Submitted 3 weeks ago by rumba@lemmy.zip to showerthoughts@lemmy.world
Could I make a little gun and just walk around through the parking lot and aisles of the supermarket and freeze all the carts in place?
Shopping cart wheels here only freeze for one reason: crappy bearrings that get jammed up when people run through pools of spilled grape juice and maple syrup.
They always make that one sound
Oh wow yeah. Like nails on a chalkboard!
I always assumed it was a powerful magnet that was buried in the parking lot. The magnet triggers the lock. That seems the cheapest, lowest cost way. They have locking carts for over 30 years so it is unlikely to be a new technology.
Im not sure exactly how the system works, but if I were designing one, there would be 3 approaches I can think of.
The first is to equip the lock with a GPS system and dictate that it locks if it’s not within range of a particular location. This one would be the most expensive to implement, but should come with minimal opportunity for messing with it.
Next down the list is each lock is equipped with a radio to connect to a wifi or sub-GHz broadcaster, and as soon as it misses enough heartbeats to a central control point, it locks the wheels. This could be disrupted by jamming the signal, but jammers of this type are highly illegal, and easily trackable.
Last is the cheapest option, which is to include an RFID module tied to the lock and a system to broadcast a signal at the perimeter. If a cart comes within range for a long enough period then the RFID tag is activated and the wheels lock.
I suspect it’s probably a sub-GHz radio situation, with the broadcast power tuned to be within a few 100 meters of the store. If you had some kind of SDR you could probably pinpoint the signal they use and repeat it, letting you wheel a cart outside the zone, but as soon as you stop the signal the wheels will lock.
It looks like they’re SubG. The lock signal broadcasts on the buried line, and at least in the one manual I found, they can also lock all, lock all in a lot, and optionally lock singles via serial number (there’s an inventory system). There’s a directional handheld device that can be used within a couple of feet of an affected cart.
Oh man I love the page view counter at the bottom.
I use this constantly by the way heh
Bluetooth speaker with a coil instead of a speaker?
I wonder the same thing. What I wonder even more, though, is why you think inconveniencing others is acceptable or funny?
acceptable or funny?
didn’t say acceptable or funny
I just wondered it.
It sounds like something you could accomplish with a device like the Flipper Zero.
Maybe, it’s actually pretty limited compared to a real SDR.
This might actually be a good opportunity for an AI answer:
–
First, understand the basic principle you’re working with:
Shopping cart wheel locks respond to a specific RF (radio frequency) signal (typically around 7.8 kHz to 10 kHz for many systems, but it varies by brand).
In normal operation, the boundary wire emits this signal constantly.
The unlocking posts emit a different coded signal to reset the wheel.
So to design a portable device that could lock and unlock wheels manually anywhere, you would need:
You must figure out the exact frequencies and modulation patterns used by the cart system.
Different companies (like Gatekeeper Systems, Carttronics, etc.) use slightly different RF signatures.
Usually lock signal = basic broadcast (constant tone) Unlock signal = special modulated unlock code or sequence.
This might require:
A spectrum analyzer
Or reverse-engineering an existing unlock station.
The portable device needs:
A programmable low-frequency RF transmitter (around 8–10 kHz range, low power)
A microcontroller (Arduino, ESP32, etc.) to generate the right modulation if the signal is coded
An amplifier to make the signal strong enough to reach the cart wheels (they usually detect it within 3–10 feet)
Optionally:
Two modes: a “lock” mode (sending the boundary signal) and an “unlock” mode (sending the unlock code).
Low-frequency oscillator (to generate 7.8–10 kHz wave)
Microcontroller (to handle switching, coding)
Coil antenna (for strong near-field transmission — like a small induction coil)
Amplifier circuit (boost the signal so carts sense it a few meters away)
Battery pack (portable, rechargeable — say 12V lithium-ion)
Mode selector switch (Lock / Unlock)
Simple microcontroller code to:
Emit plain carrier signal = Lock mode
Emit modulated code = Unlock mode
Example (pseudo-Arduino code):
if (mode == LOCK) { generateContinuousTone(8kHz); } else if (mode == UNLOCK) { generateUnlockCode(); }
Matching signal format exactly: If the unlock signal is digitally modulated (like FSK or ASK), you must match the timing perfectly.
Power levels: Too much power could fry circuits or interfere with nearby electronics. Keeping it safe is key.
Legal considerations: Broadcasting radio signals, even low-power ones, can be restricted depending on the country.
Simple Concept Sketch:
[Portable RF Cart Lock/Unlock Device]
Size: About a large walkie-talkie
Two buttons: Lock / Unlock
Coil antenna: About 3–6 inches
Range: 1–3 meters effective
Display: Small LED indicators (Locking / Unlocking / Signal OK)
In short: You’re essentially building a portable invisible “fence” generator with two channels: a “keep away” signal (lock) and a “come back” signal (unlock).
I thought about doing this the day before Thanksgiving or Christmas, imagine the shitshow
I’m honestly shocked this doesn’t happen.
That would be hilarious!
You could probably do it with a Flipper, maybe? They work on radio waves and I know you can use a Flipper to, like, change someone else’s TV channel or open their garage.
Bubbles has joined the chat.
seathru@lemmy.sdf.org 3 weeks ago
Here’s a decent rundown.
hackaday.com/…/tech-in-plain-sight-shopping-cart-…
rumba@lemmy.zip 2 weeks ago
Yeah, I just watched a youtuber take apart a modern one. Found the maker and the manual for the system. Then watched some more people taking the system apart (including the one you linked)
Neat stuff:
Looks like it’s <9 Khz signals (7.2khz in the one I looked up), at that frequency, you can do a replay attack using your phones speaker from 0 distance.
the electronics in the wheels are potted, including the battery so they’re disposable at EOL.
Apparently, the unlock signal is just an inverted lock signal. Easily detectable.
Their system has the ability to lock/unlock all carts, only the carts left in the parking lot overnight.
One of the features available is for them to identify and freeze a particular cart if it doesn’t go through a checkout.
QuarterSwede@lemmy.world 3 weeks ago
That was surprisingly interesting.
felixwhynot@lemmy.world 3 weeks ago
The hacker who did the Def Con talk linked in the Hackaday article also did another one, they are both very interesting