ActiveTech Blog

How to Read a Schematic

A schematic is a drawing of an electrical circuit which shows all of the electronic components and the electrical connections between them. Rarely are the mechanical or physical layouts or assemblies described here. Those are left for the PCB layout drawings. The job of the schematic is to provide a detailed, easy to read electrical outline of the circuit. All electronic components are given unique names (references) which identify each of them uniquely. No two components can have the same reference number. Usually, all the component types and values and other important component specific information is shown. When a circuit assembly is to be repaired or modified, the circuit changes are made on the schematic first, so it can be used as a guide to prevent errors when you finally do make changes to the PCB. Once the schematic is modified, the changes are easier to show to other people and to repeat on multiple boards. Most advanced circuit schematic editor programs can read the changes you make in a schematic and automatically transfer the changes to the PCB layout. Most systems or projects are composed of a collection of schematics each showing a specific circuit.

For those who have never read electronic schematics before, you may find it a bit over-whelming the first few times you look at a schematic with all those strange unrecognizable symbols. Finding books loaded with circuits is easy enough, but finding a book which shows an assortment of the most common symbols is likely a lot harder, although they do exist. It is hard learning what all the different electrical and electronic components do, but first, you need to recognize them on paper.

Keep in mind that the following section is intended for beginners, mainly hobbyists who have recently purchased an electronic project kit.

 

Wires

Wires are shown to be connected with a dot at the junction. They are shown to be disconnected by showing one wire jump the other. It's a good habit to always use one symbol or the other & never just draw wires across each other like "+" because someone somewhere someday might be wondering if you intended for those two wires to be connected or not. Typical schematic drawing software usually has no jump symbol, only dots, so always assume that two lines are NOT connected if there is no dot. Also, keep in mind that CAD drawings are incredibly precisely drawn, so there is virtually never any doubt as to whether a dot was drawn or not, but with handmade drawings, it can be less obvious.


Power & Ground Symbols

These vary but are pretty easy to interpret. They indicate circuit or analog and Earth or chassis. They are NOT the same thing and voltages can often exist between them. The Earth ground (green or bare copper wire in an AC outlet, middle or round prong of a plug) is often tied to the chassis of a device (such as an oscilloscope) running from the outlet. If the negative terminal or the ground terminal of your circuit gets connected to Earth ground accidentally (say by touching the oscilloscope probe ground clip to your circuit at some point), you may have problems if your circuit is running on a power supply whose negative terminal is not isolated from Earth ground. Most lab-quality power supplies provide a replaceable jumper which allows a user to connect a terminal to chassis or Earth ground. I virtually never have either power supply output connected to chassis ground simply to avoid problems like this and to keep my circuit isolated from the AC outlet completely. Misunderstanding of grounds and where they are applied is the leading cause of spontaneous combustion of oscilloscope probe cables.
You do not need to use different ground symbols for analog and digital circuits in general although you can if you need to. Sometimes, digital circuits can be electrically noisy and so it is desired to separate the power lines of the digital circuits from the analog ones.

Toggle Switches

 

The first one is open, the second is closed. Current cannot pass the open switch, but it can pass the closed switch. The switches shown are often referred to as single-pole, single-throw or SPST type. Toggle switches remain in whatever position you move them to.

Push Button Switches

These work like toggle switches except that they return to their normal state when you release them. The first switch shown is a normally-open type and the second is a normally-closed type.