I want to build a device that i can more or less mount in a hat or something like a belt that will alert me to something approaching. Something that works off heat perhaps. I work outside sometimes on projects and it gets dark on me and I would like to know if someone is sneaking up on me. I think the problem would be excluding myself from being detected. I was thinking i could use a cell phone vibrator to alert me if someone is approaching. Any ideas on circuits that might assist in the making of this project?