security detector for clothes

Friends,

I want to make CHECKPOINT FREQUENCY SECURITY TAGS detector. Suppose Tags works @8.2MHz, and are taken from EAS (Electronic Article Surveillance system) from store.



Any ideas, schematics... firmware for mcu is not problem, but hardware?

Thanks in advance,
Peja

I've just found that those tags are simple passive LC oscillating circuits @~8.2MHz.
It takes energy from alternating magnetic field of transmitting coil, then transmitting coil stop oscillating and start "listening" for oscillations of the Tag which continue to oscillate at resonant frequency for some time.

How to make transmitter/receiving coil, and detection circuit. I suppose that's made of very selective and sensitive receiver. I've found some projects for RFID, but this isn't same, coil is much greater, different frequency...

Can anyone help?

Peja

For some private purposes I want to make this receiver for detecting security TAG's (something like these http://www.securitytagstore.com/prod...d_Security_Tag).

I'll plan to build system with two wire square loop antennas, which represent inductor in circuit. first is transmitting, and second s receiving antenna.

How to make receiver for this? First, I don't know if security tags change their resonant frequency with temperature, probably YES. Also, antenna for receiver (also inductor) will change its characteristic. With some preliminary calculations for quare loop antenna, it value is about 15uH for 2 turns.
Idea s that transmitting coil will transmit 8.2MHz, then stop transmitting, and after that, receiver coil will listen if some TAG is oscillating after that.

SO, first problem is to build receiver for 8.2MHz. Can someone help please?

Peja