Circuit diagrams, aka schematics, are line drawings that show how a circuit’s components are connected together. They serve as a map or plan for assembling electronics projects, and they are easy to read — far easier than understanding how the circuits they describe actually work. This is an important point: You can read and successfully build from a schematic diagram without understanding the circuit.*
Schematics are also readily available for countless easily-buildable electronic devices. Do you hear that? That’s the sound of freedom.
Schematic diagrams are made up of two things: symbols that represent the components in the circuit, and lines that represent the connections between them. That’s it. Let’s start with the connections, since that’s easier.
Connections
Circuit diagrams depict a perfect world where wires and other conductors do not interfere with one another and have no resistance of their own. If a line runs between components, it means that they are connected, period, and it tells you nothing else. The connection can be a wire, a copper trace, a plug-socket connection, a metal chassis, or anything else that electricity will run through without much resistance. Messy details like wire or cable specifications and routing, if they are important for a project, belong elsewhere in its documentation. The length of a line also has nothing to do with the connection’s actual distance in real life. Schematics are drawn (ideally) to be clear and simple, with components and connections arranged on the page to minimize clutter, not to represent how they might be placed on a circuit board.
Lines represent connections, but where two lines cross, it doesn’t necessarily designate a 4-way shared connection. Schematics distinguish between unconnected paths that happen to be drawn with lines crossing each other, and junctions where the line crossings designate a shared connection. The most common way to make this distinction is to put a dot over the line intersections that indicate connections, which means that any line crossings without dots are all unconnected. The other method is to assume that plain crossed lines do connect, but draw small “jumps” at wire crossings where there is no connection.
As a corollary, a three-way intersection always means a three-way connection, even without a dot. Some people follow the dot drawing rule with 3-way connections and others see no need, because there’s no reason to draw a connection to nowhere.
In addition to the lines used to show connections between components, schematics use special symbols to show connections to different types of power and ground. A power or ground symbol may appear in multiple places on the schematic, but it always means a connection to the same place or conductive object. Power connections are also often shown without any symbol, just a label indicating the type of voltage, e.g. V+, 5V, 5VDC, 12V, 120VAC, with positive (+) implied for unsigned DC voltages.
Components
Each circuit component is represented by a symbol that indicates the general type of component and a label that points to (or directly lists) its particular specifications. Wikipedia’s “Electronic symbol” article shows some of the most common symbols, and Electrical What?! has a more complete, searchable collection.
Formal schematics label each component with a parts designator, which is a code made up of a letter or two identifying the component type (e.g. R for resistor, C for capacitor), followed by a unique number for that type in the circuit (e.g. resistors R1, R2, etc.). A parts list accompanying the schematic associates each parts designator with component specifications (e.g. R1: 120k Ω, 1/4 W).
(Schematic from “The Biggest Little Chip” by Charles Platt, MAKE Volume 10, p. 65)
In less formal schematics, people dispense with the parts designators and list and just label the part symbol on the drawing itself with any necessary specs.
(Schematic for “DSLR Time-Lapse Trigger” by Chris Thompson, MAKE vol. 15, p. 156)
To avoid special characters, resistor specs often drop the capital Omega (Ω) for ohms (220k means 220k Ω) and capacitor values use “u” instead of a lowercase Mu (µ) to mean micro (10uF means 10 µF / 10 microfarad).
(If you’re unclear on what ohms and microfarads are, don’t worry &emdash; you can still build working circuits from a schematic. But meanwhile, it will help to learn the Hydraulic Analogy, and keep in mind that electricity is much, much faster than water.)
Each component symbol has some number of connection points to which lines can be drawn. These correspond to the leads (or other terminals) of the physical component. For resistors, ceramic capacitors, and some other simple components, it doesn’t matter which way the leads connect. But with most components, the leads have a set orientation or perform different functions.
Every component has a datasheet, published by its manufacturer, that associates the component’s physical terminals with their functions, as indicated by the connection points on the schematic symbol.
Integrated circuits (ICs), aka chips, package electronics components into small, uniform blocks with some number of connection terminals running along the sides, either metal legs or (with some surface mount components) metal contacts underneath. Schematic diagrams represent chips as rectangles with lines coming out designating the chip’s legs. In some drawings, the rectangle symbol replicates the package’s physical layout, with the legs numbered counterclockwise from Pin 1, just left of the notch at the top. But in order to reduce line crossings and general spaghetti-factor, some schematics swap ICs’ legs around and put them on all sides of the rectangle, labeling them by pin number.
Chips are single components physically, but functionally, some chips contain multiple independent components housed in the same package. In such cases, the chip may be diagrammed either physically or functionally, using separate symbols for the functional components that the chip contains, labeled so that it’s clear they’re on the same chip. For example a 4093 chip, which contains four independent logical NAND gates, might be drawn and labeled like this:
(Schematic from Nandhopper 1-Bit Noise Synth on Instructables, by Kyle McDonald)
Notice that the functional drawing omits power and ground connections to the chip. If a circuit diagram represents a chip using its functional components, you need to remember to hook up its power and ground as well, even if the schematic doesn’t show them. Here, again, the datasheet is your best friend, and in general, ICs demand even more poring-over-datasheets than discrete components do, to make sure all those identical-looking legs are connected properly.
That’s it!
Schematics are just maps showing how to connect discrete components. The easiest way to translate most schematics into a working circuit is to use components with standard 0.1″ pin spacing, and connect them together on a solderless breadboard using jumper wires. Then you can test connections and otherwise debug and get to know the circuit with a multimeter, before you consider committing it to solder.
Reviewing the main points:
You can read and successfully build from a schematic diagram without understanding the circuit.
- Schematic diagrams are made up of two things: symbols that represent the components and lines that represent the connections.
- If a line runs between components, it means that they are connected, period, and it tells you nothing else.
- Schematics distinguish between unconnected paths that happen to be drawn with lines crossing each other, and junctions where the line crossings designate a shared connection.
- Schematics use special symbols to show different types of power and ground.
- Each circuit component is represented by a symbol and a label.
- Each component symbol has some number of connection points. These correspond to the leads (or other terminals) of the physical component.
- A component’s datasheet associates its physical terminals with their functions as indicated by its symbol.
- Some schematics swap ICs’ legs around and put them on all sides of the rectangle, labeling them by pin number.
- A chip may be diagrammed either physically or functionally, using separate symbols for the functional components that the chip contains.
- If a circuit diagram represents a chip using its functional components, remember to hook up its power and ground.
*Of course, understanding the circuit helps if you want to modify it, or if the diagram has errors– which is not unusual. Edited sources such as MAKE add value by building the projects before publishing them, ensuring that schematics and other documentation are correct.
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