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

[floobydust]

It looks like the device is for Electronic Article Surveillance (EAS).  Low frequency is 58-132kHz and used inside metallic objects that would normally block RF tags.
There are some Acousto Magnetic tags that might function like this one, incoming signal and retransmits that.

[DrG]

Anti-theft device. the coils and the two caps are resonant circuits for the theft detector at the doors. The IC and resistors are pure camouflage (very usual back then).

Camouflage for what? The device is concealed inside the speaker, what would be the point of adding gratuitous components?

They are all connected properly as the OP said. It is a frequency divider using a flip-flop. The resistors are probably for bias or something. What's odd is there does not seem to be a way to disable the device externally if it is a theft alarm trigger.

It is a puzzle and one with no schematic and with low resolution photography

A dummy 4013 is not credible to me.

I was reading about the history of EAS https://losspreventionmedia.com/electronic-article-surveillance-eas-source-tagging/ and don't see anything like this mentioned.

I have seen a circuit where a 4013 was used as an amplifier:



So, assume, for the sake of argument, that it is some kind of beacon / transponder that is detectable by something like a simple AM/FM radio. How long would that 3V last once activated? A week?

I suppose it could have some use to tell whether some returned item was "ours" (without opening it up) and had not been voided?

It would be an interesting experiment to draw out a schematic and put a fresh battery in it and see what you could detect. In any event, the difficulty in finding any record of such a device (at least so far) is, in a sense, testimony to its limited efficacy.

[mathsquid]

What's odd is there does not seem to be a way to disable the device externally if it is a theft alarm trigger.

Crazy theory: Could one of the components on the board be a tiny reed switch that is held open by a small magnet?

If so, then maybe the speaker is secured to the shelf with a thin loose cord/chain, like a pen at a bank, but instead of a pen, the end of the chain has a disk containing a small magnet. The disk is held to the speaker with a weak adhesive that holds it in place but breaks loose if it is yanked.  The coin cell would only need to work while the unit is being stolen.

[james_s]

They are all connected properly as the OP said. It is a frequency divider using a flip-flop. The resistors are probably for bias or something. What's odd is there does not seem to be a way to disable the device externally if it is a theft alarm trigger.

Maybe they used a high power signal to burn it out like they do with the passive resonant tags? Or maybe they just shut off the alarm when they know a legitimate customer is taking something out the door, or maybe it was only used to prevent people from trying to sneak stuff out the back/side entrance.

[MrMobodies]

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.

I didn't see that but wanted see a sketch.

Anyway just out of interest to see where all the tracers go:

Code: [Select]

https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWKswDZ0A4BMYDMAWfAdiIE59UFcQtIQl9qEBTAWjDACgAbEV01CAF9hWfHToQJPPqnwhxdDHUVRwayJwDus+atZyQuBFijbdRkyEPHTmgMbX5t8GFMuJseFK8+4XRw53KzQ6D2R4SD8oiKjOACcaDAwjUyxk8HS1MHgEpJS3FPSi8WzcnUNDZSczCvlqhuk6oUFq4U1m0JBqro6asEJuyUG+w0KhzJS+8ZnB8emsgfkl8BHzGdRh5c3ayZbV+Xb1xbcD1ztjlJ7B6oWUscHDPuLUmkgU3AudWhT8IvfwDtngCuiteuZqqoxCo4LsXlDSqo+uxBFgiKYUeBSF8jKgrp8FGhugS+tdDoIVn1hCthEcdLTWnQ6QoCRg-gpYWypuZDD9zm9uTogpZTMK+c8Mi4uh5zC8XPDYRKClksAgKVlnmq9qrUei4Vq0RiiYbduNqVknjz5HyxkDOI5MWBsXwiZTYv5YtEAi6KYNMe13TEYB7g3EdP7BJj2ciiez+IJo+Z4woUsnmY6-Q95MisxYdbsDNatcn88jRMXRKU7vyzYJq1U6JahVlGhNSUoOzVRvJKs4LgA3GoEvkktSlagSZAIcwE3CQPv8voE4VjU5L0WnVlNV5x2OC-KvT5hHFHw+0M7riYExPNgqnGYaq38npr2Xj+eHj-dvbs+bmX9ZL+zp9EBRoUsBb5hB+cpfrKkrQRk0JwvB1qSlYwIfGkAKJkAA

Can't read to see if they are capacitors or what values there are and wouldn't know how to implement the logic and stuff to make the circuit flow in Falstad.

[floobydust]

OP would have to give us the capacitor values. One of the polystyrene caps is 680pF. It looks like tuning to a couple hundred kHz, below AM radio band.

I think it just sits there quiescent until the RX coil gets enough signal to clock the FF which generates output to the TX coil. The two 10MEG's add a minuscule amount to the off current drain.

[james_s]

I bet a coil of wire connected to a function generator could be used to tickle it. Monitor the power consumption of the device and sweep the frequency until you see it do something.

[Cyberdragon]

I bet a coil of wire connected to a function generator could be used to tickle it. Monitor the power consumption of the device and sweep the frequency until you see it do something.

Perfect use for a dip meter.