The Amazon description and box state this is compatible with ALL Timex Heart rate watches.  But this is not true.  My watch did not work.  If you have an older Timex watch (i.e 565, you need the digital one...Of course it is pricey if you can even find it!  Amazon sells it for $42
http://www.amazon.com/gp/product/B000LC79CY/ref=ord_cart_shr?ie=UTF8&m=ATVPDKIKX0DER