I'm trying to design an alert system for a handicapped child that lives nearby me where the system will send an alert via SMS to their mother when the child gets too far from the house, and another when they re-enter the area. I assume and RF signal would be the best for that. My question is: Where can I find one reliable enough that it could detect distance of up to... let's say 150-200 ft. that could be calibrated to plus or minus 10 ft. or so? If you know which pins go where that would be helpful as well seeing as I've never worked with anything external before. Only the command terminal of the OS.
If you have a better idea for a sensor please let me know. It's for a handicapped 3 year old who figured out how to open doors, but can't communicate or reason well.
We're also looking into other options but I figured this one would be interesting for me to try.
Thanks for any info in advance!