NFC field generator - Page 1
Forum section: electronics / beginners

author: grymoire
author_rank: Contributor
timestamp: November 28, 2018, 12:30:34 pm
content: I am working on an art project, and I want to build a thin panel that can generate an NFC field which can power some NFC LEDs, such ashttps://www.sparkfun.com/products/14888.All I need is to light them up. I would appreciate any help. A thin design would be great, but I also am interested in increasing the XMIT range, so I can light them up from a distance. I don't think I need to read any information.Thanks

author: Berni
author_rank: Super Contributor
timestamp: November 28, 2018, 12:51:42 pm
content: You basically just need to power a coil of wire with the right frequency AC signal. NFC tends to use around 13MHz so those LED tags are probably optimized for that.How you generate the alternating current for the coil is completely up to you. But i suspect that getting it to work over a long range would include large coils and a lot of power put into it.

author: grymoire
author_rank: Contributor
timestamp: November 28, 2018, 01:50:44 pm
content: Yes, NFC is 13.56Mhz. I found some NFC coils that are used for smartphones. And I can get an NFC board with a build in antenna. I don't want to use a large board if I can avoid it. It's theQuotepart is where I need the help.

author: Berni
author_rank: Super Contributor
timestamp: November 28, 2018, 03:50:11 pm
content: Well you need to make an oscillator that generates the desired freqency, Any oscillator can be used or even a microcontroler can be used to toggle a pin at that rate. Whatever you are most comfortable using.That needs to be put trough some sort of power stage. This means using a transistor to switch the power to the coil at that freqency or using some sort of driver chip to do it.You also might want to have a capacitor wired across your coil and tune it to a value that makes the capacitor and coil oscillate as a LC circuit at the desired frequency. Take it step by step and play with it until you get it working.

author: judge
author_rank: Contributor
timestamp: December 16, 2019, 05:24:14 pm
content: I'm just trying to do the same thing. I found a circuit for an oscillatorherethat uses three NAND gates and a 13.56MHz crystal (and a few discretes). The oscillator basically works but the voltage is kind of low (around 2V p2p), plus the waveform looks very noisy. I'm not sure if the noise is real or an artifact of me hooking up my ancient oscilloscope.I figured I could maybe feed the signal into the gate of a MOSFET to switch the full 5V, but the MOSFETs I have to hand are too slow (rise time of around 200ns!). So I was wondering if anyone could point me at a suitable part, advise about how to clean up the waveform, tell me that this will never work and I should do something else instead?Also curious if the OP every got this to work?

author: Benta
author_rank: Super Contributor
timestamp: December 16, 2019, 07:23:22 pm
content: Do a search for "wireless charging" or "contactless charging". Several suppliers have solutions (TI, Infineon ec.)

author: judge
author_rank: Contributor
timestamp: December 16, 2019, 07:38:47 pm
content: Wireless charging works at about 100KHz. NFC is at 13.46MHz. I have tried these LED tags on wireless charging stations. They don’t work. They only work with NFC.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 16, 2019, 08:21:03 pm
content: This should get you going:https://www.aliexpress.com/item/32959673490.htmlLooks like they use an IRF530N

author: judge
author_rank: Contributor
timestamp: December 16, 2019, 09:52:47 pm
content: Quote from: PA0PBZ on December 16, 2019, 08:21:03 pmYour searching skills are way better than mine! That is one helluva amplifier! Interestingly the datasheet for that MOSFET seems to show that the total rise/fall time is 91ns, which is greater than the period of a 13.56MHz waveform (73ns). I guess it would be less at a lower voltage/current? Also the Vgs(th) is 2V which is right at the limit of what my circuit produces. Or maybe not. 2V is the p2p voltage, I think that is offset from 0V some. If it isn't, maybe I can offset it.I can also start searching for 13.56MHz amplifiers now. Many thanks.

author: Ammar
author_rank: Regular Contributor
timestamp: December 16, 2019, 10:40:51 pm
content: If you just care about lighting them up get one of these:https://www.amazon.com/M24LR-DISCOVERY-Discovery-transceiver-M24LR04E-R-powered/dp/B01EJ933VKAnd follow these instructions to get it delivering power continuously:Go here:https://www.st.com/en/evaluation-tools/m24lr-discovery.html#tools-softwareDownload "PC software for executable demo..."Connect the daughter board to your PC via USBOpen CR95HF development softwareUser commands → CR95HF Command List → 02: Protocol Select → Choose Reader protocol code 01:ISO15693 → Click send frameThat should do it.

author: Ammar
author_rank: Regular Contributor
timestamp: December 16, 2019, 10:42:42 pm
content: Only 10W??This would really get things going:https://au.mouser.com/pdfdocs/IXYS_PRF-1150.pdfReally depends what the op is after. By distance do they mean a cm or like 10 m?

author: Ammar
author_rank: Regular Contributor
timestamp: December 16, 2019, 10:43:17 pm
content: Quote from: PA0PBZ on December 16, 2019, 08:21:03 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2019, 12:09:30 am
content: Switching times are limited by device capacitances alone -- RF amplifiers' bandwidthis subject to the same limit, but the center frequency is subject to more detailed characteristics (in this case, limited by the dropping gain / rising losses of the MOSFET with increasing frequency, and stray inductance affecting stability).An oscillator with something like IRF530, at 13MHz, is pretty typical.Get out some wire, some smaller capacitors, and you're good to go. (Would recommend a Colpitts type.)Tim

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 12:59:42 am
content: Quote from: Ammar on December 16, 2019, 10:42:42 pmObsolete item. No stock. No price. Doh!

