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Bought some old speakers from ebay - here's what I found inside

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Bud:
The coils look similar to modern RFID antenna coil. I'd speculate this could be some sort of a transponder device which , when energized externally by a reading device , emits a response at a different frequency, which the reading device can receive. Now, what the purpose might be is one's guess, perhaps wireless inventory tracking or anti-theft  as was suggested.

floobydust:
The problem is the coin cell isn't going to last very long, so whatever this device does, only good for say 5-10 days max.

I can't see any mic input or the crossover network inductor being next to it, if that modulated the RF carrier for a bug.
Maybe the device is used for tracking the shipment to see where it ended up? Like a foxhunt TX.
The battery brand name is odd, Venture Technology?

edit: I sketched a schematic, only one FF is used.
The RX coil is the input clock, and the other is a TX coil at 1/2 frequency I believe. So it needs excitation say 2MHz and it transmits 1MHz perpendicular.

ucanel:
I do not think it is a spy thingy but for who search for a mic
there is a giant mic right there.

tkamiya:
Most likely is then:

The speakers were probably on-shelf display.  There are some anti-theft stuff that sticks to side of merchandise and when the item is yanked off, sensor picks it up and sounds alarm.  It makes sense that this puck is the item side of the set.

james_s:

--- Quote from: floobydust on May 17, 2021, 05:21:35 am ---The problem is the coin cell isn't going to last very long, so whatever this device does, only good for say 5-10 days max.

I can't see any mic input or the crossover network inductor being next to it, if that modulated the RF carrier for a bug.
Maybe the device is used for tracking the shipment to see where it ended up? Like a foxhunt TX.
The battery brand name is odd, Venture Technology?

edit: I sketched a schematic, only one FF is used.
The RX coil is the input clock, and the other is a TX coil at 1/2 frequency I believe. So it needs excitation say 2MHz and it transmits 1MHz perpendicular.

--- End quote ---

This makes perfect sense for an anti-theft device. There was probably a transmitter around the doors so if somebody tries to carry the thing through the door, that transmitter excites the device planted inside the speaker which then transmits a signal on a different frequency which triggers an alarm.

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