RFID help/advice needed please.

[JamesW]

Hi folks,

Can anyone give me a few basic pointers on RFID please?

I need to design something that can read an id of something from about a metre away. I've spoken to the CCS support guys regarding the development kit, but it is a low frequency kit that will not work up to the distance I need.

Whilst being an old hand at PIC's, this is an area I have no experience in at all. I can see that microchip do the 12F675F which seems to communicate at a higher frequency which may do the job. The app notes for this are a for a tyre pressure monitor which is a lot more complicated than I need.

Ideally I would like to buy an off the shelf tag and design a reciever that will read the id from a metre away.

If anyone can give me any pointers it would be much appreciated.

Thanks

James (james@winterandsmith.com)

[C Turner]

There are two types of RFIDs out there: Active and passive, with the latter being the most common and usually the cheapest.

The passive low-frequency RFID tags require enough energy from the "receiver" (which is actually both a transmitter and receiver) to power the code generator contained within the RFID unit itself and then modulate the code somehow onto the pickup coil. Some of these units actually re-radiate their code on, say, half the frequency, while others may simply short out their pickup coil to "space modulate" their code: The "receiver" detects this as a field disturbance to demodulate it.

Some of the extremely short range RFID tags are also field-powered by the "receiver" but operate in the HF range (typically in the 13 MHz region.) As you might expect, the smaller these are, the smaller their pickup antennas are as well, effectively reducing the range.

Another type of "passive" RFID tag which is truly passive (e.g. no active electronics at all) are the "tattle tags" in merchandise which are simply a resonant circuit: The would-be thief passes through a modulated RF field and the presence of a resonance is easily detected. Strictly speaking, this isn't really an RFID as it can be used only to detect the presence of such a device - nothing more. There are more complex versions of this that have multiple tuned circuits, the frequencies of which can be chosen to represent specific codes, and thus make it possible to have many different codes, to identify specific items or people. These, however, tend to be somewhat larger and thus more expensive. These can have reasonable range (up to several meters) but they also require fairly large antenna arrays to work properly and are usually installed at exits, where people will walk through the array on their way out.

With passive devices, one is limited by not only the laws of physics (e.g. trying to emit enough power so that the miniscule antenna can not only receive the signal and power itself, but also emit a readable signal of its own) but laws of the land which very strictly limit the emissions related to these sorts of devices and their "readers."

If more than a few feet is needed, one is probably relegated to active devices - those containing batteries and, mostly likely, a receiver. These often operate at higher frequencies (often in the 900 or 2400 MHz range) owing to their necessitated small size. If line-of-sight is guaranteed, infrared solutions are also possible as well.

It need not be said that the active devices are usually far more complicated and expensive - plus there is the fact that there's a battery to worry about - a factor that severely impacts reliability as well as liability.

You don't actually say what your need is- Whether you are trying to telemeter information, or simply detect a device's presence, so it is difficult to determine if there might be some alternative technology that might suit the task at hand, or, as it sometimes happens with me, trying to find a complicated solution for a simple problem...

CT

[JamesW]

Cheers for the response.

The application I need it for is for couriers collecting bags of paperwork from shops, banks etc. Whilst the contents are worthless, they keep being snatched.

My customer wants the courier to wear a tag/transmitter (maybe on his belt) and the bag to have a receiver in it.

It is activated in the shop. If the transmitter and receiver are separated by more than a couple of metres it will sound a siren which will continue to sound until the battery is flat or it is turned off. (So no data needs sending really, just a 'is it there' message.

The passive RFID seemed like a sensible idea, but I can go with active RFID if it will do the job.

If active will work, can you offer any suggestions as to choice of chip etc.

Thanks for your help

James

[treitmey]

What about a zigbee pair. set them to ping each other with a unique id. When the pair don't hear from the other(out of the 1 meter rang) then the buzzer goes off.
Don't know how you could adust DOWN the range of the zigbee tho.
Is this possible?

