Thursday, January 20, 2011

Why does my tooth brush have a wireless network?

My toothbrush (Braun Oral-B 5000) has been displaying a nagging "Replace brush head" message recently. I initially though nothing of it. Almost all consumer electronics will nag you when some consumable needs changing.

But then I thought: how does it know? It's not like the head is wired in. It's deliberately detachable for cleaning. There are no electrical contacts. Could it be there is a RFID chip in the head and a RFID reader in the tooth brush?

Sure enough the Braun Oral-B 5000 features a RFID reader and RFID chips in the heads – a toothbrush area wireless network!

Out of curiosity I wrapped about 1 meter of magnet wire (about 30 turns) around the head and connected it to an oscilloscope. There is a strong excitation signal at about 13MHz for about a second after the brush is powered up.

A quick google of  "13MHz RFID" does reveal that 13.56MHz is popular RFID frequency. Unfortunately decoding this signal seems involved. There is an open hardware sniffer project here: http://www.openpcd.org. However I don't really have the time or incentive to hack my toothbrush right now.



An oscilloscope trace from the coil wrapped around the toothbrush head. A strong signal of about 75ns period is observed for about 1 second after the brush is switched on.

2 comments:

Jenna said...
This comment has been removed by the author.
Marc said...

You didn't happen to work out what protocol the toothbrush used to talk to the "Smart Guide" by any chance?