The present invention relates to a wearable device and a system. The wearable device (20, 40) comprises an electronic circuit (21), a power supply (22), a switching circuit (23, 43) coupled between the electronic circuit and the power supply, and an electromagnetic field detection circuit (24) coupled to the switching circuit (23, 43) for detecting an electromagnetic field generated by an NFC transmitter within the detection range of the electromagnetic field detection circuit and for generating a trigger signal if an electromagnetic field generated by an NFC transmitter is detected within the detection range of the electromagnetic field detection circuit. The switching circuit (23, 43) comprises a flip-flop (231) or a transistor circuit (431) including an input transistor (432) and a fuse (433) to switch the connection between the electronic circuit and the power supply on in response to the trigger signal.