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Huge Area RFID Reader

Hordeus

New Member
Hello everyone
I'm trying to make a RFID reader with diameter of 1.5 meters.
So far i managed to work 125kHz RFID reader with 12 cm diameters.
But yesterday i tried coil antenna with 35cm diameter and amplitude of coil antenna's signal didn't dropped much almost 200mV.
119871
And there wasn't anything on the output.
So i think the signal strength is the problem.
Any ideas and advice?
Thank you
 

unclejed613

Well-Known Member
Most Helpful Member
by changing the coil dimensions, you changed the inductance, and it probably doesn't resonate at 125khz... you need to test the coil with different values of C2 until you get reasonably good sine waves across C2 when the arduino is driving the MOSFET.
 

alec_t

Well-Known Member
Most Helpful Member
The LM358 doesn't have much gain at 125kHz. Try an opamp with a greater gain-bandwidth product.
Reduce R4 and/or increase R3 to increase gain.
 

unclejed613

Well-Known Member
Most Helpful Member
The LM358 doesn't have much gain at 125kHz. Try an opamp with a greater gain-bandwidth product.
Reduce R4 and/or increase R3 to increase gain.
actually i think the op amp is amplifying demodulated AM (with the demodulator being D1, C3, and R5) which is then being cleaned up into logic levels by T2.
 

alec_t

Well-Known Member
Most Helpful Member
Good point, but the gain still seems inadequate,.
For a given current input, the larger the diameter of the transmitter coil the weaker the field strength within it will be. So the tag will receive a weak signal and its response as received by the large coil will be weaker still.
For a circular coil the field strength at the centre, in the plane of the coil, is inversely proportional to the coil diameter. If the tag and coil both had, say, a diameter of 2cm and were superimposed, then the field strength at the tag would be 150cm/2cm = 75 times the strength that would be present if the coil had a 150cm diameter.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
He asked me in a personal message last week as well; this was my reply last thursday, to give this thread continuity:
---------


It could be the card is not getting enough energy to power up properly, or that the simple receive circuit with a fixed threshold is not adequate to demodulate the data.

For a card further from the coil, it needs proportionally higher power as there are more losses both ways.

I'd probably redesign the transmit side; use a small, screened, tuned circuit driven by a tap for the transistor and another winding to couple that to the large coil.

eg. something like this:
http://electriciantraining.tpub.com/14180/img/14180_78_1.jpg
You can forget everything to the left of the transistor as you already have the drive signal to the FET gate and the limiting resistor can be between the coil and V+ as it is now, rather than in the source/emitter as they use.

The second winding means the connection to your large coil does not add capacitance to the tuned circuit. I'd try feeding between one end and a tap on the main coil with the tuning cap across the ends; like the collector coil in that diagram is driven be the transistor.

I have no idea if it will work, or can ever work... but that's what my approach would be.
 
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Beau Schwabe

Active Member
The problem I think and it has been mentioned is that there isn't enough drive current for the coil.
As it is on a simulator the drive current is about 46mA with a Peak to Peak voltage output on the coil around 40V without a load.
Changing the 100 Ohm resistor (R1) to 47 Ohm resistor increases the drive current to about 83mA producing a pk-pk voltage of 60V

I would change the front end excite circuit even further and implement a push pull drive lowering R1 even further down to 10 Ohms.
This increases the current drive to about 160mA and the output pk-pk voltage to around 140V without a load.

Now, from here lets think about what needs to happen.... We don't need the 125kHz carrier frequency, we just want the modulation that
happens. So a passive low pass filter that rolls of at about 10kHz should be perfect. To isolate the high voltage, after the filter, a simple capacitor (100nF or so) will provide a simple DC voltage block. The remaining modulated signal can then be fed through a LM386, or simply a transistor with adequate gain.

Lets get to the first stage first and see what kind of reading we get from TP1 (See attachment below) and if you can see visible modulation near the peaks of the sine wave (<- you should) when a tag is brought near the coil.
 

Attachments

Diver300

Well-Known Member
I did some work with RFID about 10 years ago. I was using an HTRC1100. It worked fine up to 10 cm diameter with around a 10 cm range. The HTRC1100 seemed a lot more sensitive than a simple envelope detector would have been. The system would work when an oscilloscope would not see any effect on the coil waveform from the return signal from the RFID chip that I was reading. I guess that the phase change was being detected.

