Course Correction

A project log for Beeping, Blinking Business-card Badge (B4)

Second generation of the NFC business card adding audible feedback

greggreg 07/30/2023 at 03:410 Comments

It has been a while since I posted on this project.  I had some inspiration at CrowdvSupply Teardown 2023 and have been putting in some more time on this.  I’ve got a few issues with the NXP micro.  It only generates 1.8V from NFC which is only enough to drive a red LED without a voltage doubler.  It is also missing support from their SDK on GitHub.  

With the WCH CH32V003 available for about a dime in volume, I could use it with a I2C NFC EEPROM for about the same price, and drive an RGB LED without a voltage doubler.  The CH32V003 is not specifically a low power device, so I decided to see if it would be feasible to drive this from the NFC harvested power.  It only runs down to 2.7V, so I need to know if the NFC chips can deliver that when loaded.  There are a few options available with energy harvesting output.  I’ve used the ST25DV in the past, and NXP has a couple similar devices like the NT3H2211, or NTP53321.  The data sheets are pretty sketchy on how much power is available, so I decide to measure it for myself.  

I designed a board to test a couple of the device.  The ST25DV and NT3H2211 are both designed with 50pF load capacitance, but the NTP53321 is designed for a different load cap, so it would require a different tuned antenna.  I already have an antenna I’ve used for 50pF load, so I designed a board to test the two 50pF chips.  Unfortunately they have different pin outs, so I had to put down two footprints in parallel.  I added the CH32V003 and a current sense amp so I could see how much current it is using.  I used my badge as a template and shrank it to save a little on the OSHPark price.  I kept the bannana plug/alligator clip pads for measuring voltage and current.  I’ll share more about what I found in another post.