How to read a schematic
Let’s face it, the maker and electronics world is not intuitive when you first step into it. One thing that intimidates a lot of people is a schematic. It’s filled with lines, symbols, and information that you may not understand yet, and can easily overwhelm you. Once you pick up some basics, schematics become simple to understand. By the end of this tutorial, you’ll understand how to get the most out of them.
What are schematics?
Think of schematics like a roadmap of electronic projects. You could theoretically finish the project without them, but they help you a lot along the way. In other words, a schematic is a simplified representation of an electronic circuit. It can be read and understood easily. Schematics standardize the display of electronic circuits, components, labels, and connections.
Everything in a schematic has some kind of meaning. Each symbol represents a component, and each line represents a wire which connects them. With that said, a schematic is not like a physical map. The circuit board will rarely mirror it. It is more of a guide when designing and constructing a circuit board. The components are typically arranged to fit the constraints like the size of the circuit board
Just recognizing the symbols of the components is not enough. You should get an idea of how a circuit works when looking at the schematic. We’ll go over the fundamental symbols and wires. You will be proficient enough to start reading most schematics after going through the following list.
Schematics can be very simple with just a few components, as well as very complicated displayed across numerous pages. An essential part of reading schematics is learning how to read symbols. Here is the list of most commonly used symbols.
A resistor is one of the most basic electronic components. It restricts the current flow. There are two schematic symbols for a resistor. The zig-zag symbol on the left is a standard in the United States, while the rectangular symbol on right is the international standard.
A variable resistor increases or decreases the resistance according to external input. There are several types of variable resistors, such as photoresistors and thermistors. The symbol is very similar to a fixed resistor, with the only exception being an arrow placed across the middle.
A potentiometer is a type of variable resistor used for adjusting the voltage and current in an electrical circuit. The symbol is similar to a fixed resistor, with an arrow pointing towards it from where the output voltage is taken.
A diode is a polarized device that allows current flow in only one direction and stops it from going in other. Polarized means it has a positive and a negative lead (anode and cathode). The flat edge of the triangle is a positive lead, while the straight line on the end of the triangle is a negative lead.
A light-emitting diode works the same way as a regular diode. The only difference is it emits light when the current passes through. The schematic symbol looks like a diode with two arrows. The arrows represent the emitted light.
A photodiode absorbs photons from light and generates an electrical current. While the arrows for the LED schematic symbol go from the diode, the arrows for the photodiode go towards it.
As the name suggests, a non-polarized capacitor doesn’t have a polarity, so it doesn’t matter how it’s connected. The schematic symbol looks like two T shapes perpendicular to one another. These capacitors can usually store less energy than polarized capacitors.
It will matter how you connect a polarized capacitor . If it isn’t properly connected, it might even explode! The positive lead is represented with a straight line and a plus sign, and the negative with a curved line. Polarized capacitors typically have higher capacitance compared to their non-polarized counterparts.
A variable capacitor (sometimes called a tuning capacitor) is a capacitor whose capacitance can be changed. The schematic symbol is like a non-polarized capacitor, but with an arrow through the middle.
A transistor is one of the basic building blocks of modern electronics. It is used for digital logic, amplifying and switching electrical signals and power. It is made of semiconductor material and has a minimum of three terminals for connecting to an electronic circuit.
The most common type of transistor is the bipolar junction transistor (BJT). The three terminals on the BJT transistor are base (B), collector (C), and emitter (E). A BJT transistor can either be an NPN (negative-positive-negative) or PNP (positive-negative-positive).
A schematic symbol for a BJT is a circle with three lines representing the three terminals going inside it. On the NPN transistor, the base is on the left, the collector is on top and the emitter is on the bottom. There is another line in the circle, and the three terminal lines connect to it. An arrow will go from the line in the circle and point to the emitter.
On the PNP transistor, the base remains the same, but the collector and the emitter switch places. An arrow will point from the emitter to the line inside the circle.
You’ll sometimes find these two transistor symbols drawn without the circle, but it is an uncommon practice.
The other common type of transistor is the metal–oxide–semiconductor field-effect transistor (MOSFET). It is the most used transistor. MOSFET ’s terminals are gate (G), drain (D), and source (S). The schematic symbol is similar to the BJT symbol. A horizontal letter T is put with only the upper half in the circle. Another line is next to it. The drain and source terminals connect to that line. Unlike the collector and emitter in the BJT schematic, the drain and source terminals are drawn with a 90° angle.
