https://www.youtube.com/watch?v=o1Ifb1Fo9SQ http://www.elektroda.pl/rtvforum/topic1655378.html http://www.swharden.com/wp/2013-04-14-simple-diy-ecg-pulse-oximeter-version-2/ https://hackaday.com/2010/01/06/pulse-oximeter/ https://hackaday.com/2010/05/02/diy-pulse-oximeter/ http://www.instructables.com/id/Simple-DIY-Pulse-Sensor/