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What sensors / networking protocol should I use for this project? Answered

Hey folks.  

I'm a relative newcomer to the maker scene but I've been into GNU/Linux, BSD, shell scripting, Java and more for quite awhile.

So here is my dilemna.

Basically I'm trying make a device in which, let's say a child or dog, comes within 4 - 6 feet of an object and an alarm goes off.  However the trick is that the alarm can only go off when the particular dog/person is close to the item;  basically I have to pair two sensors or some type of communication devices together.

I was thinking bluetooth, zigbee...unless I'm missing out on a particular sensor (or set of sensors) that could be paired together.


EDIT:  Of course RFID would work, but the max distance is about 4 inches I think?


Barring visual image recognition techniques, I guess you need the moving object with some kind of transmitter. Maybe a weak IR signal that is constantly flashing or ultrasound beeper to be detected. Otherwise you might need to reverse some of those child locator devices where the transmitter unit moves away and an alarm goes off. Good luck.

Yes, I was sure I needed a transmitter but the question is what kind.

Ultrasound seems possible, but can those sensors be "paired"?


I guess if you can pulse the ultrasonic speaker like in morse code and have the receiver listen for the distinct pattern.

If the detection area is free of other obstacles, a standard ultrasonic sensor (e.g., the Parallax PING))) sensor) should work -- and the "detected" person won't need to carry any other kind of device. (I just double-checked and the PING has plenty of range for this application.) You can tune the detection distance in software so that it doesn't trigger on objects outside the detection area.

Another possibility is the Kinect. It has built-in distance calculation and is very hackable. I think it even has body-recognition algorithms so you could distinguish people from other obstacles. Heck, it even has a built-in servo and can pan to sweep a larger area.

ultrasonic was my first thought, but I really need the sensor / module to be paired to another sensor / module and only interact with it specifically.

Thanks though, I'm going to look more into ultrasonic sensors ;)

Oops, sorry, I missed that part.

Take another look at RFID. There are several different RFID technologies available, some of which work at a distance (at least 3 feet, IIRC).

Do you happen to know the name of the particular RFID tech which has a longer range?

Thanks for the ideas folks! I'm not sure what route I'll go but I have some good places to start at least.

Thanks again.

There are devices for keeping track of a child that use the reverse idea. It is wired so that if the child gets too far away the alarm at home base sounds. If you can find a simple enough one, and reverse engineer it, you could possibly change the alarm to sound on the strengthing of the signal, rather than the vaishing of it.

Yes I have seen quite a few, most of them use bluetooth.

Hmmm, some older ones from a garage sale (I got one to play with, this way) are still "off band AM radio controlled".

RFID range is limited largely by the antenna. You can get longer ranges with bigger antennae

That is interesting, I didn't know the range of RFID could be increased with larger antennae. Do you happen to know how far?