This simple device, designed by Wichit Sirichote, measures the quality of AC voltage. The device measures 3rd harmonic which is used to find the distortion value comparing to fundamental frequency.
Device calculates DFT of signal which is captured via step down isolation transformer and 10-bit ADC. Testing showed that clean sine wave gives less than 0.5% of distortion, while square wave gives about 33%. Project description includes detailed measurement theory outline and practical examples as well. Project is built on test board which was developed for general purposes. Source code is available for download.