While digital music technologies are rapidly growing, music communities using traditional acoustic instruments are sometimes unable to take full advantage of all of the digital processing techniques available to electronic musicians. One way to include them in the latest developments is to develop interfaces connecting non-electronic instruments to the digital world. This paper presents HarMIDI, a sensor system to convert keystrokes on an Indian harmonium to MIDI. The paper presents the sensor assembly, calibration methods, and the algorithm to output MIDI. The calibration methods calibrate the notes and temporally synchronize the MIDI stream with audio. The system has been evaluated for time synchronization of onsets and offsets. The sensor setup is affordable, portable and can be used with any existing harmonium.