Hudson from NYC Resistor was inspired by Beth’s avrfid.S project to try to build a replacement for the multiple HID Prox card he carried for work. Hudson notes that “Beth’s design is simultaneously a technical tour-de-force and an example of how badly we can abuse the Atmel chips.” The entire schematic consists of an ATTiny85 and a coil!
He used the Bus Pirate to help programming the ATTiny85. “Once the fuse bits have been configured to use the RF waveform as the clock source the chip will no longer be programmable with a normal AVR ISP. One option is to use Dangerous Prototypes’ Bus Pirate, which can provide a “recovery” clock during programming. Unfortunately it didn’t work for me with the current release of avrdude; I had to make the following patches to the avrdude/buspirate.c source to get it to work.”
Read Hudson’s post on NYC Resistor’s website for complete details, code and more photos.
Kevin via the contact form.