Build Custom Object Sensors for LEGO Mindstorms EV3: DIY Light‑Based “Eyes”

While the LEGO Mindstorms EV3 kit ships with an impressive array of sensors, you can take your robot’s perception to the next level by building a custom object sensor. This guide walks you through creating a lightweight, light‑based detector that mimics a touch sensor, yet uses illumination and shadow to spot nearby objects. With a few inexpensive components from suppliers such as Adafruit, SparkFun, or Jameco, you’ll equip your EV3 with a pair of expressive on‑screen eyes that react in real time.

Inside the LEGO EV3

The EV3 is a powerful, programmable platform powered by a Texas Instruments 456 MHz ARM processor. It features Bluetooth, Wi‑Fi, USB host mode, and a full LCD interface. The four input ports (1–4) use RJ‑12 connectors to attach sensors and motors; port 1 is the one we’ll repurpose for our DIY sensor. The board’s I/O architecture, flash memory, and on‑screen GUI make it an ideal canvas for custom hardware.

Preparing the RJ‑12 Cable

To connect your sensor, simply take a standard RJ‑12 cable, cut off one connector, and strip the outer jacket to expose the six internal wires. The sensor’s relay contacts will bridge two of these wires, allowing the EV3 to detect a change in voltage when an object interrupts the light beam. This minimal wiring keeps the build compact and safe for educational use.

Programming the EV3

In the EV3 software, you’ll assemble a sequence of function blocks that: 1) monitor the input port for a voltage drop, 2) trigger a motor or alarm, and 3) render animated “eyes” on the LCD to indicate detection. The software’s rich sound library and visual effects let you create a lively, interactive robot that feels aware of its surroundings.

Follow the step‑by‑step instructions, upload the code, and watch your EV3 come to life with its very own pair of glowing eyes.

For a deeper dive, see the original tutorial: Hacking the Lego EV3: Build Your Own Object Sensor “Eyes”.