A schematic, or schematic diagram, is a drawing or an illustration of symbols that represent one idea or system. In the field of electrical and electronics, it is like a map used as a guide to build or troubleshoot circuits. It is often used for the maintenance and repair of devices. Also, it helps in the actual mounting of components for a circuit network. Schematic diagrams describe the circuit designs of electronic devices or equipment.

It is important to know how to interpret schematics. Understanding the circuit does not mean knowing only the symbols of components used for the illustration, but also knowing the functions of it in the diagram and knowing how these components are connected in the circuit. But first, one must know the basic symbols used in a schematic diagram for it is the first step toward comprehending a schematic diagram which is the main goal of this tutorial.

Here are the fundamental symbols commonly used in a schematic diagram:

POWER SOURCES

These sources supply electrical energy to a circuit. Basically, to give life to a circuit. A circuit system will not function without a power source.

DC Power Sources (Vcc)

DC (Direct Current) power source allows current flow in a constant direction.

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AC Power Sources

AC (Alternating Current) power source allows current flow in a continually changing manner.

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Batteries

A battery has two or more cells. Its symbol has parallel lines; the longer line represents the positive side while the shorter line represents the negative side.

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Ground

Ground is the common return path of a circuit, where current returns to its source. This is often referred to as the negative side in a circuit.

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Terminals

For external connections, terminals are denoted by empty circles. Terminals are connection points to external circuits. These are differentiated from nodes or junctions which have solid circles.

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Switches

Switches make or break a connection or connections in a circuit. These also allow the user to change the path of the current flow.

SPST Switches

A SPST (Single Pole, Single Throw) switch is an on and off switch. The first part of the image below indicates that it is off, thereby blocking the path of current. The second part of the image below indicates that it is on, thereby allowing the flow of current through the switch.

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SPDT Switches

A SPDT (Single Pole, Double Throw) switch is a 2-way switch. There are two routes for the current flow in this switch, which depends on the position of the switch.

How to Read Schematics - SPDT Switch

Momentary Switches

Momentary Switches are switches that only remain at a certain state unless being actuated. Push-button switches are the most common examples of momentary switches. These switches may be normally open or normally closed. The first image is a normally open push-button switch while the second one is a normally closed push-button switch.

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Multipoint Switches

Multipoint switches have multiple contacts. DPST and DPDT switches are examples of these.

DPST (Double Pole, Single Throw) switches have 2 inputs and 2 outputs. This switch allows for control of 2 outputs, either both on or both off since it is a single throw switch. The switch on the left is an open DPST switch, and the switch on the right is a closed DPST switch.

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DPDT (Double Pole, Double Throw) switches have 2 inputs and 4 outputs. The inputs have 2 corresponding outputs that the switch can make contact with.

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Resistors

A resistor is one of the most basic circuit elements, a passive one. This carries out electrical resistance that restricts or limits charge flow. The first image is the US standard, while the second one is of the international standard.

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Variable Resistors

A variable resistor is a resistor that changes, adjusts, or controls resistance value, either to increase or to decrease. Its symbol is the same as a fixed resistor, only with a diagonal arrow laid across the middle.

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Potentiometers

A potentiometer is a three-terminal resistor. The first two terminals are the endpoints of the resistor symbol and the third terminal is the endpoint of the arrow. The arrow is also called as the wiper. The potentiometer is usually used for voltage control and electric current flow control.

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Photoresistors

Also known as Light Dependent Resistors (LDR), photoresistors are a light-sensitive active component responsible for indicating the absence or presence of light.

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Capacitors

Capacitors store electrical charge, it is a passive electronic component or element. The capacitance value of a capacitor is measured by Farad.  There are two common types of capacitors, non-polarized and polarized.

Non-Polarized Capacitors

Non-polarized capacitors are capacitors without polarity. These capacitors usually have smaller values than that of a polarized capacitor.

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Polarized Capacitors

Polarized capacitors, in contrast to non-polarized capacitors, are capacitors with positive and negative polarities. These have values larger than that of the non-polarized capacitors.

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Inductors

Inductors are represented by a series of bumps or curves. In actuality, an inductor takes the form of a coil. This passive electronic component creates a magnetic field when there is a flow of current. Inductance is measured in Henry.

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Transformers

Transformers involve two coils facing each other, with lines in between them. These are usually used to step up or step down voltages. Energy is transferred between the two coils.

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Relays

Relay is an electrically operated switch. It is usually a coil and a switch. This device closes or opens a circuit.

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Diodes

A diode is a polarized device that allows the flow of current in one direction. Being polarized, it has a positive lead (anode) and the negative lead (cathode). The flat edge of the triangle is the positive side while the vertical line is the negative side.

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Transistors

A transistor is a semiconductor device that is used either to amplify or to switch. One category of transistors is called the BJT or the bipolar junction transistors. Under this category are the two basic transistors, the NPN and the PNP transistors. The NPN transistor turns on when current flows through the base of the transistor, while the PNP transistor turns on when there is no current at the base of the transistor. The first image is the symbol of the NPN transistor while the second image is that of the PNP transistor.

