How to read a schematic

Lines are wires, and dots matter

On a schematic, every line is a wire. The drawing doesn't care how long the real wire is or which way it bends; it only shows what connects to what.

Where lines meet, look for a dot. A dot (a junction) means the wires are joined, so current can flow from one into the other. When two lines simply cross with no dot, they are not connected; one wire just passes over the other. Some drawings make that extra clear with a small hop, a little bump in one line.

The battery and the resistor

A battery cell is drawn as a long line beside a short line, set across the wire. The long line is the plus (+) terminal and the short line is minus (−). A battery made of several cells is often drawn with two or more of these pairs.

A resistor is a zigzag. A resistor is anything that limits, or "squeezes," the current flowing through it. In drawings used in Europe and Canada (the IEC style), a resistor is often a plain rectangle instead.

Lamp, switch and ground

• Lamp: a circle with an X inside. Some US drawings show a circle with a little filament loop instead.
• Switch: a line with a hinged gap. When the hinged part is lifted, the gap is open and no current flows; when it's down, the circuit is closed.
• Ground: three shrinking lines, stacked. On a schematic, ground is the circuit's zero-volt reference, the point every other voltage is measured from. It isn't always tied to the earth (that's the safety ground from the last episode).

Put them together

Now read a whole circuit. Start at the battery's plus side and follow the wire: through the switch, through the resistor, through the lamp, and back to the battery's minus side. That's one loop. With the switch closed, current flows all the way around and the lamp lights.

In a single loop like this, the order of the parts doesn't change the current. The same current flows through every part.

Try it yourself

Search for the schematic of a simple flashlight or a battery-powered toy, or look at the diagram in an electronics kit's manual. Find the battery (long and short lines) and trace the wire with your finger from the plus side all the way back to minus. Count the parts you pass. Can you spot any dots where wires join?

Good to know

• This episode covers a handful of symbols so one short stays one idea. Capacitors, diodes, LEDs and transistors get their symbols in Season 3.
• The symbols here follow common US practice (IEEE 315 / ANSI). IEC drawings, common in Europe and Canada, show a resistor as a plain rectangle.
• Some US drawings show a lamp as a circle with a filament loop instead of an X.
• A long-and-short pair is one cell; a battery of several cells is often drawn with two or more pairs.
• Some drawings mark crossing wires with a small hop and connections with a dot. Older drawings may show a connection without a dot, so read each drawing's own conventions.
• The ground symbol here means the circuit's zero-volt reference. It is not always connected to the earth.
• In a single loop, the order of the parts doesn't change the current.

Answer to the comment question

Two wires cross with no dot. Are they connected? No. On a schematic, a dot marks a junction where wires are joined; lines that just cross with no dot pass over each other without connecting. That's the common convention, but older drawings sometimes skip the dot, so always check a drawing's own conventions.