Recreating circuits from photos or illustrations on the Arduino can be difficult, and for that reason, standardized symbols are used to represent the variety of components and connections that you may want to use in your circuit. These circuit diagrams are like maps of the underground: They show you every connection clearly but have very little resemblance to the way things look or connect in the physical world.

## A simple circuit diagram

Take a look at a basic light switch circuit made up of four components: a battery, a pushbutton, a resistor, and an LED.

The first thing you may notice is that this example has no battery. Because your Arduino has a 5V pin and a GND pin, these take the place of the positive (+) and negative (-) of the battery and allow you make the same circuit.

The second thing you may notice is that the physical circuit uses a pushbutton and, therefore, is not technically a light switch. This is more convenient, given that the components in most Arduino kits and the pushbutton can easily be swapped out for a switch later if you desire.

The best way to compare a circuit diagram to the actual circuit is to follow the connections from positive to negative.

If you start at the positive (+) 5V pin on the Arduino, it leads on to the pushbutton. The physical pushbutton has four legs, whereas the symbol has only two. The legs of the physical pushbutton are actually mirrored so that two are connected on one side and two on the other.

For this reason, it’s very important to get the orientation of the pushbutton right. The legs of the physical switch are this way to make it more versatile, but as far as the circuit diagram is concerned, there is only one switch with one line in and one line out.

The other side of the pushbutton is connected to a resistor. The resistor symbol on the diagram is not as bulbous as the physical resistor, but apart from that the diagram and physical resistor match up well; there is one wire into the resistor and another out.

The value of the resistor is written alongside the component, as opposed to having color-coded stripes on the physical one. There is no polarity for resistors (no positive or negative), so there is nothing else to show.

An LED, by contrast, does have a polarity. If you connect it the wrong way around, it won’t illuminate.

On the circuit diagram, the symbol marks the polarity with an arrow pointing in the direction of the current flow from + (anode) to - (cathode) and uses a horizontal line as a barrier in the other direction. On the physical LED, a long leg marks the anode and the flat section on the side of the lens marks the cathode.

The - (cathode) of the LED is then connected to the negative (-) GND pin on the Arduino to the negative terminal of the battery to complete the circuit.

## How to use a circuit diagram with an Arduino

Although it’s useful to understand this simple circuit, you will most likely be using an Arduino in your circuit somewhere, so take a look again at the same circuit powered from an Arduino.

The large, caterpillar-shaped item on the diagram is the Arduino. This is the standard symbol for an integrated circuit and is similar to the physical representation — a rectangle with lots of legs poking out. All the legs or pins are labeled so that you can tell them apart.

Also, rather than have one circuit, the diagram shows two, each running back to the Arduino. This is good to illustrate how the Arduino fits in with conventional circuits. Instead of switching the power on and off, you’re sending a signal to the Arduino, which interprets it and outputs it to the LED.

This is a great practice to adopt when you have a complicated circuit: Rather than tackle it as a whole, break it up into its components. This circuit has one input circuit and one output.