Electronics · Beginner · 8 min read
Reading a schematic
Symbols, nets and how to follow the lines from power to ground.
A schematic is the circuit's map. It shows which parts connect to which, without worrying about where they sit on the board. Engineers read it first, because once you know the map, the board itself makes sense.
The good news: you only need a handful of symbols and three rules to read most hobby schematics.
Three rules
- Lines are wires. A line between two symbols means they're electrically connected. A dot where lines cross means they join; lines that cross without a dot don't touch.
- Labels are wires too. Two points with the same name, like
VBATorIR_TX, are connected even if no line is drawn between them. This keeps big schematics readable. - Current flows from power to ground. Power rails (like
VBATor3V3) are drawn near the top, ground near the bottom. Follow a path from top to bottom and you're following the current.
Try it on a real circuit
Here's the circuit that fires the infrared LEDs in our IR Remote Cloner. Tap each symbol to see what it means, then press Trace the current.
MOSFET Q1 · AO3400A
How to read it
The transistor symbol has three pins: gate (the control input, on the left), drain (top) and source (bottom).
In this circuit
Works like a switch the chip can flip. A small voltage on the gate lets the large LED current flow from drain to source.
Read it top to bottom: battery power goes through R1, which limits the current, then through the two LEDs D1 and D2, then through the transistor Q1 to ground. The chip's pin IR_TX doesn't power the LEDs itself. It only opens and closes Q1, like a finger on a light switch.
The symbols you'll see most
| Symbol | Part | What to look for |
|---|---|---|
| Zigzag or small box | Resistor (R) | Value in ohms: 10Ω, 4.7k, 100k |
| Two parallel lines | Capacitor (C) | Value in farads: 100nF, 10µF |
| Triangle into a bar | Diode or LED (D) | Current flows the way the triangle points |
| Three-legged symbol | Transistor (Q) | Gate or base is the control input |
| Box with numbered pins | Chip (U) | Pin names along the edges |
| Shrinking lines | Ground (GND) | The 0 V return path |
The letter in front of each name tells you the kind of part: R for resistors, C for capacitors, D for diodes, Q for transistors, U for chips, J for connectors and SW for switches.
From schematic to board
Every name on the schematic appears on the board too. The Q1 in the schematic above is the same Q1 printed next to a tiny black part on the real board. In KiCad you can click a part in either view and it highlights in the other.
U1ESP32-C3-MINI-1 Wi-Fi/BLE module
4 MB flash · Module 13.2×16.6 mm · about $3.10
What it is
The microcontroller: a tiny computer that runs the firmware and reads and drives every other part.
On this board
- USB_D- → GPIO18
- USB_D+ → GPIO19
Tap any part. 24 parts on this 64 × 34 mm board.
Where Kiln Forged fits in
Kiln Forged draws the schematic for you, but it shows its work: each choice in the Why panel points at the part it's about, so you can find it on the schematic and see how it's wired. Open the IR Remote Cloner demo and look at the circuit board stage.