The exposure of the user of a mobile phone is proportional to the power radiated by the phone. The power radiated by a mobile phone depends on the propagation conditions and can be modified at the request of the base station by means of the power control on a large dynamic range which is of 30dB for GSM and of 70dB for UMTS. This study allowed to design and to realize a simple, cheap and light device which displays the level of the power radiated by a mobile phone.