Hi. I build several arduino things but I really don't know how to tackle this project.
I would like to know the distance between a "reader" (arduino uno or other) and several objects.
If one object leaves >1meter, the reader will take some action. I would like to use it on many objects so if it could be small and cheap...
I don't need to know the position of objects, no triangulation or so...
I searched a lot on this forum and Internet and that's the "solutions" I founded :
Passive RFID : Impossible because I can't get the (even approximate) distance of an object. Moreover, operating distance in theory cand be some meters, in fact it's < 10cm (or more with really big and not cheap readers)
Active RFID : Seems to work but with several objects, the reader will have collision problems while reading the closest RFID tag and "not seeing" others. openbeacon active RFID seems to solve the problem but it is a bit complicated/big for this simple one-way thing I try to achieve
Pulsing RF emitter : That's the solution which comes naturally to my mind. I imagine and RF emitter at 1khz, one a 2khz (fake values) etc... And the reader would know what object are here and their distance. But with this not-digital solution i fear the wave collisions (i'm not expert... at all).
(EDIT) New ideas :
RF simple transmitter @ 433 or 315 Mhz like DCDW (thanks tkbyd) or Virtual Wire + some noise filters
ZigBee or Nordic even if a little bit overkill it will be good prototype before RF transmitter
DASH7 implementation (doesn't exist yet)
What is your opinion about this solutions ? Do you see something else ?
This is a much more difficult task than you think. RF will not work like you think it will.
The only thing could be an IR or ultrasonic distance sensor but that will only detect the closest object.
IR doesn't go through walls and boxes (objects will be hidden) so no.
Grumpy_Mike:
What is your opinion about this solutions
Sorry I don't think any will work.
This is a much more difficult task than you think. RF will not work like you think it will.
The only thing could be an IR or ultrasonic distance sensor but that will only detect the closest object.
Your first post said they didn't?
So use them if you think you know how to do it but unless you are holding anything else back I would disagree with you.
I can't see how this would measure distance. Reading RF signal strength as a measure of distance is something that does not work well in practice due to propagation interference from the surroundings.
You could consider doing your own version of the same idea.
Great idea I saw this yesterday and it seems to do the work. "High" distance + low power + simple data. I want to put my emitters on many points so cheaper is greater, low battery consuption and small size too.
Grumpy_Mike:
I can't see how this would measure distance. Reading RF signal strength as a measure of distance is something that does not work well in practice due to propagation interference from the surroundings.
Hi again Grumpy, I understand that distance will NOT be precise. BUT what I need to know is not a precise distance. So if I know that the signal is typically high, it's ok that's because things i'm tracking are near. This is sufficient information.
About precise distance, as you said I don't see anything cheap, small, low power (neither GPS nor very complicated mesh network triangulation) which works through walls. Just ultrasound or IR if the emitter is really in front of you.
Note : I don't track any information from emitters I just want to know if several objects are near me. For example, while moving with my computer, I already managed to find a wifi access point position with only one information : signal power. It works.... But not very accurate.
Human eyes see distance because of 2 " light receivers" and parallax. Can we do this with RF ?
How about hiring a person from a low-wage country ?
If all the objects look different, I'd be considering a camera and object recognition. But that would be difficult
to develop and it is not really an arduino sort of problem.