I tried going to a much larger coil, around 1 m diameter. I added driver transistors as the HTRC1100 was clearly not powerful enough, and I had a transformer to get a large enough current in the coil, with the tuning capacitors on the primary side of the coil. The coil was 1 turn, with about 10 A flowing in it.

I got resonance, and I got the RFID card to recognise the 125 kHz and the modulation, which I detected with a separate coil near the RFID card. However I couldn't get any response for the return signal. It was too small for the HRTC1100 to detect.

As far as I can tell, all longer range RFID is done with a different frequency return signal.
 

Beau Schwabe

Active Member
Diver300, I built an RFID radio in 2014 designed to listen to the door reader itself and not the actual tag which was basically a two coil system only the two coils are spread out over some distance. The idea is pretty simple. When you place a RFID tag near a reader, the reader becomes a modulation transmitter. The whole idea was not for nefarious attempts, but to exploit and present a vulnerability at a national security conference held in Las Vegas that year. Initially, myself and a friend of mine that works as a Penetration tester conceived the idea and set out to design a proof of concept. Our goal was to come up with something that would only work about 3 feet, but instead we ended up setting a record for a valid read at 25 feet.

So as far as I know, unless you have an active tag, meaning that it is powered, you won't get the distance. Some of the higher frequency tags (13.5MHz) will go further, but the principle is the same. Unless they are powered, then they are generally limited to a few feet.


EDIT- The "pickup coil" that I used to achieve that distance was only about 3 inches in diameter.
 
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Diver300

Well-Known Member
We used a TI reader that worked fine with a door-sized coil. The cards had no battery, but they transmitted back on a different frequency. I think it was 125 kHz one way and 132 kHz the other. The reader contained lots of large capacitors, and I think there was a lot of passive tuning to extract the return frequency from the big powerful transmitting signal.

I agree that something like that is needed for long range. There is just too little energy when the card can only transmit by partially shorting the coil.
 

Mickster

Well-Known Member
Most Helpful Member
Diver300, I built an RFID radio in 2014 designed to listen to the door reader itself and not the actual tag which was basically a two coil system only the two coils are spread out over some distance. The idea is pretty simple. When you place a RFID tag near a reader, the reader becomes a modulation transmitter. The whole idea was not for nefarious attempts, but to exploit and present a vulnerability at a national security conference held in Las Vegas that year. Initially, myself and a friend of mine that works as a Penetration tester conceived the idea and set out to design a proof of concept. Our goal was to come up with something that would only work about 3 feet, but instead we ended up setting a record for a valid read at 25 feet.

So as far as I know, unless you have an active tag, meaning that it is powered, you won't get the distance. Some of the higher frequency tags (13.5MHz) will go further, but the principle is the same. Unless they are powered, then they are generally limited to a few feet.


EDIT- The "pickup coil" that I used to achieve that distance was only about 3 inches in diameter.
I kinda recall stumbling upon the above somewhat recently, when looking up BFO circuits.
Initially, I found this from yourself:
...then ended up somewhere else after clicking a few links.
 

Beau Schwabe

Active Member
Mickster ... hahaha! ... funny thing is that started out as a pilot project to sell a thing or two on eBay. I was just testing the water. I've probably sold a couple thousand of those boards. ... And best of all when I monetized the video, I made in a day the BOM cost of the board and components without doing a thing.
 

unclejed613

Well-Known Member
Most Helpful Member
As far as I can tell, all longer range RFID is done with a different frequency return signal.
the ones you see near the exit of the supermarket are usually 13.55Mhz, and those use large coils. the ping and the response are about 10khz apart. while listening for HIFER beacons, i regularly saw RFID pings and responses from a nearby BigLots store.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
the ones you see near the exit of the supermarket are usually 13.55Mhz, and those use large coils.
That's presumably the printed coil type, often hidden under a fake barcode label?


Another long distance system commonly used for detachable security tags on clothing and such works with very low frequencies - a common version uses a mechanical resonator "tag" tuned to around 58KHz with a pulsed drive coil.

A tag that has not been disabled rings for a brief time after receiving a signal pulse and radiates while doing so.
They contain a magnetostrictive resonator plus a magnet.

No data, or any electronics at all in the tags themselves, it's more like a very selectively tuned metal detector system with a resonant target.

I've always thought the are a bit close to the 60KHz MSF time signal & wondered if they desense radio synced clocks near shops that use those tags?
 

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