Switches are electrical components that can connect or disconnect the conducting path in an electrical circuit. They come in many different configurations. The most common type is made of one or more electrical contacts connected to external circuits.
An SPST (single pole, single throw) is a simple on-off switch with two terminals either connected to or disconnected from each other. A symbol for a terminal is an empty circle. An SPST switch is denoted with two terminal symbols and a line between them. The line can be either connected to one or both of them. It will depend on whether the circuit is closed or not.
An SPDT (single pole, double throw) switch has one input and two output terminals. It is useful when changing between two active states in a device. The schematic symbol is made from three terminal symbols and a line between two of them, depending on how the circuit is wired.
A DPST switch is the same as two SPST switches controlled by a single mechanism. The schematic symbol reflects that – it looks like two SPST switches.
A DPDT switch is equivalent to two SPDT switches controlled by a single mechanism. The schematic symbol looks like two SPDT switches.
An air-core inductor has coils wound on ceramic, plastic, or other nonmagnetic forms. It doesn’t have a magnetic core made of ferromagnetic material. Inductors are usually represented by a squiggly line or a series of curved bumps.
A magnetic-core inductor has a core made of magnetic material such as iron or ferrite to increase the inductance. The schematic symbol is the same as the air-core inductor, with two lines added next to the squiggly line.
Single-cell batteries have the same schematic symbol, no matter if they are lithium-polymer, cylindrical, or alkaline AA. The symbol is two parallel lines of different lengths. The longer line usually represents the positive terminal on the battery.
Two battery symbols display multi-cell batteries, regardless of the number of cells.
Solar cell (photovoltaic cell) battery is shown with a circled battery symbol and two arrows pointing towards it.
When working with electronics, you’ll be working with constant voltage sources for the most part. The schematic symbol of DC voltage (direct current) is a circle with plus and minus signs inside.
The other voltage current is AC (alternating current). The sideways letter S inside a circle represents this current.
Logic gates are electronic circuits that manipulate voltages. In essence, they manipulate the binary ones and zeros (voltage high and low) moving around in an electronic device. Logic gates usually have two inputs and one output. The NOT gate is the only one with a single input.
The AND gate will have an output of 1 or high only if both inputs are also 1. In any other case, the output will be 0 or low. The schematic symbol looks like the letter D. Two input lines connect to it on the straight line. The output line is in the middle of the curved line.
The OR gate will have an output of 1 if either or both of the inputs are also 1. The output will be 0 only if both inputs are also 0. The schematic symbol is similar to the AND gate symbol. The only difference is that the straight line in the letter D is curved.
The XOR gate, short for exclusive OR, is similar to the OR gate. The output will be 1 if either of the inputs is 1. The output will be 0 if both inputs are 0 or 1. In a nutshell, the output is 1 when the inputs are different. The output is 0 when the inputs are the same. The schematic symbol is the same as the OR gate, with another curved line added to the left.
The NOT gate is a logic gate implementing logical negation. The main function of the NOT gate is to output the inverted input circuit signal. That is why you’ll often see it referred to as an inverter. Thus, if the input is 1, the output will be 0 and vice versa. The schematic symbol is a simple triangle with a line connecting to one of its sides and coming out of the tip on the other side. At the end of the tip is a little bubble that indicates the negation of the function.
The NAND gate is short for NOT AND. As the name implies, it will be the opposite of AND gate. If both inputs are 0 or either one of them is 1, the output will be 1. If both inputs are 1, the output will be 0. The schematic symbol is almost identical to the AND gate. The only difference is the little bubble on the output line indicating inversion.
The NOR gate is short NOT OR. It will have the opposite outputs of the OR gate. If both inputs are 0, the output will be 1. If either or both inputs are 1, the output will be 0. The schematic symbol looks like the OR gate, with the inversion bubble at the output line.
Finally, the XNOR gate is short for exclusive NOR. The outputs will be opposite to that of the XOR gate. Thus, if both inputs are either 0 or 1, the output will be 1. If the inputs are different, the output will be 0. The schematic symbol is similar to the XOR gate, with the bubble on the output line for the inversion.
For easier visualization of the inputs and outputs for each logic gate, we prepared the tables below. They will help you in remembering the schematic symbols for logic gates.
Transformers are passive components that transfer electrical energy from one electrical circuit to one or multiple others. Electrical energy can be transmitted between coils without necessarily having a conductive connection between the circuits. One such example is the air-core transformer, designed to transfer radio-frequency currents. The schematic symbol is two squiggly lines next to each other.
