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    • Circuit Symbols 101

    101.jpg

    CIRCUIT SYMBOLS 101 | CONNECTORS 101 | CABLES 101 | LAMPS 101 | LEDS 101 | RELAYS 101 | SWITCHES 101

     

    Circuit symbols are used in circuit diagrams which show how a circuit is connected together. The actual layout of the components is usually quite different from the circuit diagram. To build a circuit you need a different diagram showing the layout of the parts on stripboard or printed circuit board.

    Wires and connections

     Component 

     Circuit Symbol 

    Function of Component

    Wire

    		 1.png
     

    To pass current very easily from one part of a circuit to another.

    Wires joined

    		 2.png
     

    A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.

    Wires not joined

    		 3.png
     

    In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'hump' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!


     

    Power Supplies

     Component 

     Circuit Symbol 

    Function of Component

    Cell

    		 4.png
     

    Supplies electrical energy.
    The larger terminal (on the left) is positive (+).
    A single cell is often called a battery, but strictly a battery is two or more cells joined together.

    Battery

    		 5.png
     

    Supplies electrical energy. A battery is more than one cell.
    The larger terminal (on the left) is positive (+).

    DC supply

    		 6.png
     

    Supplies electrical energy.
    DC = Direct Current, always flowing in one direction.

    AC supply

    		 7.png
     

    Supplies electrical energy.
    AC = Alternating Current, continually changing direction.

    Fuse

    		 8.png
     

    A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.

    Transformer

    		 9.png
     

    Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.

    Earth
    (Ground)

    		 10.png
     

    A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.


     

    Output Devices: Lamps, Heater, Motor, etc.

     Component 

     Circuit Symbol 

    Function of Component

    Lamp (lighting)

    		 11.png
     

    A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

    Lamp (indicator)

    		 12.png
     

    A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.

    Heater

    		 13.png
     

    A transducer which converts electrical energy to heat.

    Motor

    		 14.png
     

    A transducer which converts electrical energy to kinetic energy (motion).

    Bell

    		 15.png
     

    A transducer which converts electrical energy to sound.

    Buzzer

    		 16.png
     

    A transducer which converts electrical energy to sound.

    Inductor
    (Coil, Solenoid)

    		 17.png
     

    A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.


     

    Switches

     Component 

     Circuit Symbol 

    Function of Component

    Push Switch
    (push-to-make)

    		 18.png
     

    A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

    Push-to-Break Switch

    		 19.png
     

    This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

    On-Off Switch
    (SPST)

    		 20.png
     

    SPST = Single Pole, Single Throw.
    An on-off switch allows current to flow only when it is in the closed (on) position.

    2-way Switch
    (SPDT)

    		 21.png
     

    SPDT = Single Pole, Double Throw.
    A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.

    Dual On-Off Switch
    (DPST)

    		 22.png
     

    DPST = Double Pole, Single Throw.
    A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.

    Reversing Switch
    (DPDT)

    		 23.png
     

    DPDT = Double Pole, Double Throw.
    This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.

    Relay

    		 24.png
     

    An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit.
    NO = Normally Open, COM = Common, NC = Normally  Closed.


     

    Resistors

     Component 

     Circuit Symbol 

    Function of Component

    Resistor

    		 25.png
     

    A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit.

    Variable Resistor
    (Rheostat)

    		 26.png
     

    This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

    Variable Resistor
    (Potentiometer)

    		 27.png
     

    This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

    Variable Resistor
    (Preset)

    		 28.png
     

    This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.


     

    Capacitors

     Component 

     Circuit Symbol 

    Function of Component

    Capacitor

    		 29.png
     

    A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

    Capacitor, polarised

    		 30.png
     

    A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

    Variable Capacitor

    		 31.png
     

    A variable capacitor is used in a radio tuner.

    Trimmer Capacitor

    		 32.png
     

    This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.


     

    Diodes

     Component 

     Circuit Symbol 

    Function of Component

    Diode

    		 33.png
     

    A device which only allows current to flow in one direction.

    LED
    Light Emitting Diode

    		 34.png
     

    A transducer which converts electrical energy to light.

    Zener Diode

    		 35.png
     

    A special diode which is used to maintain a fixed voltage across its terminals.

    Photodiode

    		 36.png
     

    A light-sensitive diode.


     

    Transistors

     Component 

     Circuit Symbol 

    Function of Component

    Transistor NPN

    		 37.png
     

    A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

    Transistor PNP

    		 38.png
     

    A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

    Phototransistor

    		 39.png
     

    A light-sensitive transistor.


     

    Audio and Radio Devices

     Component 

     Circuit Symbol 

    Function of Component

    Microphone

    		 40.png
     

    A transducer which converts sound to electrical energy.

    Earphone

    		 41.png
     

    A transducer which converts electrical energy to sound.

    Loudspeaker

    		 42.png
     

    A transducer which converts electrical energy to sound.

    Piezo Transducer

    		 43.png
     

    A transducer which converts electrical energy to sound.

    Amplifier
    (general symbol)

    		 44.png

    An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.

    Aerial
    (Antenna)

    		 45.png
     

    A device which is designed to receive or transmit radio signals. It is also known as an antenna.


     

    Meters and Oscilloscope

     Component 

     Circuit Symbol 

    Function of Component

    Voltmeter

    		 46.png
     

    A voltmeter is used to measure voltage.
    The proper name for voltage is 'potential difference', but most people prefer to say voltage!

    Ammeter

    		 47.png
     

    An ammeter is used to measure current.

    Galvanometer

    		 48.png
     

    A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.

    Ohmmeter

    		 49.png
     

    An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.

    Oscilloscope

    		 50.png
     

    An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.


     

    Sensors (input devices)

     Component 

     Circuit Symbol 

    Function of Component

    LDR

    		 51.png
     

    A transducer which converts brightness (light) to resistance (an electrical property).
    LDR = Light Dependent Resistor

    Thermistor

    		 52.png
     

    A transducer which converts temperature (heat) to resistance (an electrical property).


     

    Logic Gates

    Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off).
    For more information please see the Logic Gates page.
    There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).

    Gate Type

    Traditional Symbol

    IEC Symbol

    Function of Gate

    NOT

    		 53.png
         		 54.png
     

    A NOT gate can only have one input. The 'o' on the output means 'not'. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.

    AND

    		 55.png
         		 56.png
     

    An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.

    NAND

    		 57.png
         		 58.png
     

    A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.

    OR

    		 59.png
         		 60.png
     

    An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.

    NOR

    		 61.png
         		 62.png
     

    A NOR gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.

    EX-OR

    		 63.png
         		 64.png
     

    An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).

    EX-NOR

    		 65.png
         		 66.png

    An EX-NOR gate can only have two inputs. The 'o' on the output means 'not' showing that it is a Not EX-OR gate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).


     

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