I bought a heart rate sensor last week from Amazon.  It has been several years since I have used a heart rate monitor. The battery died in the strap and I would need to send it in to get it replaced. I never sent it back in. The Polar H7 uses blue tooth to connect to a iPhone 4s.  I used it yesterday when I went for a bicycle ride. There are several iPhone apps that you can use with the H7. I used the Endomodo Pro app. The app uses the gps to track distance, speed, altitude and the heart rate monitor to get the full picture of your work out. With the heart rate monitor you can really tell if you are taking it easy or working your butt off.  Here is my ride on Endomodo's website. ​​