Hackaday picked up this story of a PCB printed directly on a quarter! Which state quarter would make the best blinky light?
As a base layer the printed five passes of insulation on the coin, before printing the traces. Holes are left in the insulation to create a form of via that connects to the coin. On the board is an ATtiny2313 microcontroller that flashes an LED, and on the reverse side of the coin is a CR2032 cell that’s secured with a set of bolts and washers. You can see it taking shape in the video below the break.
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