A schematic is the "sheet music" of an electronic circuit. Parts are drawn as symbols and connections as lines, showing at a glance which parts connect how. If you can read schematics, you can build circuits you find online, understand the application circuits in datasheets, and design perfboard layouts. This article covers common schematic symbols, the connection rules where lines cross, and the order to read a schematic. The shape of symbols can differ somewhat between standards (IEC and ANSI).
Common symbols
| Part | Symbol shape | How to read it |
|---|---|---|
| Resistor | Zigzag line (or a rectangle, IEC) | R1, R2 … with the value (10k, 330) beside it |
| Capacitor | Two parallel lines | C1, C2 … with the value (100n, 10u) |
| Electrolytic capacitor | Parallel lines with one curved or a + mark | The + side is the positive terminal. Mind polarity |
| Diode | A triangle and a vertical bar | Current flows in the triangle's direction (anode → cathode) |
| LED | A diode symbol + two arrows leaving it | Has a direction |
| Zener diode | A diode with the bar's ends bent | Fixes the voltage in reverse |
| NPN transistor | A vertical bar with three lines, emitter arrow pointing outward | B (base) input, C (collector), E (emitter) |
| PNP transistor | Same as NPN but the emitter arrow points inward | The current direction is opposite |
| Op-amp | A triangle with + and − on the inputs | The output is the pointed tip |
| Ground (GND) | Horizontal lines that shorten under a vertical line | The 0 V reference point |
| Power | An arrow pointing up or text like +5V | Supplies that voltage |
| Battery | A long line (+) and a short line (−) | The long line is positive |
| Switch | A broken line and an angled contact | Connected when pressed (closed) |
| IC | A rectangle with pin numbers | Pin numbers and names are written on it |
Logic gate symbols can be seen in 74LS and 74HC Logic Gate Basics.
Connection notation rules
The most confusing part of a schematic is where lines meet.
| Drawing | Meaning |
|---|---|
| A dot (●) where lines meet | Connected (a junction) |
| Lines cross in a plus shape with no dot | Not connected (they just pass) |
| Lines meet in a T shape | Usually connected (read as connected even without a dot) |
| A line hops over with a small half-circle | Not connected (older notation) |
Whether there is a dot or not makes the difference, so zoom in and check. When a drawing is complicated, labels are sometimes used instead of showing crossings with dots.
Labels (net labels)
In complex circuits, instead of drawing every line, the same name (for example, CLK, RESET, +5V) is put on line ends. The same name means the same wire connected. The net alias in Patterndo is a display-only feature that puts a label like CLK on a wire to make it easier to read. Unlike labels in a schematic, giving the same name does not connect them electrically, so the real connection has to be made with wiring (Patterndo Features).
Power and ground notation
- Power (+V): shown at the top of the circuit with an arrow or bar as
+5V,VCC, orVDD. - Ground (GND): shown at the bottom of the circuit with a ground symbol. All the same GND symbols are connected as one ground.
- Drawing the power and ground lines to every IC gets complicated, so sometimes VCC and GND are shown as separate symbols at the IC and the lines are omitted. Even then, you have to think of the IC's power pins (for example, VCC on pin 14 and GND on pin 7) as connected (Reading DIP IC Pin Numbers).
The order to read one
- Find power and ground. Check which voltages (5 V, 3.3 V, 9 V) are used.
- Work out the role of the ICs and main parts. You can find datasheets by name and number (NE555, 74LS00).
- Follow the signal flow. It is generally drawn with inputs on the left going to outputs on the right, with power at the top and ground at the bottom.
- Divide into blocks. Split it into the power, input, processing, and output sections and understand one at a time.
- Check part values. Calculating why a value was chosen deepens your understanding of the circuit (LED Current-Limiting Resistors, Voltage Dividers).
Reference designators
Parts in a schematic carry a letter for the kind and a number.
| Letter | Part |
|---|---|
| R | Resistor |
| C | Capacitor |
| D | Diode |
| LED | LED (or D) |
| Q | Transistor |
| U (or IC) | Integrated circuit |
| SW (or S) | Switch |
| J, P | Connector |
With numbers like R1 and R2, you can link to the parts list (BOM) and point to a part on the perfboard or PCB.
From a schematic to a perfboard layout
A schematic shows only the connections and not the positions. Where to place the parts and how to wire them is explained in Moving a Schematic onto Perfboard in 7 Steps. You can also bring a schematic photo into the editor to draw connection hints (Importing a Schematic Image).
Summary
A schematic is made of part symbols and connecting lines, and a dot means connected; a crossing without a dot means not connected. Find power and ground first, follow the signal from left to right, and read it block by block. Once you can read them, continue to the next stage in the Self-Study Roadmap for Electronics.