Standard Schematic Symbols

Schematic symbols are a shorthand notation — agreed internationally — that lets engineers draw any electronic component as a simple geometric shape. Once you know the symbols, you can read any circuit diagram regardless of who drew it or where in the world it came from.

IEEE and IEC Standards

Two main standards govern schematic symbols. The IEEE (Institute of Electrical and Electronics Engineers) standard is used throughout North America. The IEC (International Electrotechnical Commission) standard is used in Europe and most of the rest of the world.

In practice the differences are minor. A resistor drawn as a zig-zag line is IEEE; the same resistor drawn as a rectangle is IEC. Both conventions are widely understood. Ham radio literature — especially older American books and data sheets — predominantly uses IEEE symbols, and that is the style used throughout this course.

Fig 1 — IEEE symbols (left) use a zig-zag for resistors and curved bumps for inductors; IEC symbols (right) use rectangles. Both are correct.

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Passive Components

Passive components cannot amplify a signal — they only resist, store or transform energy. They are the most common symbols on any schematic.

Resistors

A resistor is drawn as a zig-zag line (IEEE) between two terminals. The value is written next to the symbol: 10k means 10 kilohms, 4R7 means 4.7 ohms. A variable resistor (rheostat) has an arrow through it. A potentiometer — three terminals with a wiper — is drawn with the arrow pointing to a third terminal at the center.

Capacitors

A fixed capacitor is two parallel plates separated by a gap. The plates are drawn as straight lines of equal length. An electrolytic (polarized) capacitor has one curved plate (the negative side) and is marked with a plus sign on the positive lead. The value is usually given in microfarads (µF), nanofarads (nF) or picofarads (pF). A variable capacitor has an arrow through one plate.

Inductors

An inductor is a coil of wire drawn as a series of arcs (or bumps in IEC). A tapped inductor has a connection part-way along the coil. An iron-core inductor has two parallel lines beside the coil; a ferrite core uses dashed lines. A transformer shows two coupled coils on either side of core lines.

Semiconductor Devices

Semiconductor symbols use a combination of lines and arrows to show the direction of conventional current flow.

Diodes

A diode is a triangle pointing into a bar. Current flows in the direction the triangle points (from anode to cathode). Variants add features to the bar or add a letter to the symbol body:

• Zener diode — the bar has a small Z-shaped bend on each end
• Schottky diode — the bar has a small S-curve at each end
• LED — arrows pointing away from the junction indicate emitted light
• Photodiode — arrows pointing toward the junction indicate received light
• Varactor — a capacitor symbol attached to the cathode side

Bipolar transistors (BJT)

A BJT has three terminals: base (B), collector (C) and emitter (E). The emitter always has an arrow on it. For an NPN transistor the arrow points outward (away from the base line). For a PNP transistor the arrow points inward (toward the base line). A useful memory aid: NPN = Not Pointing iN, PNP = Pointing iN Please.

Field-effect transistors (FET)

A JFET is drawn with an arrow on the gate pointing toward the channel for N-channel, or away for P-channel. A MOSFET adds a broken line for the channel (to indicate the gate oxide insulation) and the body is shown with an arrow. Enhancement-mode MOSFETs have a dashed channel line; depletion-mode use a solid line.

Fig 2 — A reference chart of the most common schematic symbols grouped by component type.

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Active Devices

Operational amplifiers (op-amps)

An op-amp is drawn as a triangle pointing right. The two inputs enter from the left side: the inverting input (−) is at the top-left, the non-inverting input (+) is at the bottom-left. The output leaves from the point of the triangle on the right. Supply voltage pins (V+ and V−) are often omitted on simple schematics to reduce clutter; they are assumed to be connected.

Voltage regulators

Three-terminal regulators (7805, LM317) are drawn as a box with three labelled terminals: input, output and adjust (or common). The part number is written inside or next to the box.

Integrated circuits

A generic IC is drawn as a rectangle. Each pin is a short horizontal line coming off the edge of the rectangle, with the pin number and function labelled beside it. Power and ground pins are usually shown on the top and bottom edges; signal pins on the left and right.

Logic Gates

Logic gate symbols follow the ANSI/IEEE Std 91 standard in American literature. Each gate has a distinctive shape:

The bubble (a small circle) always indicates inversion. An input bubble inverts the input signal; an output bubble inverts the output.

Connectors, Switches and Miscellaneous

Connectors

A single-pin connector is drawn as a small filled circle with a line. Multi-pin connectors are shown as a row of numbered circles (the connector body) with lines for each pin. In RF schematics a coaxial connector is usually labelled J1, J2 etc. and the connector type (BNC, SO-239, SMA) is written beside it.

Switches

An SPST switch is a line with a gap and a pivoting arm. SPDT adds a second contact point that the arm can reach. DPDT doubles the symbol vertically with a dashed line showing the mechanical linkage. Push-button symbols have a momentary action arrow or a different arm orientation to distinguish normally-open from normally-closed.

Fuses and circuit breakers

A fuse is a small rectangle or S-curve between the circuit lines. A circuit breaker is similar but with a flag symbol indicating it can be reset. Both are labelled with their current rating (1A, 5A).

Lamps, buzzers and speakers

A lamp is a circle with a cross inside. A speaker is a rectangle (the coil) attached to a triangle (the cone). A buzzer or piezo element is a box labelled BZ or LS with its reference number.

Wires and Junctions

Lines on a schematic represent wires. Two lines that cross without a dot are not connected — they simply cross over each other. Two lines that cross with a filled dot are connected — the dot is called a junction.

A net label (a short piece of text on a wire end) is used instead of drawing a line when connections would make the diagram too cluttered. Two wire ends with the same net label are connected even though no physical line joins them on the page. This is common for power supply and ground connections.