Control shopping cart wheels with your phone (2021)

Student housing near the engineering campus at University of Michigan is about 1/2 mile away from the reasonably close Kroger grocery store. When I started there in 2009, the carts didn't have the locking wheels and many people would take the cart all the way back to their apartment complex -- this meant that the Kroger store manager had to rent a moving van every so often and go collect them all as the carts cost $500~1000 new, so replacing them when they're 1/2 mile to a mile away wasn't good economics. Eventually this got so bad (never a cart at the store when you wanted one, had to go get them all the time) that they switched to the geolocking wheels, which was a pain because the geolock frequently false detected if you were at the back of the annoyingly small parking lot, so there were always carts stuck in the back ~20 spots.

Later, my girlfriend told me that the specialty foreign foods store near the Big Lots she worked at in a different Detroit suburb would intentionally come steal the Big Lots carts, rather than pay for their own (see above, expensive), so the Big Lots clerks would occasionally get sent on a mission with a moving van to get a bunch of their carts from the next shopping center over's parking lot. I think they might not have ever paid for the geolocking wheels, since Big Lots is low margin and those options are pretty expensive, but you can see the incentive to do so.

Larger stores in Sweden also use the coin system, even though as in the Netherlands it feels like use is declining in favor of just unlocked carts.

My favorite part of the system in larger stores is that to handle people not carrying cash (Sweden is pretty long-gone in this regard), you can usually go inside the store to get a free plastic token that fits the reader.

That always made me chuckle, since the entire point of the system is that you're supposed to be incentivized to return the cart to get your money back, so by replacing your money with a free plastic token that they hand out from a basket, they did .. something to the overall system design.

Still fun as an example of how the customer's overall experience is more important than the point of an entire security system, I think.

The coin insert system is to incentivise people to return the trolley though not to prevent thefts.

Aldi in the USA uses a coin system on their carts, you insert a quarter to unlock the chain. It's not so much about theft, if someone wanted a cart, a quarter is a cheap price to pay. They do it so that people bring the carts back to the corral at the front of the store to get their quarter back, instead of leaving them in the parking area. That way, the store doesn't need to hire someone to go out and gather the carts every 15 minutes.

In my view it's quite an inconvenience: who carries quarters around anymore? I rarely have any cash with me, let alone loose change.

I expect we'll soon see something where you make a small payment with your card or mobile pay app which is rebated when you return the cart.

One reason, beyond cart theft, is aesthetics.

A grocery store at Bayview Village, an upscale mall in Toronto, uses this system to stop cart travel outside the grocery store parking garage. Mall management considers carts trashy and that they otherwise bring down the appearance of the mall. This was one of the conditions when the store opened in 2005. Their cart policy may have changed 20 years since.

>Am I going to need to just start sharing my Claude chat history to prove to people

In the United States, this is for shoplifting (can you call it shoplifting if they're sprinting to the door pushing a full cart?). If the cart doesn't pass through a checkout lane, the wheel becomes disabled. The local grocery chain here has them, and it's never been deposit-for-a-cart.

I honestly do not know if the shoplifting thing was ever a real problem, or just an imaginary one that they paid a bunch of money to "solve". Occasionally, there are a few carts in the corral where the wheel in question will not roll anymore, and you have to take a new one.

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.

Homelessness in the US. They steal carts to carry their stuff around, or to collect trash aluminum cans to earn money, etc. In certain areas, it's pretty common and probably crosses the cost:benefit line for stores.

In Canada, I had one that locked itself when I left the shop as soon as I entered it, because I realized I forgot my wallet.

I went through the main gate but left without going through the cash registers. I guess it detected it and thought I was stealing.

Probably a combination of:

* In a high-trust high social cohesion culture, you can rely on people returning carts all the way to where they belong instead of just selfishly leaving them in the parking lot once they are done with it.

* In the US, the opioid epidemic means there are many more homeless drug addicts. Stolen shopping carts are useful for them to move their stuff around.

