Basic Schematic Symbols

Table of Contents

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

Electrical Grounding Schematic Symbols

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

Resistor Schematic Symbols

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

Capacitor Schematic Symbols

Schematic
Symbol
Symbol Identification Description of Symbol
fixed value capacitor schematic 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 schematic symbol Fixed Value Capacitor
fixed value polarised capacitor schematic symbol Polarized Capacitor A fixed value polarised DC capacitor usually an electrolytic capacitor which must be connected to the supply as indicated
variable capacitor schematic symbol 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 coil inductor schematic symbol Open Inductor An open inductor, coil or solenoid that generates a magnetic field around itself when energised
iron core inductor schematic symbol Iron Core Inductor An inductor formed by winding the coil around a solid laminated iron core indicated by solid lines
ferrite core inductor schematic symbol 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
single pole single throw switch symbol SPST Toggle Switch Single-pole single-throw toggle switch used for making (ON) or breaking (OFF) a circuits current
single pole double throw switch symbol SPDT Changeover Switch Single-pole double-throw changeover switch used for changing the direction of current flow from one terminal to another
normally open pushbutton schematic symbol Pushbutton Switch (N.O) Normally open contacts pushbutton switch – push to close, release to open
normally closed pushbutton schematic symbol Pushbutton Switch (N.C) Normally closed contacts pushbutton switch – push to open, release to close
relay symbol with spst contacts SPST Relay Contacts Electromechanical relay with internal single-pole single-throw toggle contacts
relay symbol with spdt contacts SPDT Relay Contacts Electromechanical relay with internal single-pole double-throw changeover contacts
relay symbol with dpst contacts DPST Relay Contacts Electromechanical relay with internal double-pole single-throw toggle contacts
relay symbol with dpdt contacts DPDT Relay Contacts Electromechanical relay with internal double-pole double-throw changeover contacts
4-pin dip switch schematic symbol 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 schematic symbol Semiconductor Diode Semiconductor pn-junction diode used for rectification and high current applications
zener diode schematic symbol Zener Diode Zener diode used in its reverse voltage breakdown region for voltage limiting and regulation applications
schottky diode schematic symbol 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 schematic 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 schematic symbol 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.
npn darlington transistor schematic symbol Darlington Pair Transistor Two bipolar transistor npn or pnp connected in a series common collector configuration to increase current gain
n-channel jfet schematic symbol 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-channel jfet schematic symbol 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-channel mosfet schematic symbol 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-channel mosfet schematic symbol 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 schematic symbol Light Emitting Diode (LED) A semiconductor diode which emits coloured light from its junction when forward biased
7-segment display schematic symbol 7-segment Display A 7-segment display used common cathode (CC) or common anode (CA) for displaying single numbers and letters
photodiode schematic symbol Photodiode A semiconductor device which allows current to flow when exposed to incident light energy
photovoltaic cell schematic symbol Solar Cell P–N junction photovoltaic cell transducer which converts light intentsity directly into electrical energy
photoresistor schematic symbol Photoresistor Light dependent resistor (LDR) which changes its resistive value with changes in light intensity
indicator lamp schematic symbol Indicator Lamp or Light Bulb A filament lamp, indicator or other which emits visible light when a current flows through it
opto-isolator schematic symbol 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 schematic 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 schematic symbol 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 schematic symbol 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 schematic symbol 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 schematic symbol 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 schematic symbol XOR Gate Exclusive-OR gate with two inputs that outputs a logic 1 (HIGH) whenever its two inputs are DIFFERENT
xnor gate schematic symbol 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 schematic symbol 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 schematic symbol 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 schematic symbol 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 schematic symbol 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
multiplexer schematic symbol 4-to-1 Multiplexer A Multiplexer passes the data on one of its inputs pins to a single output line
demultiplexer schematic symbol 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.

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