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No its not for a library. I considered posting the suggestion on the thread but being as I have little idea how to get it done and am new to the forum I held back.
Can you think of a cheap way to cut and reestablish an antenna wire? Preferably with only solid state materials (ie if a transistor could do it, that would be awesome).
I have looked into RFID reader chips and they all seem to have closed circuit antennas that are powered. I wonder what the power is for.... If indeed the antenna either fails to function or loses efficiency when power is cut (via a transistor), than that would be perfect for it's purpose I have destined it.
I hope to hear from someone with antenna experience on this forum before I invest the time and resources into a purely R&D project.
Passive RFID tags have no battery. They are powered by induction from the interrogator's RF field. The transmitting antenna must emit a relatively high field strength. Switching and feedline losses must be very small....
After lurking for a while, this question finally motivated me to reister so I can respond.
dustin, the first thing you have to decide is what kind of RFID system you want to use. There's some inexpensive 125 khz modules available from SparkFun and also from Qkits. If you want to get into 13.56 Mhz cards, then the modules are more expensive but the cards are cheaper and can have a longer range. The cheapest module would most likely be from rfidgeek.com. In most cases, you will also need a microcrontroller or a computer to receive the card data from the module and decide what to do with it.
My system used three cardreaders in close proximity so I also had the problem with wanting to shut off the RF pulse that the modules are always emitting when they are not supposed to be active. They have a tendency to de-sense each other if they're closer than a couple feet. The quick and dirty method is to simply use a transistor to drop power to the module. Or, since most of these modules have a reset line, just drop that pin to ground.
Hope this was helpful. Feel free to contact me if you need more information.
I have considered doing a time division multiplex but I am not sure this will work. I would prefer to have my nodes stay active for 100% of the time (instead of just during their turn on the multiplexer). However multiplexing is an option I'm keeping open.
The issue I ran into with the multiplexing approach is the cost. The multiplexer chips I found where pretty expensive and since I need 1000s of nodes with reversible polarity this requires 2 wires per node and jumbles of transistors/diodes. To purchase enough multiplexers to get it working would be a dramatic cost and very space-consuming.
The other approach that (I hope) is better is lining them up in a grid (like LCD) and use X and Y to activate specific nodes. This reduces the number of wires (for 1000 nodes) to 66 (33 x 33) which much more reasonable. The only trouble is actually making a grid that you can control with x / y wires that doesn't turn on whole rows accidentally (keep in mind that I need to turn on multiple nodes at once at different polarities.
The reason I'm asking what it does is often members may offer a better way of doing it.
I've never seen a passive RFID tag that was 0.35" in diameter. Also at 12" x 16" I would guess you're going to have a problem as RFID range is much larger than 0.35"
(first post in this forum, luckily google showed me the way)
i want to do something similar: i'd like to use 4 (or more) antennas on one rfid reader and switch between antennae to locate objects on a flat table. basicly a chessboard with antennae below each square. to cut back on my expenses i thought i could use a simple multiplexer to switch between the antennae.
now i colleague told me i'll probably fry every multiplexer because of the high ac/dc current that flows between antenna, rfid reader and multiplexer.