When I was a kid we didn't have this coin thing in supermarkets in France, and the problem wasn't people stealing carts (which they could do for for €1 anyway) but many people were leaving them at random places in the parking lot, basically near where they were parked. That made driving in the parking around the carts a big mess.

https://ourworldindata.org/trust Some of us live in a low trust society.

But this is such a cool talk. My local Giant (grocery store) has a wire that goes along the left side of their parking lot. Only about 20 spots there but once you cross that line, the wheel locks. It took me 2-3 trips of confusion why the cart was breaking before a store employee shouted at me "you're gonna lock it" right as I crossed the line. He came out to me, unlocked it and we had a long talk about how it all works.

I'm from The Netherlands and I've seen carts with locking wheels. Granted it's very rare, but definitely a thing.

I've lived in the US as well and have never seen them there, but it's a big country.

https://www.tiktok.com/@canal26argentina/video/7169586172967...

Argentinian fans during Qatar's FIFA World Cup 2022 using a shopping cart as a barbequeue grill.

A friend of mine had stolen one with his brothers back when we were in primary school. But stores here simply have a guard on their parking lots to prevent people from taking the carts away - haven't seen any tech for that.

The big supermarket in my town does it, but that's because students stole the shopping carts to carry booze during the year-end party and parade!

>in the Netherlands (or neighbouring countries)

Most supermarkets in Belgium use a coin but some supermarkets (notably Colruyt) lock their shopping cart wheels.

Supermarkets that have a step-less escalator (e.g. to go to the parking lot in the basement) also use these locking wheels to make sure the cart never moves on the escalator. I live near an Albert Heijn that has these.

You live in a high trust society.

For context, in The Netherlands, supermarkets are in urban areas and have no parking lot at all or a relatively tiny one.

The usual USian aspect of schlepping a shopping cart waay out to your car and then not wanting to schlep it back or to one of those bays is thus far less relevant.

Also, buying all groceries for the next week is rare here. In other words, you just pack your stuff in bags and walk off, no need to bring the shopping cart.

It's a somewhat unique situation (in NL it is not legal to have a store open to the general public except in a commercial zone, i.e. a supermarket cannot open its business in an industrial zone which means the stores kinda have to be in the middle of population centers, which then opens the door to just buying what you need every day. This then in turn means you can go by bicycle and you don't need a massive freezer either).

Interesting how such a relatively innocuous butterfly flap (the zoning laws) result in such an utter change in culture (bicycle, urbanisation, shop-for-the-day instead of for-the-week, etc).

Using a 1 pound coin is basically saying “it costs a pound to take this and not bring it back.” It works for lazy returners, but makes blatant theft quite easy.

The shopping cart locking-if-it-leaves-the-parking-lot feature is a thing in Germany (not universally, but I've seen it e.g. at ALDI).

In the Netherlands most people shop at a small store, not at a big box surrounded by a sea of parking. So no shopping carts there.

It's most common in places with lots of elderly or homeless, both groups find these carts very useful and will simply take them, homeless to keep, elderly to abandon near their home once they have transported their groceries.

It's more also common in places where people walk, since it can be hard to bring groceries home on public transport.

So yes, very localized.

The shop near me doesn't have locking wheels (they used to, but stopped), instead they have a guy in a pickup that drives around occasionally, searching for carts.

I've seen it in the netherlands as well in poorer areas

Localized to where? They are at every Aldi in Tenerife...

€0,50 actually

https://cnlohr.github.io/lolra_talk/

They left their e-mail in the talk, in case you want to increase the odds of reaching them

