Two weeks ago I [shared the idea of building a small dual-axis heliostat (inspired by Rjukan, Norway)](https://www.reddit.com/r/esp32/comments/1ovwosn/eternal_sunshine_my_next_esp32_project/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button) to bring sunlight into my daughter’s living room. I’m pleased to show this working prototype driven entirely by an ESP32S3.

The prototype uses a scaled-down pan/tilt assembly with a 20 cm mirror (the final system will use one or two PV-size mirrors). Two linear actuators allow the mirror to be adjusted up/down and rotated east/west. The stand is made of 20×20 mm aluminum profile with custom 3D-printed joints. The complete 3D models can be[ viewed, copied, and edited from this OnShape link](https://cad.onshape.com/documents/4b46a47fa7170bf2a2975312/w/01a9a9f069b44d52b21dd2cf/e/16baa0cb1f3f76c9e244c0a5?renderMode=0&uiState=692de723667477efa30c4c82).

How it works: The ESP32 calls an online astronomy API (ipgeolocation.io) via WiFi to retrieve the current sun azimuth and altitude for the exact GPS coordinates. Using the API is the most accurate method and a natural fit for the ESP32’s native WiFi connectivity. Using those values, and knowing the angular position of the target, the ESP32 computes the required mirror pitch and roll, which is then used to drive the actuators.

Since the sun moves very slowly, only tiny corrections are needed every minute (\~0.3–0.5°). This level of fine control is best achieved by moving the actuators in very small steps. The motors are pulsed at reduced speed using the controller’s PWM outputs. Best results were achieved with 50 ms steps at 10% PWM (\~2.5 V) every second (\~0.1° steps).

The mirror’s actual orientation is captured by an IMU (SINDT485) from WinMotion. The X and Y inclinations are read via RS485 Modbus RTU and are precise down to 0.001°, which is quite remarkable for a 60 € component.

The sun position is fetched and a new mirror target position is computed every minute. The actuators are then stepped until the IMU reports that the mirror has reached the desired position.

The electrical schematic is greatly simplified, as the ESP32 controller used for this prototype incorporates the RS485 driver and has 16 outputs, each capable of driving up to 1 A with programmable PWM. Each actuator draws up to \~2 A on startup, so four outputs have been paralleled to give ample headroom to drive the motors without any external drivers. Two DPDT relays are used to swap the motor wires and select the actuator direction.

All components (24 V power supply, ESP32 controller, and relays) are DIN-rail mount and fit neatly into a commercial waterproof cabinet.

The application software was 95% written using AI, with progressive prompting: moving motors first, reading the IMU via RS485, reading the sun-location API, computing mirror orientation, and integrating all parts after each was verified independently. [Source code can be retrieved here](https://www.dropbox.com/scl/fi/6vw5nsiy2edrk7fi9b7j0/sunshine_tracker.zip?rlkey=c4j1x0ro69pq8mxa9pyizhq4z&dl=1).

The results can be [viewed in this timelapse video](https://www.dropbox.com/scl/fi/jq52m93bure273ogi2j2e/EQ-sunshine_2.mp4?rlkey=cre2ytgp8n2hbjn5zgxceeoi3&st=s6lcurfx&dl=1) that compresses 1 hour into 30 seconds and compares the sun’s reflection from a fixed mirror vs. the tracked mirror. The fixed mirror drifted by around 2 meters while the tracked mirror remained on target. The test was done on a windy day and the target reflection is a little wobbly but remains centered.

Next, the system will be scaled up and installed in the spring of 2026.

Eternal Sunshine: ESP32-Controlled Heliostat Prototype Working