A heliostat designed in your browser

A heliostat is a mirror that tracks the sun and reflects light onto a chosen point. Sunspot is made up of 37 hexagonal facets with 2 mm acrylic mirrors. Each facet has a 30 mm side, the mirrors cover an area of 865 cm², and the complete mirror assembly weighs 5.64 kg.

Each facet is tilted by two servos, 74 in total. The mirror array therefore acts as a single mirror with adjustable curvature: the light spot can be focused on a target 4 m or 30 m away. The entire assembly rotates on two axes, azimuth and elevation, using 28BYJ-48 stepper motors, beneath a PMMA dome approximately 724 mm in diameter.

We provide the sunspot.hexart.io app for free, with no account required. It’s a sample of what we do today: an engineering project from requirements to a production-ready package.

The application does not call any AI model. The code calculates every value using formulas from literature and standards, which can be verified in the calculation documentation. The same set of parameters produces the same part dimensions.

The sun’s position is calculated using the NOAA and Meeus algorithm with refraction. Direct normal irradiance (DNI) is calculated using the Meinel model with the Laue correction, air mass using the Kasten and Young formula from 1989, and the shape of the solar disk using the Buie model. Wind loads are calculated according to EN 1991-1-4, gear teeth according to ISO 6336, and bearings according to ISO 76 and ISO 281.

Where AI has worked

AI helped write the code. Paul Lazniak defined the requirements, while a team of AI agents wrote the code in waves: authors worked in separate file scopes in parallel, followed by integration and review by separate agents. Feature authors used Claude Opus 5.5, while Claude Sonnet 5.5 verified individual submissions.

Seven research agents prepared 30 component cards with catalog data. One coordinating session managed 25 agents. The rule was: no made-up figures, every value must have a source.

Set the device and target on the map or by entering an address. Terrain data comes from elevation datasets, buildings from OpenStreetMap, and the city model from Google Photorealistic 3D Tiles via Cesium ion photogrammetry. The target can be any point in space: a window, a panel, or part of a wall.

Select the date and time from a calendar, or run the day in fast-forward. The same scene works in VR headsets and in AR mode with a view of the surroundings (WebXR).

The “Download print package” button provides a ZIP archive with 38 files, about 111 MB. It contains 28 STL files for 211 parts to print and cut, plus 4 SVG drawings of plates and mirrors to cut from a sheet. It also includes an Arduino sketch, a materials list in CSV and Markdown, and the project quote and parameters in JSON.

For the default configuration, the bill of materials has 107 items. Buy 80, 3D-print 20, cut 4 from sheet material, and cast or print 3 parts. The fasteners alone total 340: 70 screws, 30 nuts and 240 washers. In addition, there are 74 servos, 5 PCA9685 controllers, 2 28BYJ-48 motors and a DS3231 clock.

The entire device weighs 74.4 kg. Materials cost 8,608.73 PLN net according to the October 2026 price list, and FDM printing takes approximately 24.7 hours.

Manual and research card

Using the current parameters, the application generates a construction manual and a research report with calculation results in Polish and English, ready to print as PDFs. In the version dated October 26, 2026, the manual was 61 A4 pages and the research report was 32 pages.

The generator selects a board based on the number of mirrors: Arduino Uno for up to 61 facets, Mega 2560 for up to 432, and ESP32 for up to 864. For 37 facets, it selects Mega 2560. The sketch then has 2,128 lines and is estimated to use about 44.9 kB of flash memory.

The servos are controlled by PCA9685 modules, with 8 facets per module. A DS3231 clock provides UTC time, and a Hall sensor provides the zero position, with calibration stored in EEPROM. The controller has a SAFE button and a power limiter for the focused beam.

Virtual Arduino in the browser

The sketch core also runs in the app. C++ code is translated into JavaScript and run on emulated hardware: microsecond timing, step interrupts, Hall sensor hysteresis, servos and EEPROM. You can see what the controller does before anything is uploaded to the board.

The figures on this page come from a simulation, not from measurements of a built device. The application states this in its own footer: “Engineering simulation. Estimated results. Verify by measurement before building.”

The project documentation includes a list of 14 known gaps. For example, the controller box does not have an interior yet, and the NEMA motor will not fit in the mounts for the 28BYJ-48 motors. We tested VR and AR mode in the emulator. Testing with the Quest 3 headset is still ahead of us.

Concentrated light burns and ignites, and its reflection damages eyesight. Before installing the device outdoors, check where the beam falls throughout the day.