Marshall Brain's Blog

---

Memory sticks

I have four different Sony devices that use Memory Sticks (Sony's packaging for flash memory), and I had started using Memory Sticks as a form of floppy disk. It's the same idea as the little USB disks built from flash memory. The nice thing about a Memory Stick, as opposed to a floppy disk or a USB disk, is that a Memory Stick is small and rugged enough to stick in your wallet.

So I had a Memory Stick in my wallet and it was happy there for a long time. Then one day it stopped working. I replaced it, and then the second one stopped working too. When the second one blew out, I figured out what is happening.


Using a card reader to get in the door at the office

In a lot of offices now, the doors are controlled by card readers. These readers use electromagnetic energy to power the card, in somewhat the same way that a crystal radio uses nothing but the power of an AM signal to power the radio. The reader on the wall creates an electromagnetic field. When you hold the card up to the reader, an antenna in the card picks up the field. The field is actually strong enough to power a circuit inside the card. Once it is powered up, the card transmits the code number of the card back to the reader, a computer in the building confirms the code and the door opens.

Instead of taking my card out of my wallet, I had held my wallet up to the reader. Apparently the electromagnetic field is strong enough to destroy the chips inside a Memory Stick. By extrapolation, I would assume that these readers might also be dangerous to smart cards, compact flash cards, etc. that you keep in your wallet. Moral of the story: remove the door card from your wallet before you use the reader.

Now that I had two blown Memory Sticks, the obvious thing to do is take one apart. Here's what's inside:


Inside a memory stick is a tiny printed circuit board

The chip on the left is Fujitsu part number 30LV0032 0052 M75 PFTN, which is the flash memory. The chip on the right is Fujitsu part number MB86188, which is a microcontroller.