LFM

Reading a schematic

A schematic is a drawing of a circuit that uses a standard symbol for each part. Reading one is four skills: recognize the symbol, decode the name and value, follow the nets, and use the layout. Work the tabs in order, and click anything on the page to read what it is.

+
BT
Battery
R
Resistor
C
Capacitor
D
Diode
D
LED
Q
Transistor
U
IC / chip
SW
Switch
GND
Ground
Click any symbol
Start here

The symbol library

Every part has one fixed symbol, the same in every schematic you will ever read. Click a symbol to learn what it is and the letter that names it on the drawing. These nine cover every part in this module.
The letter under each symbol is its reference-designator letter: R for resistor, C for capacitor, D for diode, Q for transistor, U for an IC, SW for a switch, BT for a battery.
+ BT15 V R1220 Ω D1red LED GND
Click BT1, R1, D1, or the ground symbol
Reading a nameletter + number
How a part is named

Name = type letter + number

Each part carries a name and a value. The letter gives the type and the number makes it unique, so the first resistor is R1 and the second is R2. The value sits beside the name: 220 Ω for a resistor, 5 V for the battery. Name and value together tell you exactly which part sits there. Click each part to decode it.
+5V R110 kΩ Vout R210 kΩ GND
Click the dot, the crossing wire, or a net label
Wires and nets

A dot means joined

A line is a wire. Where wires join, a filled dot marks the connection, and everything touching that dot sits at the same point in the circuit. Where two wires cross with no dot, they are not connected; they only pass over each other. A connection point can also be given a net name instead of a long wire, and every point with the same name is joined as if a wire ran between them. Click the dot, the crossing wire, or a net label.
+ BT15 V R1 220 Ω D1 signal → + supply ground / return
Click the top rail, the bottom rail, or the signal arrow
The convention

Power up, ground down, signal left to right

Schematics are drawn the same way every time, so you can find your way around an unfamiliar one at a glance. The supply runs along the top, the return or ground along the bottom, and a signal flows left to right, from its input to its output. Every figure in this module follows it. Click the top rail, the bottom rail, or the signal arrow.
+5V +5V Rc220 Ω D1LED Q12N2222 Rb1 kΩ GPIO 3V3 GND C1100 nF GND
Click any part or net label
Base current2.5 mA
LED current14 mA
Collector VCE0.04 V
The whole circuit

A transistor-driven LED node

One circuit, every part you have met: a resistor, a capacitor, an LED, and a transistor. Read it the way you read any schematic. Find the supply at the top and the ground at the bottom, follow the named nets instead of long wires, and name each part by its symbol and its designator. Click a part to read it. The three readings on the right come from a SPICE simulation of this exact circuit.