I've managed to rig up my ID12 RFID chip to my arduino and it reads fine but the distance is kinda shoddy. I know there is a way to attach an external antenna but i can't figure it out for the life of me even with the example in the datasheet. Google has no answers so i'm wondering if anyone has any experience?
Here's the datasheet if anyone needs it:
That data sheet shows how to connect up an external antenna (ID2 but ID12 will be the same) so I am not sure what you don't know.
The EM RFID card works at 125KHz so the external antenna should consist of a coil and capacitor that resonate at this frequency.
Fr = 1/(2 PI SQRT(LC) )
To get a good range the coil should be as large as possible. You can develop several hundred volts across it at resonance so make sure the capacitors are at least 500V working.
Well i'm pretty new to arduino and electronics as a whole. What is LC in that equation?
L is the value of inductance of the coil in Heneries
C is the value of the capacitor in Farads.
I've managed to rig up my ID12 RFID chip to my arduino and it reads fine but the distance is kinda shoddy.
How shoddy is it?
I have a ID20 and it works at a range of 18cm which is well within the range of 16+cm given in the datasheet.
Eberhard
I have a ID20 and it works at a range of 18cm
This is a good range, you won't get much better than 25cm.
These readers use the magnetic field and that drops off with the inverse cube of the distance as opposed to the inverse square of the electric field.
This is a good range, you won't get much better than 25cm.
Yes I was also suprised by the performance. The scanner is now mounted behind a 45mm thick wooden door and it still has a range of 10cm (including the wood).
Eberhard
Yeh well sure it does give some distance but i only have the ID12, not the 20 and an antenna would be handy just for mounting purposes.
So at what distance do need the sensor to work?
I bought this one too:
http://www.seeedstudio.com/depot/125khz-rfid-module-uart-p-171.html?cPath=40
It comes with a external antenna but stops reading after about 15cm.
Also the antenna wires are very thin and fragile. So you need some extra care when mounting the antenna. The wires that connect the antenna to the receiver pcb are roughly 8 cm long, but I don't think you can extend them without getting into trouble.
Maybe you should rethink you mechanical design.
I connected my ID20 to the arduino board with a shielded cable of about 30cm length. Works without flaw.
Eberhard
The scanner is now mounted behind a 45mm thick wooden door and it still has a range of 10cm
I am very impressed. 
What diameter is the coil?
I once made one with a 30cm square coil with comparable range but at those sorts of range other things matter as well.
I once had one customer complained about the range of an RFID reader but it turns out he had installed it behind a fire extinguisher .
The 10 cm include the door of course, so its 5 cm from doorfront level.
It's the ID20 with a build in antennea. I guess the coil matches the size of the housing which is 4x4 cm.
And also there is no metal near the circuit at all. I put the thing into a plastic housing. It is only the door-opener for a bicycle-garage here in the house, where you don't expect much "electrical noise" to be around.
Eberhard