Proximity sensor linked to cell phone?

[RussA]

I want to put together the following;

A small battery powered sensor/dongle (which I'm thinking would be some form of bluetooth device) that could be attached to a physical object - say a power-drill or rucksack etc.

This sensor/dongle would be communicating with my cell/mobile phone such that if the physical object is moved beyond a preset range (say 6 feet) from the cell/mobile phone its communicating with, then the phone would detect this "out of range" event, and, via an app on the phone (which I'd write) would cause the phone to show/sound an alert of some sort on the screen and/or audibly.

So, a couple of questions;
1) Is there anything that does this already?
2) Can anyone recommend a suitable sensor/dongle that could communicate with the phone - I'm thinking a basic bluetooth-connected sensor of some sort?
3) Is bluetooth the right way to do this?

Many thanks!
Russ.

 

[kellys_eye]

You can get movement alarms that are self-controlled and sound off if the items are moved. Given the limited range of a bluetooth solution it would seem more appropriate to go for the KISS principle and just listen for the alert!

 

[RussA]

I can't rely on the alarm being sound-only, as it may be in a high noise environment, and it needs to trigger the phone come what may...

 

[kellys_eye]

Take your pick:

https://www.alibaba.com/showroom/bluetooth-vibration-sensor.html

 

[BobK]

People ask for this sort of thing all the time. There answer is that there is no simple way to detect how far two objects are apart. If you want an accuracy of about 30 feet, a GPS could do it.

Bob

 

[RussA]

Just to be clear I'm NOT trying to measure an actual specific distance, I'm trying to detect when something has been moved out of a roughly pre-set distance - say beyond 2 metres etc. Imagine it as a theft alarm; for example I want to know if someone's walking off with my bag that I left on the back of my chair, and hadn't realised its being stolen - that would then trigger an audible and/or visual alert on the mobile/cell phone.

 

[BobK]

If I understand you, here is what you want. You have a cell phone. You have another object which holds some kind of sensor that can communicate with the cell phone. When that object is further than 2 meters from the cell phone, an alarm is activated on the cell phone.

If this is not correct, please clarify it for us.

If it is correct, tell us how you think an electronic device might achieve this without knowing the distance between the two objects.

There are ways to do this under some special circumstances, but solving it in general is very difficult. For instance if the space between the two objects is always clear, you can do it but if there are other objects in the way it becomes much more difficult.

Bob

 

[RussA]

If I understand you, here is what you want. You have a cell phone. You have another object which holds some kind of sensor that can communicate with the cell phone. When that object is further than 2 meters from the cell phone, an alarm is activated on the cell phone.

If this is not correct, please clarify it for us.

If it is correct, tell us how you think an electronic device might achieve this without knowing the distance between the two objects.

There are ways to do this under some special circumstances, but solving it in general is very difficult. For instance if the space between the two objects is always clear, you can do it but if there are other objects in the way it becomes much more difficult.

Bob

Yes, your understanding is correct. My thought was that the phone would detect the electrical strength of the bluetooth signal (not a measurement in feet/meters etc.,) and if it reduces by a quantifiable amount - say 20%, or xMicro-volts (or however signal strength is measured) then alert of that.

 

[BobK]

The stregth of the signal is affected by all kinds of things in the environment. In a controlled environment, such as in the middle of a huge flat empty parking lot, with the orientation of the two devices strictly controlled, it could work. In the real world, not so much.

Both the transmitter and receiver have antennae that are somewhat directional. And even if they were not, large metallic objects would distort the radiated field significantly.

Do you begin to see the problems?

My guess is that an attempt to use Bluetooth signal strength as a proximity detector might trigger the alarm at anywhere from 3 to 30 feet in different environments.

Bob