Kevin Tang made this circuit board business card with an NFC chip and LEDs. One nifty trick of the card is that it’s battery-less; when tapped to a phone, in addition to sharing your info, the card will draw power and light up the LEDs!
Here’s what’s actually happening, and it took me a while to stop finding it magical (I haven’t, really). An NFC phone doesn’t just listen for a tag—it broadcasts a strong radio field to power the tag, because the tag has no battery either. The chip in a normal NFC sticker wakes up on that borrowed energy just long enough to answer, then goes dark the moment the phone leaves. What I didn’t appreciate until I built one is that some chips will hand you the leftover power on a pin, to do whatever you want with. NXP’s datasheet lists a typical harvested output of about 5 milliamps at 2 volts under one test setup[2]—not much, but plenty to light an LED, or blink a low-power sensor, or wake a tiny microcontroller for a fraction of a second.
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