Mobile Eye-Tracking Display Setup With EEG Amplifier Photodiode Communication

This project was developed as part of the broader SYNCC-IN initiative, which investigates biobehavioral synchronization between child–parent pairs. A key aspect of this work was recording eye-tracking data during stimulus presentation while keeping it temporally aligned with EEG recordings.

For this purpose, we used Pupil Capture software, which relies on AprilTag markers to achieve high eye-tracking accuracy and precision. Because our research often took place in changing laboratory environments, we needed a setup that was both robust and easy to recreate. The solution presented here offers one of the simplest and most affordable DIY approaches to setting up eye-tracking with AprilTags.

Note: If you build a similar setup, chances are you’ll be working with a different type of display. This guide is meant to give you the general idea - you’ll likely need to adjust some details to match your specific equipment.

For the software side of the procedure, including experiment control scripts, see the companion repository: syncc_in_et on GitHub.

To build this setup, you’ll need a few common tools and materials, along with some simple custom parts for mounting and display integration:

1. Standard printer - for printing AprilTags
2. 3D printer - for fabricating mounts for the AprilTags and photodiode
3. Double-sided tape - for quick and secure attachment of the mountings
4. Target display - in our case a GIGABYTE monitor (your display may differ)
5. 8× neodymium magnets - for stable, detachable fixation of the AprilTags.
6. Acrylic glue - for permanent bonding between the magnets and printed moutings

AprilTags are recommended by many eye-tracking devices for reliable detection and high precision. The idea here was to place four markers at the corners of the display, as suggested in the device guidelines.

Important: Make sure that you are not using CRT display.

The mounts needed to be sturdy enough to survive transportation, yet quick to attach during setup. The solution was to permanently fix part of each mounting to the display corners and then attach the removable tag plates only during recording. Each plate connects to its permanent mounting via two small neodymium magnets, ensuring stability while remaining easy to detach.

Substep 1: Print the permanent mountings. For my display, each mounting was shaped to fit a specific corner (e.g., BL = bottom left, TR = top right) so they wouldn’t obstruct the screen. These are simple, quick-to-print models; use an infill of at least 30%.

Substep 2: Print four tag plates. These are symmetrical, so you only need one model and can reuse it for all corners.

Substep 3: Glue the neodymium magnets to both the tag plates and the permanent mountings. Important: make sure the poles are aligned so that they attract rather than repel.

Substep 4: Attach the AprilTags to the tag plates. Important: AprilTags have a defined orientation, which may matter for your eye-tracking device. To avoid mistakes, first connect the plates to the permanent mountings before gluing the tags in place.

Substep 5: Line the inside of each permanent mounting with felt and apply double-sided tape. Fix them to the display casing (not the screen) using the largest possible contact surface for stability.

Done. For convenience, I also added a small piece of tape on each tag plate to mark its correct orientation.

As part of the procedure, EEG recordings needed to receive markers and triggers corresponding to the visual stimuli shown on the display. To achieve this, we reserved a small portion of the screen to communicate with the EEG amplifier. A black-and-white box blinked in specific frequencies or counts, encoding a binary signal. This was detected as high/low input by a photodiode directly connected to the amplifier.

Since the corners of the display were already used for eye-tracking calibration (via AprilTags), we placed the photodiode at the middle of the left screen edge. A dedicated mounting ensured the sensor stayed securely attached without blocking the stimuli.

Substep 1: Print the photodiode holder. Use >30% infill. This is a simple, fast-to-print model.

Substep 2: Line the inside of the mounting with double-sided tape (similar to the AprilTag mounts) and fix it to the display casing.

Note: Avoid sticking it directly onto the screen surface, as the tape could damage the display layer.

Substep 3: Our photodiode was cylindrical, with a slightly smaller diameter than the mounting hole. To ensure a snug fit, we wrapped the photodiode with tape until it could be firmly pressed into the holder.

This setup provides a simple, low-cost way to combine eye-tracking with AprilTags and EEG synchronization via a photodiode. While the design is robust and easy to replicate, a few limitations should be kept in mind:

1. Display type matters: The exact mounting shapes and tape placement may need to be adjusted depending on the casing and size of your display.
2. Durability vs. portability: Permanent taped parts can wear out over time if frequently removed—consider keeping a spare set of mounts ready.
3. Device-specific details: AprilTag orientation and photodiode sensitivity may differ across eye-tracking and EEG systems, so always check compatibility before recording.
4. Screen safety: Never attach mounts directly to the active screen surface, as adhesives can damage the display.

Overall, this approach is meant as a general solution. Treat it as a flexible framework you can adapt to your own equipment and experimental requirements.