So i managed to shoot a cheap ikea rothult on eBay.
I can “add” other tags or my phone, but I’m not able to read x series tags.
I took it apart to see what I can do.
Here you have the pics in case anyone was curious how it looks from the inside.
First approach will be adding a different antenna coil.
I’ve got one in a similarly disassembled state sitting on my desk, too! I haven’t spent much time messing with it, but I have found that the two pins that don’t seem to be attached to anything on the antenna board (right-most, if you are looking at the front) are the electrodes for the capacity sensing feature of the NFC chip (seems like there are two versions out in the wild, mine was an ST25R3911B).
It’s likely the low-power card detect feature of the chip is using the capacitive sensing feature; it also seems that sensor calibration is tuned more for real cards than for hands. When I was able to trigger the capacitive sensor (and wake the rest of the chip), the read range was improved a good amount (I didn’t measure the distance, but initially I had to press my implant directly against the PCB antenna, and even then it wasn’t reliable). I would like to try changing the electrode geometry to see if the sensitivity could be increased, but haven’t made the time to experiment. On the other hand, there is always the option to reprogram the board entirely
That antenna board looks so easy to switch out. It’s just screaming for a replacement
Also looks similar in size to a pn532 pcb
I really want to try and make a book safe, that is held shut by one of these
Range would need to be increased, along with someway of adding some form of structure to something made of cellulose
Glue! Lots of glue!
Right, but at some point the tension force will just cause the latch to delaminate
Glueing something to paper with any meaningful resistance is tricky
Insert a wooden box into a larger book. The lock goes on the wood, not the book. It probably doesn’t even need to be a box, just the sides of the box.
I think you’re missing what I want,
I want the cover of the book to stay shut when locked,
If someone pulls on the cover with even light pressure it the cover will likely pull away from the glue, like a shitty sticker when you try to peel it because paper sucks… and will just delaminate
Attach the rothult to the cover, with the latch so that it slides into the paper edge of the box opposite the spine. Glue a metal or wood strip with a hole in it over the edge of the box. The latch will slide into the hole, locking the book.
Had some copper tape sitting around, so I tried the most dead simple thing to change the capacitance electrodes:
The change is minor, but I can get consistent reads off of a Spark 2 and xM1. Removing the tape left me with the normal sometimes-does-sometimes-doesn’t read behavior. Seems like you need to trigger capacitance and one of the other detect methods before the chip will power up for a real read.
Interesting
you may notice significant loss of battery life 
Just wrap your hand in copper foil and… 
You mean I can’t make up the power draw difference with hopes and dreams?
Thinking about the book-safe use case, it would be neat to jam a Qi charger (or, I guess, receiver?) in the book cutout with the rest of the guts to address the battery issue. I wonder if you could integrate the antenna (and capacitance sensing) into some sort of faux gold-leaf decoration for/on the cover. I’ve got the copper tape and a vinyl cutter (which can do ‘okay’ at cutting circuit traces), guess I need a passable antenna design
Very disappointing the range on this - Ikea show a metal locker using this but I couldn’t get more than pressing the card directly on the device. Shame as it’s nicely made, so I bought a AAA battery USB 5v substitute and control that with a Sonoff USB Smart plug. Adjusted to “inching” on-off at 2.5 seconds it opens and closes remotely with my phone or Alexa.
