EMP shopping cart locker

Step 3 Determining the locking signal (informational, you can skip this step)

There will be a wire buried around the parking lot of the store. A controller somewhere in the store sends a changing current down the wire. A changing magnetic field is created around the wire, which contains the signal. When a cart passes over the wire, a resonant antenna inside the wheel receives the signal. A microcontroller then recognizes the signal and determines that the wheel should lock.

Because the magnetic signal is in the audible frequency range, earphones can be used as a transducer. To record and analyze this signal yourself, follow these steps.

1) Plug earphones into the microphone port of a laptop
2) Fire up a sound recording program (Windows Sound Recorder used here)
3) Find the saw cut with the buried wire in the parking lot of the store
4) Put the earphones on top of the line on the pavement.
5) Wait until there isn't much audible background noise from cars driving by and such.
6) Record ~10 seconds of sound from the earphones.
7) play it back, you should hear a quiet chirping
8) Filter out noise using a sound editing program such as Audacity
-high pass at 7kHz
-low pass at 9 kHz
-amplify a lot
-use remove noise tool (works pretty well)
-Fast Fourier Transform to find frequency
9) Inspect your work, and write code to simulate it (provided in a later step)

The screenshot is of the signal for the GS2 system. The little blue blobs are actually a 7800 Hz sine wave, according to the FFT. The unlocking signal for the GS2 looks similar, but the middle 8 blobs are played backwards. Notice the pattern of long-short-short-short-long-
long-long-short is the same whether played backwards, or inverted. Interesting...

GS2.ogg is what the signal sounds like, if you had magnets in your ears.

Get Audacity here

The CAPS signal is much simpler, bascially an 8 kHz sine wave multiplied by a 33.3 Hz square wave. Another way of thinking about it is 120 sine wave cycles followed by 120 cycles of silence. The unlocking signal is a pure sine wave with no modulation, but unlocking a CAPS equipped cart requires the boot to be reset by hand. Don't try this at home, unless you like getting caught.

We also have good reason to believe that all GS2 wheels may be controlled with CAPS signals. It worked reliably on at least one store, anyway.

Comments

[Zerotology]

About the bike lock. Simple solution. When the bike is in motion, there's plenty of energy to charge a battery. Or, You could have a solar cell. All you'd need is enough current to trigger a transistor which disconnects from a magnet lock and the mechanism drops into place. Well, I guess the bike would be in motion so it would have to act upon centrifugal energy to lock.

[Prometheus]

Hey bellylaugh.....ever consider using this system in a more practical application, like perhaps an anti-theft-device for bicycles with a coded x-mitter with the hub-lock locking the wheel automatically in the event of the removal of the battery powering the hub-lock, in-so-much-as a "normally-locked" position for "power-off"?

I have taken great delight in watching your system work on the naive, as they seem so confused when the cart locks a wheel when they attempt to leave the lot with it (of course leaving it right in the middle of the driveway entrance like the fine, upstanding, retarded hairless-monkey-people that are confused by the obvious, as they are....I think lead poisoning passes through the womb for generations)....

This system easily identifies those afflicited with S.A.R.S. (Stupid A** Retard Syndrome), and I congratulate you on it....I think this might sell for bikes though, with the rider carrying a transmitter emitting a weak but unique signal that will unlock the hub when they get near. A thief would find the bike otherwise unrideable. Food for thought....

[Moto42]

The only real problem would be if the device locked up while the wheel is in motion if you engineered it such that the device would only lock when the wheel is stationary then the safety issue would vanish. (If your battery died mid-ride, you would be able to ride up to the next stop-sign, but then your stuck.)

[Evilthingamabober]

Put a charge indicator on the battery. Also, have a Five second delay before the wheel jams so the thief suddenly crashes. Hehehehehe...

[JamesRPatrick]

That sounds like a FOB to me. They have that system on some cars.

[_soapy_]

Until you are doing 30 downhill and the battery fails. Then you are dead, because cycle helmets don't protect against that "easily foreseen" scenario. Remeber, fail safe!

[JamesRPatrick]

What if the system only allowed for someone to use a switch to unlock it? Removing a battery would keep someone from switching it to locked or unlocked. Or have the system know the difference between a slowly dying battery and a removed battery.

[_soapy_]

I'd still be very worried about the safety of anything that can lock the wheels when they are turning. Perhaps a low level brake effect would work better - it would be like riding with flat tyres if the power failed. This would be in addition to a lock that you flicked on and off physically, for preference. It's really up to the designer, though. I wouldn't trust it, but it's your choice.

[JamesRPatrick]

I'm sure someone could come up with a logical and safe system.

