Control shopping cart wheels with your phone (2021)
29 comments
·August 22, 2025Freak_NL
What makes this such a localised phenomenon? Locking shopping cart wheels just aren't a thing here in the Netherlands (or neighbouring countries). It used to be that most required a €1 coin inserted to unlock its link tethering it to the next car in the row, but then covid happened and a lot of shops simply disabled those locks and concluded that the system worked better without — probably driven in part by an increasing number of people who don't carry any cash.
Losing a cart is expensive, but it doesn't seem to happen at the scale that would make a full blown locking wheel solution cost effective.
isoprophlex
There used to be a supermarket that had these near my student housing complex in Utrecht (the Netherlands). Only place I ever encountered them. This was 20 years ago tho.
It was completely counterproductive, too. The edge of the zone was about 50% of the way home. Out of spite, we'd push the cart up to the edge, and leave it stranded there, carrying everything the last 200m ourselves.
Not proud of that in retrospect; it goes to show that you can't stop assholes with technology.
ars
[delayed]
stevage
I didn't watch the talk, but wondering if someone can explain this line from the post:
> Since 7.8 kHz is in the audio range
What is "the audio range" in the context of radio frequencies?
yonatan8070
I believe this is referring to the human ear's frequency range, so 20Hz-20kHz, which is a range that phone speakers can produce pressure waves at. I didn't watch the talk either, but I'm assuming that one of the following cases is true:
1. The phone's speaker generates a small amount of EM intereference at the audio frequency it's playing at 2. The sound waves hitting the locking electronics cause them to vibrate at that frequency and pick up random noise from the environment as a signal.
Either way, by using a frequency between 20Hz and 20kHz, everyone has some kind of "transmitter" that can generate mostly arbitrary waveforms.
jadamson
> Since 7.8 kHz is in the audio range, you can use the parasitic EMF from your phone's speaker to "transmit" a similar code by playing a crafted audio file
The range of human hearing is about 20 to 20000 Hz. As a by-product of producing physical vibrations at those frequencies (i.e. producing sound) via an electromagnetic coil, a speaker will produce an EMF with the same frequencies.
luca4
It's not really because it's "in audio range" but rather "in a range which your phone speaker can transmit" which "happens" to be in audio/hearing range we can mostly hear for obvious reasons.
tclancy
That talk was incredible. Thanks for posting this and now I want to find them in the wild.
al_borland
The Kroger by my house as these (or ones that look very similar). I generally avoid that store for many reasons, but I’m tempted to go there just to try this out. This is a few years old now; I wonder if they changed the tones.
mofunnyman
If Defcon talks about hotel security have taught me anything, it will never be changed until the store is bulldozed to build a bigger store.
Liftyee
This perfectly embodies the kind of hacker spirit that I love.
Reminds me of the LoLRa project from cnlohr that transmits LoRa without a radio transceiver.
null
3eb7988a1663
Author's bio line says they are a "flat mooner". Which gave me quite a chuckle.
MangoToupe
What's the meaning? Like, they have a flat ass?
DrAwesome
They're (jokingly) saying they believe the moon is flat. Like a flat earther, but the moon instead of the earth.
ta8903
They believe the moon is a flat circle (not a sphere).
jmpman
I despise these wheels. About 15 years ago, my wife and I went to Target and first went to lunch at the far end of the parking lot. After lunch we headed into the store, grabbed a cart, now loaded with our newborn in his car seat, and our two year old sitting in the cart. A quick shopping trip later, we headed back to the car. When crossing the Target parking lot, the wheels locked up, in the middle of the road. Cart wouldn’t budge. Traffic all over the place, and now I have to pull both my children out, along with the shopping, and carry them all to my car. Pissed is an understatement. After my wife and kids were secured back in the car, I retuned to Target, complaining to the manager. A shrug was the best I received. Why did they need to put the wire in the middle of the road???
I hope someone attaches Bluetooth speakers to their shoes and locks every cart in target, so they have to remove the system.
toomuchtodo
Huh, I wonder if it works if you play it over the PA system.
https://hackaday.com/2016/03/04/social-engineering-your-way-...
Edit: looks like an Ardunio can do this with PWM too
varenc
> I hope someone attaches Bluetooth speakers to their shoes and locks every cart in target, so they have to remove the system.
Friends did this college in like 2005. Cambridge area, Shaws Market I think. I imagine the hardware setup was a bit different. All the details are hazy but I recall their lock transmission signal had a huge range and locked all carts in a wide area.
MrFoof
Based on what's still around, likely the one (now rebranded a Star Market, same holding company) in Porter Square, right by Porter Square station.
Based on what I recall, I believe there was one on the southeastern end of Green Street, a bit between Central and Kendall Square, barely northwest of MIT's primary campus area on the corner Massachusetts Ave and Vassar Street. That location has apparently closed in recent years.
tecleandor
Around that year, or maybe even earlier, I remember reading an article about how to DIY one of those devices with a PIC microcontroller and wreck havoc on the store. It might have been something very similar to this:
https://www.instructables.com/EMP-shopping-cart-locker/
It might been the same text that somebody copy/pasted there, sounds vaguely familiar.
alostpuppy
Avoiding these stupid wheels is probably the biggest reason I shop at Costco
asdfa456sdf33
I suppose now I can admit that we did this in college in 2003 (with RF, not audio), and had great fun seeing a grocery store descend into utter pandemonium, until the power electronics overheated and burned the signal carrier to whose chest the circuit had been taped, who started yelping in the store and drawing a lot of suspicion to himself.
axiolite
You can also take a wrench with you, to quickly remove the locking wheel from your cart. Maybe replace it with a non-locking wheel from another cart.
Shouldn't be difficult to find carts left near or beyond the edge of the parking lot.
I find the locking wheels annoying, because they're so often defective and make it a noisy struggle to get your cart through the store. But years ago I also had a neighbor in my apartment complex who would walk home with a cart every week, and would just leave (a dozen of) them there... she couldn't be bothered to push the empty carts back to the store, not even once. I'd think a $1 deposit/return system for carts would work better, and give the homeless in the area some gainful employment.
stevage
Huh, years ago ago living overseas my sharehouse all did that. But we'd take the trolley straight back to the supermarket because we weren't totally degenerate.
mattmaroon
Aldi does it for a quarter and it works pretty well to get people to return them.
Original DEF CON 29 (2021) talk: https://www.youtube.com/watch?v=fBICDODmCPI