Measuring the distance between a fixed object and a moving object

Hello;

I have a son of 10 who keeps forgetting his school bag in the school.

I am planning to construct an arduino-based solution that will have a device that will be carried in his school bag and another in his pocket.

The device in his pocket will sound a warning alarm when the distance between the 2 devices gets over a certain limit. Say 4 meters (12 ft).

I looked into several approaches;

-A pair of ultrasonic sensors used as a transmitter (in the school bag) and a receiver (in the pocket) could do the job but will not work if there are obstacles between the 2 devices. Furthermore they are not omnidirectional.

-An infrared solution will have the same problems plus distance limitations

-Two GPS captors will need a data link to communicate. They may also be problematic if the bag is in a locker.

Would you be kind enough to let me know of what you think?

I repeat the constraint.

Distance between the 2 devices= 4 to 10 meters.

The devices will need to communicate omnidirectional.

The devices could have obstacles between them.

They have to be childproof. But that is another issue!

I thank you in advance.

Would you be kind enough to let me know of what you think?

None of your proposed solutions will work (indoors) for measuring distance.

If there were a good solution to the problem, don't you think that the solution would be readily available through bazillions of retail channels?

PaulS:
If there were a good solution to the problem, don’t you think that the solution would be readily available through bazillions of retail channels?

+1

Exactly.

Perhaps come kind of gamma-ray source...

Most such devices use Bluetooth and sound an alarm when contact is lost. This will not work through metal.

Perhaps a better solution is to teach your son to check that he has his bag when he leaves the school.

Thank you for your time to answer me. I will look into the Blutouth approch and , of course, will let you know if successful. Teaching my son not to forget things is, believe me, easyer said than done.

If he shows up without his bag, immediately send him back to get it. The lesson should sink in, after some number of repetitions.

Perhaps a GPS device that hangs around his neck. When he gets away from the school it sounds an alarm to remind him to check that he has his bag with him. This could cause false alarms if the allowed radius is too small to contain glitches.

Similar but using contact with school WiFi (assuming they have WiFi). This is more prone to false triggering.

Many schools have the bags in a separate area, more than your set range.

Wouldn't like to have to "please explain" the false smoke alarm warning. :o

5 works for me............

johnwasser: Perhaps a GPS device that hangs around his neck. When he gets away from the school it sounds an alarm to remind him to check that he has his bag with him. This could cause false alarms if the allowed radius is too small to contain glitches.

Similar but using contact with school WiFi (assuming they have WiFi). This is more prone to false triggering.

Thank you again. Your suggestions are constructive.