Integrated CR2032 Holder Keeps This PCB Business Card Ultra Slim

Printed circuit board "business cards" are popular among engineers, with each rendition having a unique take. Sometimes they are purely decorative, with clever uses of solder mask, silk screen, and copper layers to achieve an eye-catching design. And sometimes, they contain low-cost clever circuits. So, it is interesting when a new idea comes up, like this one. Dennis Kaandorp's PCB business card has an integrated battery holder, an LED matrix, and a microcontroller.

Kaandorp's card measures the same size as a typical credit card. On the front are 42 dual-colored LEDs, four push buttons, and a microcontroller. The back has a QR code that links to Kaandorp's LinkedIn profile. But the most innovative feature is on both sides: the battery clip holder.

PCBs are typically made from FR4 and are somewhat stiff. However, this design uses a cutout to achieve a flexible arm design. As a result, the arms are large enough to hold a coin cell CR2032 in place but also flexible enough to replace the battery! And for safety, Kaandorp even included a MOSFET for reverse polarity protection.

The LED matrix has an intriguing design choice as well. Typically a matrix like this one would use the Charlieplexing technique. A Charlieplexed matrix would only need 10 GPIOs to control 42 LEDs in this case. However, there are some trade-offs with that type of matrix, such as limited voltage available for the forward voltage drop and complicated routing. So, Kaandorp adopted a hybrid approach of Charlieplexing and traditional matrix, which required only three more GPIOs.

Why 42 dual-color LEDs? Kaandorp picked the value because it is the same number of elements necessary for a Connect 4 game board.

A Microchip ATtiny1616 drives the matrix and reads the input buttons. These 8-bit microcontrollers have up to 22 GPIO lines which worked out great in this PCB business card design that needed 17.

The assembled cost is $5.12 in prototype quantities and $3.50 in quantities greater than 25. You can download the design files from the links at the bottom of this Bigfoot Creations blog post. You will, of course, want to change the contact info to yours!