.  Hill Country Research Innovations in Telecommunication and Automation     Cell Phone Sensor

This sensor detects cell phones at a range of about 100 m (300 feet).  It indicates field strength/ proximity on the LED display.

The unit runs on a  9 V battery.
 
 

This device can also be used

•  to detect wireless cameras, bugs, etc., wide coverage from 50 to 3000 MHz.
•  to test microwave ovens