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Integrated Circuits

An integrated circuit also referred to as IC, is like a chip or microchip that holds circuits containing hundreds to millions of resistors, capacitors, and transistors. An IC can function in different ways, it can function as an amplifier, timer, microprocessor, and a lot more. It consists of pins that have different functions. Three of the most commonly used ICs are the 555 timer, LM386, and LM358.

The 555 Timer

The 555 timer’s most common use is to provide time delays. However, it can also be used as an oscillator and as a flip-flop element. This IC is still widespread for it is cheap and stable. It is a robust 8-pin device that is very popular and useful. The first image is the actual pin arrangement of the 555 timer with the interior circuit of the IC. The second image is the schematic symbol of the 555 timer used in diagrams.

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The LM386

The LM386 audio amplifier is an IC that allows a user to amplify sound. This 8-pin chip is a low power audio amplifier. Being low-powered, it is suitable for battery-powered devices such as guitar, amplifiers, and radios. It is also a popular and widely used IC. The first image is the actual pin arrangements of the LM386 and the second image is the schematic symbol of the IC used in diagrams.

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The LM358

The LM358 is a dual operational amplifier IC powered by a common power supply. Its applications include that of transducer amplifier, integrator, differentiator, voltage follower. The first image is the actual pin arrangements of the LM358 and the second image is the schematic symbol of the IC used in diagrams.

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Operational Amplifiers

Operational amplifiers are also referred to as Op-Amps. They are voltage amplifiers with inputs and usually one output. Its typical symbol is that of a triangle with a flat edge on the left and the top of the triangle at the right.

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Logic Gates

Logic gates are an electronic device that process signals which represent true or false. The four standard logic functions are the AND, OR, NOT, and EXOR. In contradiction with these are the logic functions NAND, NOR, and EXNOR.

AND

The output of the AND gate is true when all of its inputs are true.

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OR

The output of the OR gate is true with at least one of its inputs is true.

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NOT

The NOT gate gives the inverted version of the input at its output, that is why it is also called as the inverter. Therefore, the output is true when the input is false.

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XOR

The ‘Exclusive-OR’ or the XOR gate has two inputs. The output of the XOR gate can only be true when its inputs are a true and a false.

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NAND

The ‘NOT-AND’ or the NAND gate can have two or more inputs. The output of the NAND gate is true if any of the inputs is false.

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NOR

The ‘NOT-OR’ or the NOR gate has two or more inputs. The output of the NOR gate is true when all of its inputs are false.

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XNOR

The ‘Exclusive-NOR’ or XNOR gate has two inputs. The output of the XNOR gate is true only when both of its inputs are true and when both of its inputs are false.

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Optoelectronic Devices

Optoelectronic devices are devices that deal with optics and electronics. These devices can be divided into two, light-sensitive devices and light-generating devices. The first image is an example of the light-sensitive devices called the Photodiode. The second image is an example of the light-generating devices called the Light Emitting Diode (LED).

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Speakers

A speaker converts electrical energy to sound energy. Its symbol takes a form similar to a real-life speaker.

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Microphones

A microphone converts sound energy into electrical energy.

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Fuses

Fuses are safety devices that provide over-current protection to an electrical circuit. This essential device melts when there is too much current flowing through it.

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Motors

A motor converts electrical energy into kinetic energy. Its symbol has a letter “M” encircled with the positive and negative terminals on the left and right.

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Antennas

An antenna is a device that receives or transmits radio signals.

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Connections

Now that you are familiar with the basic fundamental schematic symbols for schematic diagrams, you must now know what a wire connection looks like and what a wire crossing another wire without connecting looks like. Wires are represented by lines. Understanding the connection of these wires might be a little confusing at first but it is really pretty easy.

If you see a connection like the images below, they are connected wires. The dots over the intersections are called nodes. The absence of a node simply means that they are not connected and are just passing by each other.

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However, there are still some who use the connection without the dots. But, in most circuit diagrams, lines intersecting each other without a dot means the wires are not connected.

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There is another way of showing unconnected wires. It is the jumping of wire over a certain wire to avoid contact. This is shown with the use of a semi-circle. Even though the simple intersecting of wires without a node or a dot automatically means no connection, this is also a safe way to do it.

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Now that you’re familiar with the basic schematic symbols and the connection of wires, you are now ready to read a simple circuit. Remember to be mindful of the polarities. Below is a simple circuit that consists only of three elements – a battery, an LED, and a resistor.

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The 9V battery powers the circuit, and the resistor limits the battery’s current so it doesn’t burn out the LED. Remember that the positive side of a diode is at the flat edge of the triangle and the negative side of the diode is the line. Therefore, the LED will light up because of the correct connection to the positive and negative terminals.

Understanding a circuit will also help you modify it if you want. The first way to modify it is by simply changing the values of some components. However, you must be careful. You don’t want to ruin your first circuit, right?