More details on B5

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

Second generation of the NFC business card adding audible feedback

greggreg 04/20/2021 at 01:190 Comments

My progress been delayed by samples getting stuck in customs.  I would have preferred to have tested out the design before sharing more, but I can at least provide some of the theory of the design that I ordered from OSH Park.  It looks really good in after dark.

Here is the schematic for the design:

As I mentioned in the last post, I included two buzzers as a stuffing option.  You can see how they overlap in the picture of the board.

Another interesting thing to note are the three resistors, R5-7.  These are another stuffing option I will be testing.  As shown they are three different load resistors that I could use for impedance measurements.  I am hoping that the loading of the current to digital circuit will be sufficient for my measurements so that these are not needed.  If they are not needed as loads, I may use them as reference values for calibrating the impedance measurements.  I expect reference impedances will be more useful than the programmable loads.

The current to digital circuit measures current integrated over time which is perfect for counting coulombs into a capacitor.  The voltage rise per coulomb is proportional to the capacitance.