Wednesday, August 12, 2020

gherkin express

What's in your wallet?

A tiny credit card sized TMK powered gherkin.

The Gherkin Express is running an ATmega32U2 running at 16MHz. There are 30 5.2mm tactile switches. (31 including the reset switch). A PTC fuse to protect from power shorts and a power LED. All surface mount components are 1206 sized.

There are no diodes. Unless you have really tiny fingers you can only press 2 keys at a time, and ghosting will not be a problem. The MATRIX_HAS_GHOST flag is turned on in the firmware, which should prevent erroneous keystrokes if you do manage it.

There is a footprint for a ICSP header which I left unpopulated. Only needed if the factory USB bootloader on the ATmega32U2 gets corrupted, or maybe install a mass storage bootloader.

The 4 unused pins are brought out to little pads. (TX, RX, PD5, PD6)

There is a BOM in git. The TMK firmware is also on git.

The PCB is slightly smaller than a credit card. The PCB material is 0.6mm thick. Fully populated it is 2.6mm at it's thickest.

JLCPCB will produce a 0.6mm PCB at the same price as the common 1.6mm, but only in green solder mask. Due to lower volume, any other color comes with a setup premium. Since this is under 100mm square it cost only $5 for 10 pieces.

The back has no components. Everything is surface mounted on the front of the PCB. The circles and holes are cosmetic.

The tallest components are the 16MHz ceramic resonator and the 4.7uF capacitor. I have ordered some lower profile parts. Hopefully I can get the total thickness down to 2.1mm. (0.6mm PCB and 1.5mm tall switches.)

This is considerably thinner than the Flanck.

I am using the USB PCB connector library from 

Pinuct also has USB-C connectors.

Another project using this library is the Fluff M0 project.

The 0.6mm thick PCB slides right into the micro USB connector.

This connector is not practical for heavy use. For an emergency input device it should be fine.

Get PCB pricing on PCB Shopper.