Electronic Circuits and Components

An electronic circuit is, as the term implies, electricity moving in a path very much like a circle. Each circuit has a beginning, a middle, and an end (which is usually very close to where it began). Somewhere in the middle, the circuit often runs through various electronic components that modify the electrical current in some way.

Each device in this book is a circuit that combines Arduino with different electronic components. Some of these essentially manage the power and path of the electricity; others sense certain conditions in the environment; and still others display output about those conditions.

Let’s take a look at some of the components we will be using in our circuits:

Light emitting diodes (LEDs)

An LED is a lamp made of various rare-earth metals, which give off a large amount of light when a tiny current is run through them. The composition of the substances within the LED determine the particular wavelength of light emitted: green, blue, yellow, red, and even ultraviolet and infrared are among the possible colors.

Technically, the LEDs used in our gadgets are “miniature LEDs,” tiny lamps with two wire leads, one long (called the anode) and the other a bit shorter (called the cathode). These come in various useful forms, including single lamps from 2mm to 8mm in diameter, display bars, and alphanumeric readouts, and can serve as indicators, illuminators, or even data transmitters.

You’ll learn how to use these different types of LEDs while building the different environmental sensors in this book.


Resistors are the workhorses of the electronics world. What do resistors do? They simply resist letting electricity flow through them, and they do this by being made of materials that naturally conduct electricity poorly. In this way resistors serve as small dumb regulators to cut down the intensity of electric current.

Resistance is valuable because some electronic components are very delicate, burning out easily if they’re powered with too much current. Putting a resistor in the circuit ensures that only the proper amount of electricity reaches the component. It’s hard to imagine any circuit working without a resistor, and with LEDs resistors are almost mandatory.

While building the projects in this book, you’ll learn various creative ways to regulate current with resistors.


Soldering involves heating up conductive metal, called solder, and then using it to fuse other pieces of metal together. In small-scale electronics, we use an electrical tool called a soldering gun, which has a small tip, to heat up thin wires of solder and drip the solder onto the components we wish to join into the circuit.

Soldering creates a very stable circuit, and that stability can be a drawback. Fusing together components can make it difficult to reuse or reconfigure circuits. You also must be very careful to not short-circuit components while soldering. It is beyond the scope of this book to to go into the details of soldering, which can be a very useful skill in DIY electronics. If you’re interested in learning how, this online resource is a good place to start.

The alternative to soldering is to use a breadboard.

Solderless breadboards

Solderless breadboards are small plastic boards studded with pins that can hold wires. (More about these below.) These wires can then be connected to other electronic components, including Arduino.

Solderless breadboards make it much easier to design circuits, because they allow you to quickly try out various assemblies and components without having to solder the pieces together. While solderless breadboards typically are intended for use only in the design phase, many hobbyists keep a breadboard in the final version of a device because they’re so fast and easy to use.

If you don’t feel like soldering circuit boards, solderless breadboards are the way to go. Each gadget in this book uses a solderless breadboard.

Wire is the most basic electronic component, creating the path along which electrons move through a circuit. The projects in this book use 1mm “jumper wires,” which have solid metal tips perfectly sized to fit into Arduino and breadboard pins, and come sheathed in various colors of insulation.


Get as much jumper wire as you can afford, in several colors. When building circuits with Arduino, you can’t have too many jumper wires.

We order most of our electronics components from these online retailers:

Maker Shed, from MAKE and O’Reilly Media, sells books, kits, and tools, as well as many of the electronic components needed to build the projects in this book. Maker Shed also supplies convenient bundles for many of the projects in this book (you can find more information about these bundles in the individual project chapters).

Don’t count out your friendly local RadioShack, though. While writing this book, more than once we ran out to RadioShack for a last-minute component.

For years RadioShack cut back on its electronic components inventory, apparently seeing a better future for the business by featuring cell phones and other consumer electronics. But the company has recently begun to embrace the maker movement; at this writing, some stores around the country are even carrying Arduinos. We’re hopeful RadioShack is on the return path to being the hacker heaven it was years ago.

Get Environmental Monitoring with Arduino now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.