So I'm working on an RFID project, and to cut down on costs I'm going to use 2 or 3 RFID Antennas per RFID Reader. I want to make my Arduino UNO read the tags on an antenna, and then switch over to another antenna, etc. I figured the easiest way to do this would be to use a relay or something like that to turn the connection between the RFID Reader and multiple antennas on and off.
So my question is, am I going about this the right way? And what kind of relay should I use/build to work easily with the Arduino.
More information about the reader and antennas such as datasheets would enable people to give useful information - we cannot guess which parts you are talking about.
Here's a StackExchange question that's pretty similar to what I'm trying to do here
Yes they were rubbish answers, most of them were wrong.
As you have a 13MHz signal switching it with a relay is possible but not just any old relay. This is because the capacitance between the contacts on a normal relay will make the actual switch part irrelevant. You have to have a relay designed to do this, like one used on a ham short wave transmitter.
Next by switching the relay on an off continuously you are going to generate a lot of acoustic noise and the relay will not last very long.
Next by removing the field each time you have to give the token enough time to establish correct operation, this limits the speed of switching to about half a second at the shortest.
With half a second dead time and about 200mS to transfer data from a token you could be looking at over a second and a half before a presented token is read. This sounds a short time but feels a long time.
The main problem with using solid state switching is that the antenna has to act as a transmitter and a receiver, so your circuits need to be capable of handling high voltages. You can get over 300V across a resonating coil that is only fed with 5V.
So the short answer is yes you can do it but it is not so easy.
Near-field 'antennas' are not supposed to radiate and I would have thought resonance would be avoided for this reason. That module doesn't indicate the "antenna" voltages but I'm sure 300V would fry it...
Grumpy_Mike:
As you have a 13MHz signal switching it with a relay is possible but not just any old relay. This is because the capacitance between the contacts on a normal relay will make the actual switch part irrelevant. You have to have a relay designed to do this, like one used on a ham short wave transmitter.
Do you know where I can read up more about such relays?
Next by switching the relay on an off continuously you are going to generate a lot of acoustic noise and the relay will not last very long.
Next by removing the field each time you have to give the token enough time to establish correct operation, this limits the speed of switching to about half a second at the shortest.
With half a second dead time and about 200mS to transfer data from a token you could be looking at over a second and a half before a presented token is read. This sounds a short time but feels a long time.
The project I'm working on deals with inventory tracking, and for the scope of the experiment a few seconds is okay for delay and I already took the time it would take to switch between antennas into account
The main problem with using solid state switching is that the antenna has to act as a transmitter and a receiver, so your circuits need to be capable of handling high voltages. You can get over 300V across a resonating coil that is only fed with 5V.
I'm sorry, I don't quite understand what that means, could you elaborate?
Yes I had a look at that.
First off it refers to HF readers, these are in the region of 125KHz. You have a RF reader that goes at 13MHz, this makes a big difference.
Secondly those multiplex ideas simply do not work, I have tried them.
The cross talk is such that a card on one antenna shows up as being on both.
As I said before the voltage on a typical HF reader coil is about 50V on reception which would fry the multiplexer.
When I tried it I kept the voltage on the multiplexers down with some diode clamping. I was trying to read 8 coils at one time and I had a separate reading and exciting coils to keep things simple and not run into the current limits imposed by the multiplexer.
Do you know where I can read up more about such relays?
Try looking for Ham radio short wave transmitter / receiver change over relays.