Quantity   Component name
1 × Kinetis KL25Z64 Arm Cortex M0+ STM32F4 also works
1 × Custom PCB supporting USB plug or Socket A KL25Z freedom board works with simple modification, a Teensy-LC might but has a slightly different chip
5 × Buttons leads twistetd 90 degrees to align with rather than destroy breadboard contacts
1 × (optional) Second KL25Z wired as a USB device and used as an SWD programmer to update the first