Most symbol charts online are redrawn by hand, and a few get the details wrong. Every chart below is a real schematic sheet instead: KiCad 9 plotted it from its own symbol libraries, unchanged. Under each symbol is its common name and the name to search for in KiCad. The notes cover what actually trips people up when they read or draw a schematic: which end is which, what the arrows mean, and which pin is which.
Short answer: a schematic symbol is the drawing that stands for one part in a circuit diagram. A resistor is a box (IEC) or a zigzag (ANSI), a capacitor two plates, a diode a triangle against a bar, and a transistor a circle with an arrow on one lead. Each symbol carries a reference designator: a letter for the kind of part (R resistor, C capacitor, D diode, Q transistor, U chip) and a number. Its pins are numbered to match the pads of the part's footprint on the PCB.
Resistors
The plain resistor has two drawings: the IEC box and the ANSI zigzag. Both mean a fixed resistor and neither has a polarity. A diagonal arrow across the body means a variable resistor. A potentiometer has three pins, the two ends and the wiper, and KiCad numbers the wiper 2. A trimmer is a potentiometer you set once with a screwdriver, which is why its wiper ends in a flat bar rather than an arrow.
Three resistors respond to something other than current. In the NTC thermistor, the ϑ marks temperature and the two opposed arrows mean resistance falls as temperature rises (KiCad's PTC version draws both arrows pointing the same way). The photoresistor (LDR) has light arrows pointing in. The varistor (MOV) clamps voltage spikes. Its bent diagonal stroke is the IEC mark for a resistance that changes non-linearly, and the U beside it says the change follows voltage.
Capacitors, inductors and crystals
A capacitor is two plates. If one is marked +, the capacitor is polarized (electrolytic or tantalum) and must be fitted the right way round. In the ANSI drawing the curved plate is the negative one. An inductor is a row of loops; two straight lines alongside mean an iron core. A ferrite bead, which filters high-frequency noise off a supply line, is a small block drawn across the wire.
The transformer is two windings either side of a core. In KiCad's symbol the primary is pins 1 and 2 and the secondary 3 and 4. A crystal is a quartz block between two plates; the crystal for a microcontroller's clock usually sits beside it with a small capacitor from each pin to ground.
Diodes and LEDs
Every diode is a triangle pointing at a bar. Current flows the way the triangle points, from anode to cathode, and the bar is the cathode: the same end as the band printed on the part. The bar's ends tell you the type: bent ends for a Zener, which holds a fixed reverse voltage, and S-shaped hooks for a Schottky, which drops less voltage forward. KiCad's D_TVS draws a bidirectional TVS (two diodes back to back) for clamping spikes either way.
Arrows leaving the symbol mean light out (an LED). Arrows entering mean light in (a photodiode).
The classic mistake: an LED drawn backwards does not light. The cathode bar must face the lower voltage for an LED or a rectifier, and the higher voltage for a Zener or TVS doing its job. Check the footprint's cathode mark against the symbol before you order boards.
Transistors and MOSFETs
A bipolar transistor has a base, a collector and an emitter. The arrow is on the emitter and shows conventional current: out for NPN, in for PNP. A MOSFET has a gate, a drain and a source. KiCad's MOSFET symbols draw the channel, the arrow (pointing in for N-channel, out for P-channel) and the body diode that almost every power MOSFET has between source and drain. A phototransistor has no base pin, because light drives it.
Same symbol, different pin order. The symbol never tells you the package's pin order. SOT-23 MOSFETs are commonly gate 1, source 2, drain 3, and small SOT-23 bipolar transistors base 1, emitter 2, collector 3, but other parts in the same package differ. Check that the symbol's pin numbers match the footprint's pads for the exact part number.