[_soapy_]

This is true, but it needs to be safe enough for people to use daily, and trust with their lives. Not ideal for a homebrew device! Have a go, and post it as a collaboration, and I'll happily lend a hand.

[JamesRPatrick]

Like a forum?

[btop]

If you only locked the back wheel, it would prevent anyone from being flung over the handlebars. But with a bike it wouldnt be that hard to just whip a wheel outl

[bellylaugh]

Congratulations, Orthonormal. I am the engineer who designed the CAPS electronics ten years ago. You have "broken into" a really simple, completely unsecured system. Please leave the carts unlocked when you are done playing. If you screw them up, you are stealing from stores that don't make much of a profit margin. And from the company (actual human beings) that did something about carts being left all over neighborhoods. They don't make much money, either. Retrieving carts turns out to be a significant expense for stores. There are companies that charge money to pick them up. (Which always seemed like a potential protection racket to me.)

[moep]

Stealing? Are you kidding me? Please check the dictionary for the proper definition of "stealing". You designed a technological solution to a sociological problem, and it's doomed to failure. A few years ago all the grocery stores had corrals to keep the carts from "escaping" the store. They are all gone now because all they did is treat honest people like criminals. Your device is a little better, but still inadequate.

[adramolek]

Meop, if cart corrals make you feel like a criminal, it sounds like you have a pretty nice life. You should be out enjoying it instead of spending time locking people's shopping carts in a store. Well, unless, of course, you feel that locking the wheels on shopping carts will cause a massive social revolution ending the poor treatment of the average citizens by evil shopping cart nazi's like Loew's and Target. In that case, carry on; society could use some more motivation for things like that and this would serve as a great example to everybody.

[bellylaugh]

Meop, What's being stolen if you cause havoc with this is the time of all the people involved. Reminds me of when I tossed little rocks at cars at about 9 years old - look, I can do this! A temporary suspension of causality that happens in kids minds. As far as "doomed to failure", it's been quite successful for years. It can't be unbeatable, it has to be cheap. And I was always amused that some people react like "It's my right to steal this $150 cart and leave it in somebody else's front yard!". I agree it would be better to pay a kind old lady to stand day and night in the parking lot and chase after cart thieves on her wheelchair. She's tap on your shoulder and kindly say "Now, sonny, you know you're not supposed to do that." And of course, you would be happy to pay her salary.

[adramolek]

Bellylaugh, you are not acknowledging the fact that, whether you like it or not, people will get a kick out of locking peoples shopping carts in the store. Granted this is wrong, but you of all people should understand that this mischief takes place, because you, of course, designed an entire system to prevent similarily immature things (stealing shopping carts) from happening.

In effect, by making the CAPS system so incredibly easy to "hack", you have only converted one sociological problem (the stealing of carts) into another (the stealing of customer's time). Next time you design a system like this, make it a little more hack proof.

While it's definitely no excuse for the immaturity of actually building this device and using it, and wasting peoples time and endangering yourself and others, the ease of "breaking into" CAPS/GS2 seems to be a huge design flaw, especially in a system designed by engineers who make a living understanding and preventing such sociological problems.

In other words; I don't know how much of a purpose it serves to criticize this device. What you should be getting out of this device is a little bit of experience so that the next time you design a system, one improvement on the old sytems is that it's not as easy for some kid to attach an MP3 player to a bolt with wire around it and lock people's carts (see the related Instructables in the comments below).

[bellylaugh]

adramolek, You make understandable assumptions. The caster electronics, designed with ten year old technology, had to run for 5 years on a cheap battery and cost $5. The possibility of pranksters (or competition) triggering them was foreseen and correctly judged. Any coding system that smart nerds can't break would have been too much to implement in a few hundred bytes of code using 9uA of current. On the other hand, if you have a suggestion for an unbreakable algorithm that can be implemented, THAT would be useful. Just to prime your thinking, any code has to be passed in the short time a cart takes to pass over the 3' zone around the line. And due to receiver complexity and power consumption limits, the max modulation rate is limited.

[Moto42]

You're both acting like children.

[steelmaverick]

How do you generate the signal for the CAPS system? I know how to generate waves in audacity, but multiply them? You got me there. Maybe you could also post it up on the page, if it isn't too much trouble.

[madprgmr]

Note that the "blobs" actually represent the individual bits comprising the byte. I believe that the "long" blob is a 1 and the "short" blob is a 0 (not tested, but I think that's the "normal" way of doing it). The rests between bits are there simply to signal the end of the bit. Note that the smallest unit of time is 4ms, so everything is a multiple thereof.