Maybee bluetooth is a better answer.??
http://www.rfsolutions.co.uk/acatalog/Toothpick_Bluetooth_Module.html
another url
http://www.ibtechnology.co.uk/

[C Turner]

One thing that might work could be an ultrasonic proximity scheme: The courier wears something that sends out a "ping" and the bag contains a receiver that listens for it: If several pings are missed, it starts to yell. (Of course, the transmitter could be in the bag with the courier wearing the alarm: with a bit of fancier footwork, one could use that signal to locate a lost bag.)

If one did this sort of scheme, "windowing" the receiver (listening only when the ping was to be expected) would be one way of minimizing power consumption as well as reducing falsing: If several pings were missed, it could stop the windowing in order to attempt to re-synchronize. If a "ping-back" scheme were implemented, one could even determine the absolute distance from the bag.

This scheme would not be able to uniquely identify the bags (aside from the timing/sequence of a "ping cycle" which could be unique for each TX/RX pair) and it would avoid having any sort of RF circuitry at all (aside from the PIC itself) - something which may save one from having to jump through regulatory hoops.

The obvious "gotchas" are that one would have to make sure that it was robust enough to handle normal conditions where extra noise or signal attenuation (due to clothing, something being set atop it, the bag itself or its contents, etc.) could cause the signal to be lost or distorted for a short period. - but this could just as easily happen with any sort of RF-based wireless scheme.


CT

[ckielstra]

Devices for this purpose already exist, used for tracking luggage, cars, pets and children. Prices starting at $19.
Searching Google for 'distance monitor' and 'alarm' I found many links, including:
http://www.szwinslim.com/product1-d/child%20guard/cg.htm
http://www.1ststoptravelstore.com/AH_distance_monitor.htm
http://www.greatbabyproducts.com/Angel_Alert_Child_Safety_System_p/trav-82562t.htm

[JamesW]

Cheers for the tips.

My customers looked at what can be purchased commercially, but wants something a lot more rugged with a much louder siren in it.

He was originally going to provide one of the above for us to strip apart and use the innards as the basis for his alarm, but the one he had his eye on has been discontinued. (Hence the RFID possibility).

Thanks

James

[John676]

Hi,

I didn't read all of the responses, so please forgive me if I am stating something that someone else stated here already. I saw someone talk about xbee, we use them and they are very good and easy to use, but a little expensive perhaps for your app.

What we have used for similar apps is the tellicontrolli receivers and trasmitters. These are the same types you would use in a gate remote and transmitter. They are low cost and their range is about 30 meters. They are not as easy as the xbee modules, but they are not difficult to work with and they are smaller.

Regards,

John

PS, I am in South Africa, so can't give you a supplier name that would be of any use to you. Try googling it.

[JamesW]

Thanks for the suggestion. The Xbee stuff will work out a little too expensive for my customer.

The main reason I had my eye on a pic based solution was the cost side of things.

I may try to modify the microchip app note design based on the tyre pressure sensor unit, and see what sort of range I can get out of that.

Any other suggestions gratefully received

Thanks

James

[Ttelmah]

You can push the range massively.
Some time ago, when the RF PIC stuff was 'new', I was involved in a project using these, where high range was needed. We ended up managing to cover a gap over 8 feet wide. However the antennae needed at both ends were much more expensive, and complex than the simple ones used in the 'examples'. The one at the tag, ended up twice the size of the example antenna. If I remember rightly, becoming 1/4 wave, instead of 1/8th wave. The 'gate', ended up as an 'H' structure with two tuned dipoles. This made the tag larger and more expensive than the examples, and the gate a lot more expensive. Depending on the quantities involved, it may be possible that this type of design would be acceptable to your customer, but on smaller runs, the 'off the shelf' Xbee stuff may well be cheaper...

Best Wishes

[JamesW]

That sort of range sounds ideal.

If you have any information on the antennas it would be much appreciated.

Thanks in advance

James

[Ttelmah]

Unfortunately (regarding telling you how to do it!), the product is commercial, and on sale (if you look at the RF tagging systems being sold for airport conveyors, you should find it)....

Best Wishes