circuit design for electronic sensor devise

We store and manage diverse types of items in a warehouse-like facility. We are currently in the process of automatizing our warehouse management system. The new system will use a simple stand-alone electronic measuring devise. We now require the design of a basic electric circuit board to build the prototype of such devise. General description Compact, battery-powered device to be used in a confined space, to periodically detect when an item has been placed within this space; and then communicate the presence of the item to a nearby wifi router or comparable device capable of transmitting information to off-site server.   Key elements (i) Power (battery); (ii) time measurement (clock); (iii) sensor (ultrasound/laser/other?); (iv) signal emitter only (Bluetooth; wifi; or GPS/GPRS)   Parameters Confined space: box of varied sizes, none larger than 30x30x30cm Sensor: ultrasound or laser detector (or other?); one direction, linear measurement <30cm Detection accuracy: sensor to measure <1mm variation from previous reading (first reading is the empty box) Detection frequency: 2 times per day (every 12 hours) starting at time of initial configuration (reset button) Communications capabilities: devise communicates with router to indicate new item in the box Battery: replaceable, minimum life span of 1 year