All About Circuits Textbook

What it is

The All About Circuits textbook is a free online electronics textbook in six volumes: Direct Current (DC), Alternating Current (AC), Semiconductors, Digital, Reference and Experiments. It began as "Lessons in Electric Circuits" by Tony R. Kuphaldt, released under an open licence, and All About Circuits has reformatted and updated it for the web. The DC volume starts with the basic ideas of electricity and Ohm's law, the Semiconductors volume covers diodes, transistors and amplifiers, and the Digital volume covers number systems, logic gates and digital circuits.

Why embedded learners should read it

You do not need an electrical engineering degree to build devices, but you do need a handful of circuit ideas: voltage, current and resistance, series and parallel circuits, voltage dividers, capacitors, diodes and transistors used as switches. Most beginner wiring mistakes, such as a missing current-limiting resistor, a floating input or a motor driven from a pin, come from gaps in exactly these topics. This textbook explains them patiently, with worked examples, and it is free.

What to read first

• From the DC volume: basic concepts of electricity, Ohm's law, electrical safety, series and parallel circuits, and divider circuits.
• From the Semiconductors volume: diodes and rectifiers, and the chapters on bipolar and field-effect transistors, focusing on using them as switches.
• From the Digital volume: numeration systems and binary arithmetic, then logic gates.
• The Experiments volume has small circuits to build that make the theory concrete.

Prerequisites and time

School-level algebra is enough to start. Reading the recommended DC chapters takes a few evenings; the transistor chapters take longer. Treat it as a reference you return to when a project needs a new idea, rather than a book to finish cover to cover.

What you can do after it

• Calculate LED resistors, divider outputs and power dissipation with confidence.
• Understand why inputs need pull-ups and why inductive loads need a flyback diode.
• Choose a transistor or MOSFET to switch a load from a microcontroller pin.
• Read simple schematics and data from component datasheets.

Free or paid?

The textbook is free to read online. The site is supported by advertising and also hosts articles, forums and other resources.

Common questions

Is it too academic for hobby projects? Some chapters go deep, but the early DC chapters are very practical. Skip what you do not need yet.

Is there a PDF? The original Lessons in Electric Circuits is available in other formats from its open source distribution sites, but the All About Circuits version is read online.

Best for: software-focused students who need the electronics basics behind embedded and IoT projects.