Precise simulation of rainfall is crucial for physical modeling of rainfall-related geohazards and geotechnical failures. The practicality of conventional rainfall simulators was often compromised by several limitations, such as restricted real-time adjustability, significant cost, and a lack of versatility across different testing needs. This study introduced the development and implementation of a cost-effective, modular rainfall control system designed to simulate complex rainfall patterns in geotechnical model testing. The system integrated Arduino Uno boards to establish a dual-mode control framework, enabling manual adjustment for controlling parameter calibration and automated regulation for programmable rainfall scenarios. Employing pulse-width modulation (PWM) for pump control and a Hall-effect sensor for flow monitoring, the system provided finer, real-time modulation of rainfall intensity. It reliably achieved rainfall intensities from 74 to 440 mm/h, with a required flow stabilization time of 8 to 15 s after each adjustment. Under the system’s control, the custom-designed nozzle array delivered highly uniform rainfall, maintaining a Christiansen’s uniformity coefficient above 90 %. The system was subsequently validated through a series of reduced-scale 1 g physical model tests that simulated rainfall-related seepage erosion in a braced excavation. Test results confirmed that the system not only controlled rainfall intensity variations effectively in accordance with predefined temporal patterns but also achieved spatially uniform rainfall distribution across the model surface. Due to its scalability and cost-effectiveness, the control system supports a wide range of geotechnical model setups requiring precise and repeatable simulation of time-dependent rainfall conditions.