I am looking for a sensor to check when a door is closed that must be wireless and possibly batteryless.
Normally such check is performed by a magnetic contact (the ones used for antitheft systems).
I would avoid using cables to connect the sensor to the main board, that will be at most 2 meters away from it.
So I was thinking about an RFID application. An RFID tag that has a magnetic contact would be perfect.
But I cannot find such thing on the Internet and I am unsure it even exists.
I heard that there are RFID tags that performs more than just sending their IDs, but can store information and send it as well. But I found only ID tags.
I know about EnOcean products. They are very interesting (wireless, batteryless and with a RF range of 300 meters!)
But I am looking for samething simpler and cheaper.
Here are some of them:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0189188
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0189009&cm_vc=prev_uk
Do anybody know about wireless and batteryless contact sensors?
Thank you for any help.
An RFID tag that has a magnetic contact would be perfect.
I am not sure what you mean by this. Do you mean an RFID tag that can read a magnetic contact switch and report back the state of that switch.
If so I don't think one exists but it would be easy enough to make. However, you are not going to get a 2 meter range from a passive RFID tag of the sort that you could make yourself.
Thank you for the response.
Do you mean an RFID tag that can read a magnetic contact switch and report back the state of that switch.
You got it!
If so I don't think one exists but it would be easy enough to make.
How could I do that? Can I Build an RFID tag by myself?
I thought to take an existing tag and break one of its circuits (to be determined: which circuit) and connect it to a common magnetic sensor. Could it work?
However, you are not going to get a 2 meter range from a passive RFID tag of the sort that you could make yourself.
I read that RFID passive tags can cover 20 feets [edit](6 meters)[/edit] (How RFID Works | HowStuffWorks). This is absolutely an indicative measure.
It would be nice to identity a tag/gayeway cuople that covers 2 meters (and is Arduino-compatible).
Optical. Put a little reflector on the door so that when the door is closed, it reflects a laser beam back to your receiver. Of course, it will be very much more expensive and fiddly than just running wires or replacing a battery every 6 months. Now you know why wires are popular even though perhaps inconvenient.
Really interesting. In my case, the doors to check are at the left and the right of the point where the main unit is, at a distance of about 1 meter each one. So it would be entirely possibile.
I can just use a photocell like the ones used in gate automation and a reflective surface or sticker on the doors!
Thank you very much! 
I am still interested in the RFID solution, because I would learn something about this technology and this would be a chance.
That site says:-
These tags are read up to 20 feet away,
I used to design RFID readers for a living and the only way you can get that from a passive tag is to be right up in the microwave region. Most RFID tags you see operate at 125KHz or 13MHz.
I thought to take an existing tag and break one of its circuits
Inside an RFID tag is a coil attached to a small chip measuring about 1mm square. You can't connect anything to it.
Can I Build an RFID tag by myself
Yes you could it is quite simple. However there is little point if you can't get the read range you want. You can increase the read range by making the coil larger but I am not sure you will be able to get what you want. That is the tricky bit.