RANCANG BANGUN ALAT PENGUKUR LEVEL AIR TANGKI REAKTOR KARTINI BERBASIS SENSOR HC-SR04 DAN MIKROKONTROLER ESP32NIM.: 21106020029Muhammad Akbar FitriantoauthorKartini Reactor is a research reactor that requires accurate monitoring of the tank water level to prevent the risk of overheating, meltdown, and radiation leakage. The existing monitoring system is only active while the reactor is operating and can only be accessed from the control room, making it insufficiently flexible for realtime monitoring. This research aims to design, build, and test a water level measurement device for the Kartini Reactor tank based on the HC-SR04 sensor and the ESP32 microcontroller. The device design was carried out by creating the hardware and software design using EasyEDA, SketchUp, and Visual Paradigm software. The device construction was carried out by assembling the hardware and developing the software using Arduino IDE (version 2.3.4) software. The device testing was carried out by collecting water level data on the Kartini Reactor tank using the constructed device, then processing the data to obtain the accuracy and repeatability precision of the device. The design results include a circuit schematic design, a PCB design, a housing design, and a programming flowchart for the software embedded in the water level measurement device. The construction results consist of a hardware subsystem comprising the sub-subsystems of two HC-SR04 sensors, an ESP32 microcontroller, an MCP4725 module, an ILI9488 TFT display, and RS485 communication. This subsystem is integrated through a software subsystem in the form of program code consisting of several sub-subsystems, ranging from HC-SR04 sensor reading, data processing using the moving average method followed by conversion into voltage, data display via RS485 communication and the ILI9488 TFT LCD, to data transmission to the IoT Google Drive platform via a Wi-Fi connection. The test results show that the device achieves an accuracy of 98.29% and a repeatability precision of 99.87%. Based on these results, the water level measurement device for the Kartini Reactor tank based on the HC-SR04 sensor and the ESP32 microcontroller has been successfully designed, built, and tested.530 Fisika2026-07-23UIN SUNAN KALIJAGA YOGYAKARTA;FAKULTAS SAINS DAN TEKNOLOGIThesis