Body sensor

A body sensor for measuring body activity of a user, wherein the body sensor comprises:

• a sensor part comprising a housing having a first surface and a second surface opposite to the first surface, said housing enclosing components of the sensor part inside the housing, the components comprising:
• a circuit board provided between the first surface and the second surface within the housing,
• an optical sensor configured to measure body activity by optical measurement, the optical sensor being provided between the circuit board and the first surface within the housing,
• a first magnetic coupling part,
• a transparent window provided on the first surface of the housing and directly facing the optical sensor such that the transparent window is configured to allow light pass from the optical sensor through the first surface to outside of the housing and from outside of the housing through the first surface to the optical sensor, and
• one or more other components provided inside the housing;
• and
• a separate connecting part being independent from the sensor part and comprising a second magnetic coupling part configured to provide a magnetic coupling with the first magnetic coupling part via the first surface of the housing for securing the sensor part to other side of a user's body part.