It has occured to me that rather than the complex instructable linked below, it might be easier to purchase an off-the-shelf BTL (high voltage => greater power) amplifier to drive 2W over a small (0.3m dia) air-loop antenna. The AC input signal from the phone's USB DAC is about 0.77V RMS. Let's power the amp with two consumer off-the-shelf 9V batteries in series (18V). Assuming ~1-2 (1.5 here) ohms of internal resistance, the total current draw should not exceed 250mA or the voltage will drop below 8.6V. The loop antenna therefore needs 10 ohms of impedence, and a tuning capacitor to make it resonant (fully resistive). The AC signal needs a coupling capacitor to remove any DC component, followed by a bias resistor to tie the amps signal input to ground. Each amp signal output needs a coupling capacitor because no way will I be able to correctly calculate the tuning capacitance correctly, and also because the antenna won't be a fixed loop. The most efficient use of those 10 ohms is with litz wire, which at 20 awg comes to 300 meters with a 450nF tuning capacitor... which is not happening. Let's shoot for 30m of 20-awg non-litz wire and make up the difference with a 8.8 ohm resistor. 2w across 32 coils @ 10 ohms corresponds to 0.45 A, which at a meter's distance comes to about 0.6 uT... ridiculously tiny. Will it work to activate the circuit? I don't know. However this antenna is simply wasteful - it would be better to switch to a ferrite core design. The magnetic moment of a ferrite core increases more with coil diameter than coil length... but to target 10 ohms resistance at resonance might require a coil length greater than diameter. I haven't really looked into it.

To conserve battery you need an amplifier with a shutoff pin. These are always logic pins (DC on/off), which entails a diode/capacitor/resistor combination tuned to detect and convert AC input to on/off with a configured decay time, i.e. keep the amp powered on for X seconds without signal. Unfortunately the logic pins require high voltage relative to the power source (battery) and 0.77 Vrms just won't cut it. This then entails adding a transistor to shunt power to the shutdown pin from the battery. Then I had a look at a sample schematic of the TPA3116 amplifier, and there are so many pins to wire. At this point the project isn't any simpler than the linked instructable.

Then I found this:

https://www.rockvilleaudio.com/headrock/

According to the website it requires a minimum load of 16ohms, but that might be ideal for ferrite-core antennas. Perhaps I'll give it a try!

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.

That is pretty funny and an interesting scar to explain.

> 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.

https://www.amazon.com/s?k=croc+speaker

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

A market near me included the car park within their geofence, but did not include the (distant) bike racks or the route to them. Sigh... Never before has bringing a full load of groceries home been such an awkward hassle.

Avoiding these stupid wheels is probably the biggest reason I shop at Costco

What's the meaning? Like, they have a flat ass?

That's a different mechanism. There are rubber "feet" at the bottom the of the carts. The wheels are thin slices that sink into the moving sidewalk grooves, therefore the cart rests on the feet instead of the wheels. It's a passive mechanism, there's no actual locking of the wheels. (Try lifting the cart to see that in action.)

As other people have pointed out "audio range" is generally 20Hz-20kHz. Your phone (and other audio equipment) is therefore built to be able to transmit those frequencies. The way a speaker creates sound is by passing electricity through a wire, creating a magnetic field, and pushing against a permanent magnet. Either the magnet or the wire is attached to a membrane that will then get pushed out. Doing this between 20-20k times a second and you make sound. However when charged particles (like the electrons in a wire) accelerate they create radio waves, so the magnetic coil in the speaker will also create a small amount of radio waves in the same frequency as the sound it is producing. This is what's called parasitic EMF, and in this case it turns out that this small amount of radio signal is enough to interact with the radio in the wheels.

> 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.

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.

Has several definitions, usually 20 Hz — 20 kHz: https://en.m.wikipedia.org/wiki/Audio_frequency, but sometimes may refer to the range of specific codec (telephony might have something like 300—3400 Hz for example, so not this case). TFA means it as “I have a peripherial readily available”.

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.

This is an alternate (probably similar) app to help your atomic clock watches get a signal from your phone rather than waiting for the vagaries of radio signal propagation (WWVB / https://www.nist.gov/pml/time-and-frequency-division/time-di...).

https://apps.apple.com/us/app/radio-wave-sync/id1484233572

...basically: Turn up your phone volume, wiggle the phone speaker (which has magnets), magnets == signals => watch gets the right time from your phone instead of remote radio waves.

I've used the app a few times before and it's generally pretty reliable if you follow the instructions!

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.

Aldi does it for a quarter and it works pretty well to get people to return them.