Ground, power and batteries
Ground and supply symbols are not parts: each one is a net name. Every GND symbol on a sheet is the same net, so you rarely draw a ground wire across the page. GNDA (analog ground) looks the same but is a separate net, which is the point: analog and digital grounds stay apart until you join them in one place on purpose. Earth (three shrinking bars) and protective earth (the circled version) are for mains-connected equipment. Supply rails such as +5V and VCC work the same way as ground: the name is the net.
A battery cell is a long thin plate (positive) and a short thick one (negative). Several cells drawn in a stack mean a multi-cell battery.
Switches, relays and fuses
A push button closes only while it is pressed. An SPST switch (single pole, single throw) is a plain on–off switch. An SPDT switch connects its common, pin 2 in KiCad's symbol, to one of two outputs. A relay is a coil and a switch drawn in one box, with a dashed line showing the coil moves the contact. Relays use standard numbers: the coil is A1–A2, the common contact 11, normally closed 12 and normally open 14.
A fuse is a box with the wire running through it. The resettable fuse (polyfuse, a PTC that resets when it cools) adds the same bent stroke as the varistor: a part whose resistance changes non-linearly, here sharply as it heats.
Motors, speakers, lamps and antennas
A motor is an M in a circle; a DC motor marks its + pin, which sets the direction it turns. A speaker is a small box with a cone. A buzzer is a half-disc whose + pin matters for the self-oscillating kind. A microphone is a circle with a bar on one side, a lamp a circle with a cross, and an antenna a short fork.
Logic gates and op amps
These are the ANSI (distinctive-shape) symbols: a flat-backed D for AND, a curved shield for OR, a triangle for NOT, and an extra curved line for XOR. A small circle on an output inverts it, which turns AND into NAND and OR into NOR. KiCad draws each one here as a real single-gate chip from the 74LVC1G family, so you can also see the supply pins (VCC on pin 5, GND on pin 3). The IEC style draws every gate as a rectangle with a label inside instead: & for AND, ≥1 for OR, 1 for a buffer or inverter, =1 for XOR.
An op amp is a triangle with two inputs: + is non-inverting, − is inverting.
The letters: reference designators
Every symbol gets a reference designator, and the same designator is printed on the board next to the part, so R7 on the schematic is R7 on the silkscreen. The ones KiCad's libraries use, most of them following IEEE 315 and ASME Y14.44 practice:
| Letter | Part | Letter | Part |
|---|---|---|---|
| R | Resistor | Q | Transistor |
| RV | Potentiometer, trimmer, varistor | U | Chip, logic gate, op amp |
| C | Capacitor | J | Connector |
| L | Inductor | SW | Switch |
| D | Diode, LED | K | Relay |
| Y | Crystal | F | Fuse |
| T | Transformer | BT | Battery |
IEC or ANSI?
Two standards cover most schematics. IEC 60617, common in Europe and Asia, draws a resistor as a box, and logic gates as labelled rectangles. ANSI/IEEE 315, common in North America, draws a resistor as a zigzag and logic gates with distinctive shapes. Capacitors, diodes and transistors look much the same in both. Neither style is more correct, but mixing them on one sheet confuses readers, so pick one per project. In KiCad the IEC resistor is R, the ANSI one R_US.
How the symbols connect
- A dot is a connection. Wires that meet at a dot are joined. Wires that cross without a dot are not.
- Names connect too. Two wires with the same net label, or two power symbols with the same name, are one net even if no line joins them.
- An X on a pin means not connected on purpose, so the electrical rule check does not report it.
- Pin numbers become pads. When the schematic becomes a board, pin 1 of a symbol connects to pad 1 of its footprint. A mismatch there is the most expensive mistake a correct-looking schematic can hide.
The full chart, with the rest of the reference designators and the sources, is at pcbeditor.com/schematic-symbols. If you want to draw your own next, the circuit diagram maker runs in the browser.
Chart images are rendered from the KiCad symbol libraries and, like the libraries, are shared under CC BY-SA 4.0.
Which symbol confused you most when you started? For me it was ground: three different-looking symbols, and it took a while to learn that the name, not the drawing, is what connects them.
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