Mid-range proximity alarm

Hi all,

I am currently attempting to build a proximity alarm that would ideally go off at somewhere between 5 and 20 meters.

The main requirement is that it must work through walls and barriers, which according to my research is very difficult to achieve. I would like for there to be one main unit and another 'beacon' unit.

I understand that methods such as ultrasonic and IR distsance sensors are relatively simple, i am currently at a loss to find a suitable method, having researched bluetooth, to no avail, it seems as though the pairing process would simply be too slow...

I have an arduino uno, but if any others would be more suitable, please let me know... Has anyone built something like this, or does anyone have any ideas on how it might work?? Any help is much appreciated!! :)

Please do not cross-post. Other thread removed.

Proximity of what?

The proximity of two units, a main unit that will be the one with the alarm, and another one, that will activate that alarm once it comes within anywhere from 5 to 20 meters, im really not fussy about accuracy... Thanks

If the process must work through walls and barriers, then radio is your only option.

Unfortunately radio is extremely unreliable, easily shielded by metallized barriers, and distance measurements are nearly impossible to implement.

This is a very difficult problem and certainly not one that an Arduino can solve.

Time-of-flight with radio is possibly the only workable solution. The new Pozyx and decaWave UWB localization systems might be something to look at.

Thanks for your help! That sounds like a possibility...

Important parameter: do both devices actively cooperate, or is it one that has to detect whether another passive one is nearby?

One unit should detect another passive unit...

You just increased the difficulty vastly.

What property, if any, would be detectable from the passive device, through walls and other obstacles?

The aim here is to create a system, with two devices, in which one unit will alert another of it’s proximity, and set off an alarm. With further review, i think that if one unit would be constantly producing a signal, or letting off intermittent bursts, while the other one is reading for this signal.
Is there a way i could use bluetooth, as it seems to have a suitable range, and should remain acceptably constant even if walls etc are between the two units. As i mentioned earlier, the pairing process seems to be too slow, but is there a way to simply detect the presence of a signal, and then set off the alarm, without having to pair the units, as they aren’t required to share any information.
Any suggestions are appreciated!!

Bluetooth, like all radio waves, will be affected by walls, trees, and other obstacles. Line of sight Bluetooth can reach 100 meters or more; with a reinforced concrete wall in between maybe no reception at all. There are BT sniffers out there - you don't need to connect to a device to detect whether it is within range, however there is NO WAY of detecting how far it is from you.