Finally found what I want – hRange | a 5k automatic heart rate trainer that works with Polar, Garmin, Scosche and MIO bluetooth monitors Review

You can set a max and min heart rate and it'll talk to you when you get out of range. You'd think this would be easy to find but it's not. This is perfect. Hallelujah.
Review by jubishop on hRange | a 5k automatic heart rate trainer that works with Polar, Garmin, Scosche and MIO bluetooth monitors.