1. Introduction

Printed circuit board (PCB) design is a great skill to have as an engineer in the electrical realm. There are many different programs used to design these boards, but the materials and design rules are the same.
We will be using Eagle PCB to design our circuits. Eagle is a free software by AutoDesk, the same makers of AutoCADD.
AutoCADD is a popular software used to design mechanical components and create professional dimension sheets. If you are familiar with the AutoDesk products, you may have an easy time learning the user interface of Eagle.
Eagle is not like LT Spice in the way that circuits are designed to be simulated. With Eagle, the circuits are designed as blueprints.

For this project, the circuit from version 1 (V1) of the Line Follower project will be turned into a PCB design. Gerber files will be created, and those may be sent to a maufacturer to be created!

2. Materials

• A Computer (capable of running Eagle PCB)
• A Free AutoDesk Account
• Eagle PCB Software
• A Robotic Line Following Car (without a circuit)
• These Eagle Libraries: SparkFun and LineFollowerV2
• The Following Components:

3. Instructions

If Eagle software is not already installed on the machine, it can be found to download for free here. If the machine is on the Fort Lewis campus, it can be found by doing a quick search for "Eagle" through the start bar.

Part 1 - Drawing the Circuit

To begin, it may be a good idea to draw out the circuit before drawing it in Eagle. Let's look at the schematics of some of the components from the circuit of V1.

The DC-DC Voltage Converter:


This has extra components on it that we will not need; Components like the USB jack, the jumper pins, the 3.3V regulator system, and the rail outputs.
We will not be using rails since we are not building the circuit on a breadboard. Let's see if we can simplify this by removing those components.

It should now look something like this:

The schematic for the IR Sensor module with TCRT5000 sensors can be found here. For our purposes, we will remove the analog out pin, and we will switch the connection of the non-inverting terminal and the inverting terminal.
This will allow us to remove the hex buffer/inverter! We also will only need one op-amp and one potentiometer to calibrate the sensors, so we will remove one of each from the circuit for the second TCRT sensor.
The IR sensor circuit should now look something like this:

The pin out maps of all the parts used in the circuit for V1 that will also be used on the PCB can be found here.

When put altogether, the circuit should look something like this:

Part 2 - Drawing the PCB Schematic

Now the schematic that will be used for the PCB design can be drawn.
Create a new project in Eagle PCB; I will be calling mine "LineFollower"

Create a new schematic within the project, and place a frame aligned with the crosshair. This will set the board's orientation. Add the proper components needed for the circuit to the schematic sheet.
These will be outlined in red. The crosshair on the symbol is where you can click to move the piece around. Once they are placed, you can begin adding the wires and junctions.
These will be green, and the junctions are represented with circles. The junctions are where the wires connect.

Mine looks like this:

Part 3 - Designing the PCB Layout

Now that the schematic is completed, the layout of the board can be arranged! This is the fun part! It's like a giant puzzle with so many different ways to solve it!
There are a few rules you'll need to follow to ensure the circuit works as intended. Rules such as: not letting wires cross unless they are on opposite planes to prevent a short,
and ensuring that the proper amount of space is between components to make sure no through holes or pins connect to anything undesired. These rules, and more, are also covered in the Eagle PCB tutorials.

Move the components around in a way that flows well. It is good practice to leave at least 1.5mm of space between components. They would be soldered to the PCB by hand,
and it would be hard to do so if the components are too close together. Once the parts placement is complete, you can begin laying the wires!
Utilize vias and both planes of the PCB to wire everything together, and once you're done, check for errors with the "ERC" error checking tool. Once that is clear with no issues, run the "Ratsnest" tool to check for errors as well;
When no errors are present, it should return that there is "nothing to do!"

Mine looks something like this:


When there are no errors, and everything is wired properly, it is time to layout the copper pour. These copper pours are a sheet-pour of copper on each side of the board to connect the grounding wires together.
To do so, use the polygon tool to outline the edge of the PCB. Ensure that the top layer of the PCB is selected. Once it is outlined, run the ratsnest tool again.
This should "pour" the copper onto the board. Do this again on the bottom layer.

Mine looks like this after the copper has been "poured." The view from the top layer on the left, and the bottom layer on the right:


Once the copper is done being "poured," the PCB should be ready for the Gerber files to be generated and to be sent off to be made!

Congratulations on designing your first PCB!!

Now you can generate the Gerber files and submit them to be made! A tutorial on generating Gerber files can be found here.

Part 4 - Soldering the components to the PCB

Soldering the components to the PCB can be done a few ways. You can use solder paste heated with a hot air blower, or you can solder it by hand with a soldering iron.
Either way, you will need flux to prepare the metals to be bonded. Flux is a slightly acidic paste that is used to remove the thin, oxidized layer of the metal so the solder can bond with the metal.
Once the flux has been added to the pads and through-holes, you can then place the part where it’s to be soldered and begin soldering.

I suggest applying the flux as you go instead of putting it on all the pads before soldering any components. It's also a good idea to go from shortest component to tallest when deciding what to solder next.

For surface mounted devices (SMDs) like the op-amp, diodes, resistors, etc., you can place the component on the board with the tweezers to steady in place.
Then get a small amount of solder on the tip of the iron. Heat the pad under the component while putting the solder to the pad. You will want the solder to wrap around the pins of the component.
If the component is on there, but not flush with the board, you can usually put the iron to the pad, without any solder on the iron, and heat the pads.
Usually gravity will do its thing, but sometimes you may have to use the tweezers to adjust it.
Also, do not worry if you get solder too much solder on there, you can always try to remove excess by wiping it off with your clean soldering iron.
You also won't get the pads shorted together if there is solder mask between them to keep them separate. Solder mask is a plastic, so the solder will not bond to it.

For through-hole components, like the TCRT pairs, driver holster, DC jack, etc., you can place the pins through the holes from the top of the board, steady it with your fingers,
and flip the board over, and slip your fingers out once you have the component steadied on a surface.

You don't want to be touching the component while soldering, especially if it's small; they get hot as you heat the pins, and you may burn yourself.

Once the component is secure, you can take a clean soldering iron, and heat the pins.
After a few seconds, the pins should be hot, and that is when you can take a piece of soldering wire and touch it to the iron and pin to coat the pin/fill the hole with solder.
If the through-hole is larger than the pin being soldered to it, you can fill the hole with solder!

Just be careful and watch your fingers on the solder wire; You don't want to accidentally touch the iron!


After all the components are soldered to the PCB, you can test the circuit with a multimeter.
Power up the circuit and use the voltage probe setting of the multimeter to check that the voltages are correct at each node of the PCB.
Make sure to calibrate the sensors by ensuring the potentiometer is outputing 2.5V to the comparator! Once you know it does, your PCB should now be ready to put on the car!
Check to see that your emitters on the TCRTs are working properly. Do this by powering up the PCB, and looking at the sensors with a camera. The camera will be able to pick up the infrared light that you cannot see with your eyes.

Since the sensors are on the PCB, we need to get the sensors closer to the ground. To do this, we will use more stand-offs.
Assemble two pairs of 2 standoffs together like you did in the V1 tutorial, and attach one to each of the stacked standoffs that are already on the chassis of the line follower after removing the TCRT5000 sensor modules from V1.
You may need to drill two holes in the chassis for the bottom holes on the PCB. Assemble two pairs of 4 standoffs like you just did, and finish securing the PCB to the car.
The line follower should be ready to go!