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[on:]

Hi all, my project compass has turned in the direction of having a play with RFID.

Before I spend money on something I can't do anything wit,h I know that there are some RFID experts on here so perhaps you could confirm a couple of things for me? (or laugh out loud and correct me)

Does the antenna need to be a particular size and shape? For example could I arrange it on the bottom of a dinner tray in a format that would pick up any tags placed on the tray.

I assumed that I could only read one tag at a time with the cheaper models, is this correct? Just read a few posts about 'collision detection', does this mean that if I spend more I could detect multiple tags on the tray?

In this case I don't need any great distance but would want the whole of the base of the tray to be sensitive.

Many thanks in advance.

[Reply #1 on:]

Most RFID tags can only read one tag in the field at one time. Some employ collision detection that means the tag backs off sending if it detects another tag. So with some type you can't actually read more than one tag in a field but you don't get any false readings. With some you can read two or three tags in the field tops.
The UHF tags can read multiple tags in the field at the same time up to a hundred or so.
Your bad news is that those sorts of tags are very picky on the type of antenna you use and what is in close proximity to them.
The lower frequency tags (125KHz) need a coil which you can cover the base of a non metallic tray with but you need carefully chosen capacitors to get the resonant frequency right.

[Reply #2 on:]

as ever...thanks Grumpy Mike for the info. I'd need to be able to have it ignore a previous tag rather than read multiples so that gives me some hope for not over spending.

I'll have a seach on line for capacitors and antennas thanks!

[Reply #3 on:]

I'd need to be able to have it ignore a previous tag rather than read multiples

It's the same thing, if a tag is in the field it is interfering with the field and unless it is smart enough to back off and not transmit you can't read anything else.

I want to do a similar project where I have a physical sequencer using RFID tags. I could do this with 32 readers but that is a bit silly. An idea I have had is to use one energising coil over the whole area and multiple pickup coils in specific locations. Then multiplex the coils. In that way each tag will in effect have its own reader but I only need one actual reader. I hope to work on this over the summer break, so you may be able to adapt that idea (if it works) to your project.

[Reply #4 on:]

I just had a look at a PDF for antenna design.

hahahahahahahahahahahahaha

page after page of formula. :o

Ah the life of a newbie is a frustrating one

[Reply #5 on:]

It depends on the frequency of the antenna you want to use. For 125KHz RFID tags you just need a coil. Then whit what ever inductance it has you add a parallel capacitor to bring it into resonance at 125KHz. The formulae for that you can remember in a rhyme:-
one over two pi root LC - gives you the resonant frequency