Wireless sensor networks (WSN) are large scale networks of unattended devices, aimed at monitoring environmental parameters. Their extremely scarce hardware resources constitute a huge limitation to the use of standard security protocols to secure communications, so that custom ones must be designed. In this article we describe the development of a novel access control system for WSN based on a distributed threshold scheme. Our model gives support for mobility and limits the needed communication and consequent energy drain, which is a fundamental parameter for the lifetime of WSN.