A wearable heart activity sensor device includes a substrate of optically transparent material arranged to face a skin of a user when the sensor device is worn by the user； at least one light emitting diode, LED, arranged on the substrate and arranged to emit light through the substrate； at least one photo sensor arranged on the substrate as spatially separated from the at least one LED and arranged to absorb light through the substrate, wherein the at least one LED and the at least one photo sensor are included in a photoplethysmogram sensor of the heart activity sensor device； and an overmold of thermoplastic material covering the at least one light emitting diode, the at least one photo sensor and a space between the at least one light emitting diode and the at least one photo sensor.
