A wrist-worn apparatus for measuring a user’s heart activity includes an optical heart activity sensor configured to measure the user’s heart activity by optical heart rate measurement; and a wrist strap adapted to physically couple the optical heart rate activity sensor against the user’s skin by attaching the wrist-worn apparatus around the user’s wrist. The contact region is arranged outside the switch housing. A first bonding pad, a second bonding pad, a third bonding pad and a fourth bonding pad are disposed clockwise in the peripheral area of each chip pad in sequence. WTs wake-up, receive the first paging signal and quickly ascertain whether its paging group should expect a second paging signal, if so, the WT is operated to receive the second paging signal; otherwise, the WT goes back to sleep conserving power. Analogously, the invention relates to an audio system, or to an integrated consumer video andor audio circuit for this type of audio system, to control a stand-alone external audio reproduction device by means of control signals from the audio system or consumer video andor audio circuit.