A supplemental device for attachment to an injection device. A supplemental device for attachment to an injection device, the supplemental device comprising: a housing an electromechanical switch arrangement, the electromechanical switch arrangement comprising: a main body and a switching member, the switching member having a protrusion arranged to protrude from the main body so as to contact a surface of the injection device while the supplemental device is attached to the injection device and a biasing member configured to bias at least a part of the main body of the electromechanical switch arrangement against the injection device while the supplemental device is attached to the injection device.