The problem with air-core transformers is that they let a lot of flux go to waste. Only a portion of the electrical energy will go from one winding to another. That is why an iron-core transformer is used. It doesn’t let flux vanish into the air. Thus more electric energy is transferred between the windings. The schematic symbol is two perpendicular lines in between the two squiggly lines.
Transformers can have one or more winding on both the primary and secondary sides. Those that have more than one are called multiple winding transformers. Below is an example of a transformer with two secondary windings. Secondary windings will have a gap between them on the schematic to display they aren’t connected.
Center-tapped transformers are types of transformers with two secondary windings that are connected. They share a common connection, but each one provides a separate secondary voltage. The schematic symbol is very similar to an iron-core transformer. The only thing differentiating them is the line on the secondary winding denoting the center tap.
The current transformer is used for producing an alternating current in the secondary winding proportional to the current measured in the primary winding. They reduce the high voltage currents to a much lower value for monitoring and protection purposes. The symbol is a squiggly line with a straight long line going through it.
The number of integrated circuits is vast. Different integrated circuits do many different things. Because of this, a unique schematic symbol for every one of them is essentially impossible. Most of the time, an integrated circuit will be displayed as a rectangle with pins drawn on the sides. Each pin has a function and a number. These are very important because they help in identifying which IC is displayed on the schematic.
Wires and grounds
Wires are used to connect two or more terminals. Without them drawn, the schematic becomes practically useless. Wires can be either connected or not. If they are connected, a dot is added at the intersection.
There are three ways to display unconnected wires. The simplest and most common way is to leave the dots intersected. Another way is to draw a semi-circle or an arch over the point where the wires cross. Lastly, unconnected wires can be displayed with one of the wires cut off at the point of the intersection. No matter how it is displayed, you can easily remember it as no dot equals no connection.
In the schematics design, it is a good practice to avoid intersected wires. This can’t always be done, so some intersections must be made. In that case, having them displayed the same way is a great way to stay consistent.
If you’ve worked with microcontrollers, you know we always connect one wire to the ground. The ground path is where the current returns to its source. We say that it is the negative side of an electronic circuit. There are three different grounds.
Earth ground is, as the name suggests, the earth beneath you. A rod or wires (or both) are driven into the ground. When the electric energy goes through the circuit, it will end up in the ground and disperse. The schematic symbol is three perpendicular lines, with the longest being on top and the shortest on the bottom. It indicates an arrow or triangle. Schematics can sometimes use this symbol to signify the signal ground.
Signal or common ground is a reference point from which the signal is measured. They are used as return paths for signals and power in an electronic device. There can be multiple signal grounds in a circuit, but many simple electronic designs have a single return for all signals. The schematic symbol is a simple triangle pointing down.
Chassis ground gets its name from the metal enclosure of a device. In this case, the enclosure is defined as a reference point for the electrical circuit. This type of ground can be most notably found in cars and other vehicles. The schematic symbol looks like a skewed letter E.
Names and values
The final pieces of the puzzle in understanding how to read a schematic are names and values. These will tell you all the information you need to properly understand the schematic. Without them, you’d have a drawing of the circuit and the components, but you’d hardly know what is what exactly.
The component’s name is usually a combination of one to two letters and a number. This is called a reference designator. The letters tell the type of a component. The number helps in identifying which exact component it is if there are more of the same type. If we have a component named R10, we know it is one of at least 10 resistors on the device.
Another letter can follow that sequence to indicate grouped or matched components. Let’s say we are looking at the four resistors R10A, R11B, R12B, and R13A. By looking at the last letters in each sequence, we’d know that R10A and R13A are one group, and R11B and R12B are another.
Lastly, values next to the component will give more information to understand the schematic better at just a glance. If you see 10k next to a resistor, you know for a fact that is a 10k-Ohm resistor.
You can find the most common component name identifiers (letters) in the table below. Find a longer list of the reference designators on this link.
These names are standardized, but that doesn’t mean the schematic provider needs to follow them. You can find IC instead of U for integrated circuits, for example. The symbol should tell you enough, but keep in mind that they aren’t the same everywhere, and you’ll sometimes need to use your assessment.
Resources and go further/Reading the schematic
The ability to read schematics becomes essential the more you work with electronics. The more complicated a project is, the more crucial the skill becomes. It is a must-have skill when designing your PCB .
Did you know that we add the freely available schematics for all the boards we design? Now that you know how to read them, why not test that knowledge by looking at some of our schematics? Find a product that interests you (e.g. Dasduino Core), and find the schematic for it on the links to the left.
Products used in this tutorial
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