Wireless sensor detection system

[Alaza]

Hello there.

I'm thinking of a project where a user needs to keep a little box on him/her, and whenever the user comes in a range, of for instance 5m (16 ft.), a receiver needs to put out an analog signal to turn on a alarm/speaker with a puretone.

But what wireless type of sensor would I be looking at? The box the user carries shouldn't be much bigger than a mp3 ideally.

Thanks

 

[dknguyen]

An RFID system should work. I think you may need an active RFID system rather than passive.

Passive RFID Is good because the tag requires no batteries (using the RF energy emitted by the transmitter), but they have poor range (of like 2 feet max). BUt, I don't know too much about what is available out there. You *might* be able to find a passive RFID system with the range you need (transmitter power be exponentially higher with distance increases though). And the tags would be credit card sized or even smaller. Very small.

Active RFIDs need batteries and have much longer range and probably about the size of an MP3 player if not smaller. You'd have to experiment with power levels to figure out the range you need....I think active RFID can have a range considerably longer than 5m.

Needless to say, if you you have the choice between passive or active...go passive!

EDIT: According to this:
http://www.autoid.org/2002_Documents/sc31_wg4/docs_501-520/520_18000-7_WhitePaper.pdf

Passive RFID is usually 3m or less. Active RFID can be 100m or more.

 

[Alaza]

I've been reading through the .pdf you linked to, and it's very interesting indeed. I probably could live with a range of 3m or so, so I could go for a passive RDIF. I've looked at Farnell and their supply of RDIF components, and it's quite hard for me to determine what I'm looking for, seing I've only heard about RDIF briefly in the newspaper.

Could you point me in the direction of a low cost, passive RDIF kit? I only need the reader to turn on a loudspeaker with when the tag is in range, and the reader should be able to be supplied from the grid with 230v.

 

[KMoffett]

Google: suitcase briefcase child proximity alarm

These alarm with increasing distance between the transmitter and the receiver. You could hack one, invert the output, and make it alarm when the transmitter is in-range.

Ken

 

[Alaza]

Kmoffett:

Thanks, but I can't use anything that commerciel I'm afraid, and I wouldn't know how to hack it.

But out of pure curiosity, do you know if it's based on RDIF too?

 

[KMoffett]

I don't think it's "RFID". By the way is this a school project?

Ken

 

[Alaza]

Okay, you got any idea what it is then?

At the moment it's not, but it will be if I can get my arms around it, mostly in the way of it not being to complicated. That's once of the reasons why I'm more looking into the RFID thing.

 

[Nigel Goodwin]

Okay, you got any idea what it is then?

It's just a low power transmitter and a receiver, both battery powered - so range depends greatly on what's in between the two.

 

[KMoffett]

I think it mostly works on the presence/loss of an RF signal. The signal is probable modulated with a code (like garage door openers) to differentiate from like units. There is an active transmitter and active receiver, unlike the passive RFID "cards".

ken

 

[Alaza]

It's just a low power transmitter and a receiver, both battery powered - so range depends greatly on what's in between the two.

So it's just another radio signal? What kind of equipment would you look at if you would transmit a signal like that yourself?

I know these questions are for beginners, but I had an idea that would be really cool to realise, but I've never had any courses regarding wireless or radio transmission at all.

So my plan is to get a feeling of what I would need to a project like this, and then go to my superviser at my university and let him look through my ideas and hopefully let me know that I have a change of completing it - maybe with his help.

So what I'm asking is, what kind of equipment could be used for the needs I put forward in #0, if not RFID. And if RFID, could you then guide me towards my next step of action?

To give you an idea of my experience, I'm only at 5. semester of my bachelor in electrical engineering.

 

[Nigel Goodwin]

So it's just another radio signal? What kind of equipment would you look at if you would transmit a signal like that yourself?

Like KMoffett said, they probably send an ID code, the receiver continually looks for and checks this ID code, and if it fails to find it for a specific length of time, then it triggers the alarm.

 

[KMoffett]

So what I'm asking is, what kind of equipment could be used for the needs I put forward in #0, if not RFID. And if RFID, could you then guide me towards my next step of action?

So, how low a level do you want to go in the electronics/programming? If you do some searches on RFID you will get an idea of the capabilities of commercial RFID components. The "module" level is probably the lowest level you can go for major parts using RFID. For encoded RF level-sensing like the suitcase alarms, you might be able to go the the IC and discrete component level, and will probably need microcontrollers for encoding/decoding.

Ken