Abstract: An infra-red transmitter comprising transducer means (10) for sensing heart beat and producing a detectable electrical signal (10P); infra-red transmission means (28) that is energisable from a battery (30); signal processing means (14-20) that is responsive to transducer output signals representative of heart beats, and serves to produce therefrom short pulses (20P); and means (32-36) responsive to said short pulses (20P) for gating electrical energy originating with said battery to the infra-red transmission means (28) at voltage(s) and duration(s) that secure detectable penetration of a layer of clothing along with acceptably low battery power consumption.