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13.56MHz RFID power transmission wearable

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leumas95

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So I have a project I'm working on a project to light some NFC fingernail lights.

They light up when in the presence of a 13.56MHz RFID field. The issue is other than my phone, I have no way to light them day to day.

I would like to develop a device I can hold, like fist semi closed and it would light up the fingernails through my fingers.

I have had some success with a Pierce Occiliator, infact it 100% works. The issue is to get a brightness I am happy with I need to provide ~30v from my variable power supply. Unfortunately given I want this to be compact I don't think this circuit will work for me, or at least I don't know how to tweak it to work better for my usecases.

I haven't had much luck finding anything helpful in the internet. Everything relating to RFID at the 13.56MHz frequency focuses on reading and writing that I found.

Also a stretch goals for me would be to instead have a bracelet that if I curled my fingers towards the wrist it would still be able to power it but I'm not sure how realistic that is.

Any guidance would be much appreciated. I am fairly competent at wiring things up / following schematic, also have some experience making my own antennas (thanks to this project) and that sort of thing but the RF stuff kinda goes over my head.
 
If I understand correctly seems that you need a small spiral antenna (tag) for 13.56MHz which is fed by an RF signal coming from a crystal oscillator.
To get the maximum efficiency from this system (maximum radiated power), the spiral antenna should be tuned on 13.56MHz using a capacitor.
The application note from ST shows you how to build the antenna, and provides values of the resonant capacitor and of the coil:
You need a simple LC meter to measure the values of the coil inductance and of the capacitance (Ctun), because is important that they have the right values, otherwise you loose efficiency.
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00221490.pdf
https://www.aliexpress.com/item/32827008101.html

If you don't want to build them, you can buy the oscillator and the antenna tag from here:
https://www.digikey.com/products/en...onators/oscillators/172?k=13.56MHz oscillator
https://www.digikey.com/products/en/rf-if-and-rfid/rfid-antennas/855?k=13.56MHz antenna
 

Thanks for all that info ?.

I have an LCR meter, the circuit and coil I made is correctly tuned to 13.56MHz. I have also tried multiple antenna shapes. As I said I have a working system, I just am struggling to get the range I want through my hand given the size of the antennas on the fingernails without providing quite a lot of input power.

Sorry it seems the core of my question was lost, I just wanted to give enough information.

A more susinct way of phrasing my question is:

What is the best way to create a large amount of RF output from a relatively compact portable power source.

For example I have considered making a DC to DC multiplier but I'm not sure if it would provide some sort of interference etc.
 

You can try adding a transistor amplifier to amplify the signal from the oscillator, and if it works as you want can let as it is. Increasing the DC supply voltage will increase somehow the output power that goes to the antenna, but this method should be the last option.
The most important thing in a system like this is the efficiency of the power transfer between the tag (coil) transmitter antenna and the receiver antenna. Have to check that the transmitter antenna resonate with the tuning capacitor on the 13.56MHz, as is mentioned in the ST app note that I posted. Beware that the resonance of the transmitter circuit is affected by the hand which hold it (as you mentioned), so some tuning may be necessary for this situation.
 

Wireless power transfer at these frequencies is by inductive coupling, so you want to maximize the k factor between the coils, and maximize inductor Q factor (low AC series resistance).
 

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