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 01:01:57 am
content: I'm asking, here... Could a directional antenna be built?If the airport scanner would make them blink, that could be fun...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2019, 03:34:10 am
content: Taking NFC literally, no. You can make an asymmetrical field, but you can't make a field stop arbitrarily (no matter how many times the textbooks draw diagrams as if you could). To be exact, the field satisfies Laplace's equation, \$\nabla^2 \phi = 0\$.A very large antenna (in human terms; a dozen meters or so) can have directional gain (i.e., better than dipole) in a propagating field, but that's not near-field.The usual approach is to place a coil over a ferrite plate, which acts to short out the magnetic field lines on that side. This reduces the volume of field (reducing the magnetic path length more than the effective area is reduced, hence the inductance goes up), giving less range on the open side, and higher mutual inductance up close.The shield is as much practical as anything: it improves coupling between coil and target, and shields the circuitry underneath, allowing a compact build.Tim

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 04:51:24 am
content: That's a good explanation, Tim; thanks!

author: Berni
author_rank: Super Contributor
timestamp: December 17, 2019, 06:09:13 am
content: Quote from: Ammar on December 16, 2019, 10:42:42 pmWow that is quite the powerful little RF amp.I feel like the datasheet for that should include at least a few big red warning boxes around this monstrosity.

author: judge
author_rank: Contributor
timestamp: December 17, 2019, 02:44:13 pm
content: Quote from: T3sl4co1l on December 17, 2019, 12:09:30 amFortunately bandwidth isn't something I need to be worried about. I assume. I wish I knew more about how these LED tags work. There's a coil and an LED of course, but there are some discretes on there too, and not just capacitors! It would be nice to know what sort of voltage a typical NFC reader shoves through its coil so I know what I am aiming for. And I am assuming voltage is important in this application because although LEDs are current-limited devices, they also need a minimum voltage to get them going. And, of course, I am assuming here that it is the 13.56MHz carrier that is lighting these things up.Also, I read a lot about 'antenna tuning' for NFC readers, but my understanding is that these coils aren't antennas at all, just inductors, and I never read about 'inductor tuning', so I am curious as to what the deal is here. There is clearly more about getting the efficiency of these coils up, like apparently wanting the self-resonant frequency to be above the driving frequency, balancing range v. efficiency, high 'Q' v. bandwidth. It is hard to know which of these are relevant to my application, which has no requirement for bandwidth. Plenty to learn, and experiment with.

author: judge
author_rank: Contributor
timestamp: December 17, 2019, 02:45:53 pm
content: Quote from: Berni on December 17, 2019, 06:09:13 amI read elsewhere that 13.56MHz is used for more than just NFC. Something about welding and induction coils?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2019, 05:22:24 pm
content: Yes, 13.56 is the free ISM band in that range. It's rather narrow though, which severely limits applications: induction and dielectric heating needs an active tuner (mmm, servo driven vacuum variable caps!) or an amp with a huge tolerance for SWR; NFC and other transmissions must be low bitrate; etc.NFC at least being near field might permit more data rate up close (it could also be driven with QAM rather than ASK, giving more bits per symbol), while still meeting the limits in the adjacent shortwave bands; although I don't think anything can do that. If nothing else, a subtle modulation scheme wouldn't be easy to manage with self-powered hardware.Tim

author: judge
author_rank: Contributor
timestamp: December 26, 2019, 11:47:38 pm
content: I'm not getting anywhere making an amplifier using a single IR530, either in LTSpice or in reality. Reality matches what I get in LTSpice though (which is at most 1:1), which is always nice. On the other hand, I did manage to light up the tags by adding capacitance in series with the coil to form a resonant circuit:

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 01:39:27 am
content: Quote from: Berni on November 28, 2018, 03:50:11 pmMaking the coil a tuned circuit allows for much higher circulating current between the coil and capacitor while lowering the current through the amplifier so a less powerful amplifier can be used.

author: Berni
author_rank: Super Contributor
timestamp: December 27, 2019, 08:36:57 am
content: Quote from: judge on December 26, 2019, 11:47:38 pmYes resonant LC circuits are the way to go for wireless power transfer.The resonant nature of it causes the amplitude of the signal to rise significantly despite a small input power. This is because such circuits can 'recycle' energy really well. The inductor and capacitor exchange energy back and forth without you providing any input power. The amplitude of the oscillation only dropping slowly according to resistive looses and as much energy is transferred out to something else. So you only need enough input power to cover the system looses + transferred power.Same trick tends to be used on the receiving side too. Being a resonant LC circuit it can develop a large amplitude from a tiny input power, so very little magnetic coupling is needed to the transmitting coil in order to develop enough voltage for powering a LED. However this only works well if both the transmitting and receiving circuit are tuned to the same frequency.The lower losses you can get in your transmitting tank circuit (Low resistance coil, high Q caps etc) and the closer you can match the transmitter and receiver frequency the less input power you will need to get the same range/performance.

