bit on its single input when EN enable pin is LOW and outputs the data bit transparently when the EN enable pin is HIGH
	4-to-1 Multiplexer	A Multiplexer passes the data on one of its inputs pins to a single output line
	1-to-4 Demultiplexer	A Demultiplexer passes the data on its single input pin to one of several output lines

Here we have seen a number of basic electrical and electronics schematic symbols in graphical form used by engineers to show how a particular circuit is connected together and operates by the types of symbols used within it so that other engineers may understand.