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February 10, 2008

Shoplift Alarm Sensors

I have seen many times those little anti-theft devices stuck to PDAs, digital cameras and all sorts of small consumer electronics products and I always wondered how they worked and imagined they were very complicated systems involving delicate glass switches (for detecting violent tinkering), optical proximity sensors (for detecting the change of distance/reflectivity between the device and the protected product), impedance meters on the alarm system base (for detecting the change of impedance when someone replaces the device by an equivalent circuit), etc.

The other day I was lucky enough to see (and even photograph) a broken shoplift "sensor", as they are called. I discovered that they are very simple and not that secure.

Here are my findings

Original Pictures:

How I think the system works:
The above diagram is self explanatory and it shows how simple and easy to defeat they are. The system only involves a simple metal switch and a big plastic button to operate it.


cibomahto said...

Wow, thanks for sharing, that is pretty eye opening. I also assumed that they would be more complicated! Of course, if they are watching for the diode drop then you can't just short out the connection at the wire base. If thats the case, then it is probably still a good deterrent; I would imagine that glue tape is hard to remove (of course, with the proper solvent...)

However, if all they were designed to do was to make it so someone needs to take an extra 30 seconds to remove a device, then maybe they are an adequate design.

Carlos said...

The sensors are connected to a base with many connections. When not all connections are used, the empty ports (RJ-14) are filled with connectors containing only a loop of wire (that closes the circuit presumably). So I don't think they are actually monitoring for voltage drop across the LED. Rather, the circuit must be perfectly closed (all ports are closed) in order for the alarm not to be triggered.

amine said...

I want to say that I have enjoyed your blog (actually I went through many many of your posts). Nice Job. Keep on posting.
Good Luck.

Carlos said...

Thank you amine. I'm glad you like my posts.

sandipsingh said...

Hello Mr.Author {Carlos} ;

i really read your blog for the first time and found this blog very attractive + powerful. I also want to let you know that you really post a good content + the one about Alarm Systems is very good; it really points out the security holes people face now.
So, keep the good work and post and post and post so that we can read and get new information from you and get to see new content each time we visit your blog.
Enjoy Blogging !!

Sandeep, An Autoresponder Designer

Carlos said...

Thank you for the kind comment Sandeep.