Schematic and Breadboard Diagrams
You may remember, if you studied electronics as part of your science course at school, that there is a specific way of drawing circuit diagrams. Each component has a particular symbol (which may vary slightly depending where in the world you live) that allows anyone to look at the drawing and build the circuit. It’s the architect’s plan for the house that allows the builder to know exactly what goes where.
Just the same as a plan, these can get pretty detailed and end up being confusing for someone like you and me – given that we don’t have our Master’s degree in Electrical Engineering! We don’t really need schematics for our early projects as they’re fairly easy to build, but I’d like to use these to start introducing you to them, as they’re going to become very useful later in the book. So without further ado, let me introduce you to the schematic for the Blink project:
It’s pretty straight-forward, don’t you think? On the left are the Arduino’s pins – can you spot the 6 Analog pins (A0 – A5) at the top, the 14 Digital pins (0 – 13), and finally the power – GND, 3.3V and 5V connections? As this is not a physical design, you’ll notice that the Arduino pins aren’t set out in the same way that they are physically arranged on the Arduino itself. Let’s get straight into understanding what the schematic is telling us. Start by finding pin 13.
The current flows from pin 13 (which in this project is being used as an OUTPUT pin) to LED1. The straight line represents a connection between the pin and LED1: this could be a wire, a track on a breadboard, or a printed track on a circuit board. Look at the parts list at the beginning of the chapter, and you’ll see that LED1 is the label for an LED (pretty obvious in this case, but not always!) If you don’t already, you’ll start recognising the symbols for these components as we work through our projects – the symbols are also in the component reference at the end of the book.
From LED1, the current flows to R1. This label is a little more cryptic, so refer to the parts list, and you’ll see it’s a Resistor, with a resistance of 330 Ohm. It’s a good idea to show the value of resistors, capacitors, etc. on schematics, as it helps others to build the circuit correctly.
From the resistor, the current flows to the GND pin on the Arduino – GND being the same as the negative terminal on a battery – and completes the circuit.
Now that you understand how the parts connect together, let’s look at one way to lay them out physically on a breadboard.
The above is the first look that you’ve had at how the “theoretical” flow relates to the physical world. I find it much easier to understand things when I see them physically – often schematics can be a little too vague and hypothetical to me. I want to see what the circuit looks like and how I’m going to physically build it.
Looking at the breadboard layout above, you should be able to link it to the schematic quite easily. Let’s trace the circuit again. Pin 13 on the Arduino is at the top of the diagram. Trace the green wire from there into the breadboard – it’s inserted into a hole in column 12. You’ll see that one leg of the LED is also in column 12. Remember that all the breadboard holes in a column are connected – so the current will flow from the green wire, through the breadboard, into the LED.
The current flows through the LED, and out the other leg which is in column 13 on the breadboard. One leg of a resistor is also connected to column 13, so the current flows through the breadboard to the resistor (where its flow is restricted) and out of the other leg of the resistor – onto the lower half of the breadboard. Remember that the top and bottom halves of the breadboard are not connected. Finally, the current flows into the black wire which connects it to the GND pin on the Arduino.
There are a couple of things that I want to mention:
The LED: Remember that an LED only allows current to flow in one direction. This means that you have to connect the LED correctly – the Anode must be connected to positive current, and the Cathode to the ground current. On the Arduino, the digital pins always output a positive current, so the anode (longer leg) must be connected to pin 13. The cathode (shorter leg) therefore is connected to GND. Don’t worry if you’ve connected it backwards, you won’t do any damage to your circuit. All that will happen is the LED will prevent current from flowing and won’t light up.
The Resistor: The resistor is placed with one leg on either side of the divider on the breadboard. It’s very important that the two legs of the resistor aren’t inserted into holes that are connected inside the breadboard. Current is lazy, and chooses the path of least resistance, so it would in that case flow through the breadboard conductors and bypass the resistor altogether. We therefore need to force the current to flow through the resistor – the only way the current can cross the divide in the breadboard, is through the resistor. To put it another way, the resistor should not be connected in parallel to a conductor of lower resistance as the current will effectively bypass the resistor. The resistor should be in series to the component into which you’re trying to limit the current flow. (We’ll touch on Parallel and Series circuits shortly.) If you don’t connect the resistor correctly, there is a risk of blowing your LED – they can only handle so much current, so if the resistor is bypassed the LED could receive more current that it was designed to and blow.