> For the complete documentation index, see [llms.txt](https://docs.protoboard.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.protoboard.xyz/tutorials/line-following-robot.md).
# Build a line-following robot
Build the drive train of a line-following robot from library parts, connect power and motor control, organize the canvas and get a clean DRC
## Goal
Build the **Main controller** board of the Line-Following Robot demo project by hand: an Arduino Nano, an L298N motor driver, two gearmotors and a 12 V battery. You'll connect power and control, group the motors, leave a note for your team and finish with a clean DRC. Every other tutorial uses this project.
{% hint style="info" %}
**Time:** 15 to 20 minutes. **You need:** a free account. If you haven't used the editor yet, do the [Quickstart](/start-here/quickstart-10-minutes.md) first.
{% endhint %}
## What you're building
| Part | Role |
| ------------------ | ------------------------------------------------------------- |
| Arduino Nano | Controller: reads the line sensor and drives the motors |
| L298N motor driver | Switches battery power to the motors under the Nano's control |
| 2 × N20 gearmotors | Left and right wheels |
| 12 V battery | Powers the motor driver |
The line sensor comes later: in [Design with ProtoBot](/tutorials/design-with-protobot.md), ProtoBot adds an IR sensor array to this board.
The finished Main controller board.
## Steps
{% stepper %}
{% step %}
### Create the project and the board
1. On Home, click **+** next to **PROJECTS**, name the project `Line-Following Robot` and click **Create**.
2. Open the project from the sidebar and click **New Board**.
3. Name the board `Main controller` and click **Create board**.
{% endstep %}
{% step %}
### Add the parts
Open **Parts Bin** in the left rail and add each part with its **Add** button:
1. Search `arduino nano` and add the Arduino Nano.
2. Search `L298N` and add the L298N motor driver.
3. Search `N20 gearmotor` and add it. Right-click the motor and choose **Duplicate** to make the second one.
4. Search `12V battery` and add a battery.
Parts land in free space near the middle of the canvas. Drag them into a rough left-to-right layout: battery, driver, motors on one side, Nano on the other.
{% hint style="info" %}
Can't find a part? Click **Can't find it? Request a part** under the search results. See [Request or fix a part](/parts/request-or-fix-a-part.md).
{% endhint %}
{% endstep %}
{% step %}
### Connect the battery to the driver
1. Drag from the battery's edge handle to the L298N.
2. In the Inspector, open **Interface pair** and choose the power pair.
3. Rename the harness `12V motor supply` in the harness name field.
{% endstep %}
{% step %}
### Connect the driver to each motor
1. Drag from the L298N to the left motor and choose the motor output pair.
2. Rename it `Left motor drive`.
3. Repeat for the right motor and name it `Right motor drive`.
The L298N has two motor channels. If you connect a third motor, the DRC reports that the interface is over capacity.
{% endstep %}
{% step %}
### Connect the Nano to the driver
1. Drag from the Arduino Nano to the L298N and choose the control pair for one channel. Name it `Left motor control`.
2. Create a second harness for the other channel and name it `Right motor control`.
3. If a harness stays amber, open **Function Links** in the Inspector and click **Auto Assign**, or pick pins in each row. See [Resource allocation](/editor/resource-allocation.md).
{% endstep %}
{% step %}
### Group the motors
{% embed url="" %}
Group selected parts
{% endembed %}
[Animated GIF](https://cdn.protoboard.ai/docs/clips/group-selection-light.gif) · Steps:
1. Drag a box around both motors (with nothing selected, dragging the canvas box-selects).
2. Click **Add group** in the canvas toolbar. The group wraps the selection.
3. In the Inspector, name the group `Drive motors` and pick a colour.
{% endstep %}
{% step %}
### Leave a note for the team
{% embed url="" %}
Add a note and connect it
{% endembed %}
[Animated GIF](https://cdn.protoboard.ai/docs/clips/note-add-light.gif) · Steps:
1. Click **Add note** in the canvas toolbar. The note lands in free space.
2. Type `Check the L298N heat sink at stall current`.
3. With the note selected, use **Connect annotation** in the Inspector and choose the L298N, then click **Create**.
{% endstep %}
{% step %}
### Check the DRC
1. Open **Board Review**. Fix anything under **BLOCKED** first, then **PARTIAL**.
2. Expand an issue and click the target icon to jump to it.
3. When **// DRC Issues** reads "No DRC issues found", every harness is configured.
See [Fixing DRC failures](/validate-and-build/fixing-drc-failures.md) if an issue doesn't clear.
{% endstep %}
{% step %}
### Describe the board
1. Open **Board Identity** in the left rail.
2. Add a description, the tags `robotics` and `line-follower`, and a requirement such as `Runs 30 minutes on one charge`.
ProtoBot and your teammates read this when they work on the board. See [Board Identity, requirements and kits](/editor/board-identity.md).
{% endstep %}
{% endstepper %}
## Result
You have a validated drive train with named harnesses, a group and a note. Next, [let ProtoBot add the line sensor](/tutorials/design-with-protobot.md), or [price the BOM](/tutorials/price-and-order-your-bom.md).
## Common failures
The motor harness is red: "No compatible interface pairs found"
You connected two parts that share no interface, for example the battery straight to a motor. Delete the harness and connect through the motor driver.
A control harness stays amber
Some functions have no pin assigned. The issue in Board Review names them ("Side A function ... not yet assigned to a resource"). Use **Auto Assign** or pick pins in **Function Links**.
Add group created an empty group
Nothing was selected. Select the parts first, then click **Add group**, or drag parts into the empty group.
## Related pages
* [Design with ProtoBot](/tutorials/design-with-protobot.md): add the IR sensor array with ProtoBot
* [Harnesses and function links](/editor/harnesses-and-function-links.md): what each harness holds
* [Groups and notes](/editor/groups-and-notes.md): organize larger boards
* [Share a project with your team](/tutorials/share-a-project.md): bring a teammate into the project
