Leakage current detection is a promising method for measuring pollution levels on insulators of overhead transmission lines. This paper describes the development of a leakage current sensor based on a light emitting diode (LED) as well as the development of a processing module that detects and stores the most important components of the leakage current signal. The relative humidity is also detected and stored for further analysis. The sensor is constructed with a single LED coupled to an optical fiber. The processing module uses a PIC 16F877A microcontroller and can register leakage current peak rates during two months in three different levels.