author: judge
author_rank: Contributor
timestamp: December 27, 2019, 09:06:44 pm
content: Further messing around with LTSpice shows me what kind of circuit I need to make the amplifier work, but with the frequency, peak-to-peak voltage and possible bias voltage I have available, the IRF530 isn't much use. The peak-to-peak input signal is around 2V, which just doesn't seem to be enough to completely turn the mosfet on/off.A mosfet I've used for other projects - the AO4294 - is much faster and has a lower gate threshold voltage. I might see if I can get that to work in LTSpice, but does anyone have ideas for a plan B? I want the circuitry to be small and unobtrusive - this is for a wearable project.

author: judge
author_rank: Contributor
timestamp: December 27, 2019, 09:52:00 pm
content: I knocked up a LTSPice model from the AO4294 datasheet andthis web site, and the results of the simulation were pretty spectacular, so I need to make up a bread-board friendly adapter for it and try it out.

author: judge
author_rank: Contributor
timestamp: December 28, 2019, 03:50:07 pm
content: This is what I have so far. Naturally it doesn't work. If I connect the source resistor (R3) the signal at pin 8 of the 74LS00 disappears. If I increase the value of R3, the amplitude of the signal at pin 8 will increase too.Before I abandon this and exploreother oscillator circuits, I would really like to understand why connecting the source of the MOSFET has this effect.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 28, 2019, 04:29:31 pm
content: Quote from: judge on December 28, 2019, 03:50:07 pmThe input capacitance of your mosfet is about 2.4nF or 5 Ohm at 13.5 MHz

author: judge
author_rank: Contributor
timestamp: December 31, 2019, 04:53:16 am
content: On to a Pierce oscillator, or at least that what this is supposed to be. The oscillation dies down after about 10s, but until then I am getting good range from the coil (L1), with the LED tags lighting up at about 1cm from the coil.The values of RFC, C2 and C3 are pure guesswork. 'RFC' is not really an RFC, it is just an inductor I happened to have lying around. C2 I set to 2x the load capacitance, minus a bit for stray capacitance. I also tried 1x the load capacitance and I also tried nothing - it doesn't make any difference. C3 was just a capacitor I happened to have lying around.So I have a couple of questions:How do I calculate what values these things should have?Why do the oscillations die out?

author: judge
author_rank: Contributor
timestamp: December 31, 2019, 06:37:43 pm
content: I think the answer to (2) is that the mosfet overheats...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2019, 06:42:49 pm
content: Would guess it's marginal and temp rise causes gm loss. Crystal has much too high an impedance (~kohm) for this (the transistor is ~ohms between its terminals), and even without matching network(s) the drive level is too much for most crystals (i.e. it's likely to shatter).Replace the crystal with an L+C and tune as closely as you can? A buffer circuit would be neat but would require more components and matching LCs to realize.Tim

author: judge
author_rank: Contributor
timestamp: December 31, 2019, 08:24:35 pm
content: Quote from: T3sl4co1l on December 31, 2019, 06:42:49 pmThanks for nudging me to use L+C, I was already concerned that I was probably abusing the crystal. Part of me wants to get something going with the crystal - so I can understand what I am doing. At the moment I am just blindly following recipes, so I stand no chance of figuring something out if it isn't working right. On the other hand, I'm pretty sure that using L+C is what I would end up doing anyway. I doubt that the exact frequency matters much in this application. I guess I'll find out!

author: judge
author_rank: Contributor
timestamp: January 02, 2020, 04:15:01 am
content: As I now understand it, the temperature rise is because the MOSFET is never turned fully on. If I replace the crystal with a series L+C resonant pair, I get the same behavior. If I replace it with a parallel resonant pair the MOSFET runs cool, but it oscillates at around 4MHz instead of the 13.56MHz the pair are tuned for. This is with a supply voltage of 5V.I'm going to also try some JFET-based oscillators (using the J310), but maybe I should also play around with RF amplifiers like theBGA2866?

author: Berni
author_rank: Super Contributor
timestamp: January 06, 2020, 07:00:16 am
content: You likely want the oscillator and power output stage to be two separate circuits anyway.Using logic gates around crystal is a pretty good way for making a stable oscillator. But a MOSFETs gate needs a fair bit of current to turn on and off that quickly. So you need a extra logic buffer between your oscillator and the transistors gate. A common trick is to parallel multiple gates of a logic buffer or hex inverter chip to get more output current capability.The MOSFET choice seams pretty reasonable as it does have fairly low gate capacitance for how powerful it is, but that is still 2.4nF and so still needs a fair bit of current to switch at >10MHzAlso take care with doing >10MHz RF work on a breadboard. The crappy parasitics of it can certainly make this fast analog circuits act weird.