Mystery Frequency Disrupted Car Fobs in an Ohio City, and Now Residents Know Why

Part 15 (unlicensed frequencies used for communication) still has rules. There is a hobby exemption for up to five devices--however, that is an exemption to the testing requirements, not the rules themselves--and you are still required to use "good engineering practice" like not designing it to have the transmitter run continuously. There are also pre-certified modules available for things like wifi, zigbee, xbee, etc.

There is almost always a power limit, and there are often spread spectrum, usage type, antenna directionality, and other requirements. Additionally, licensed users need For 433 MHz in the US, you have two allowed usage rules: 47 C.F.R. 15.240 [0] and 47 C.F.R. 15.231 [1]. The former is for transportation use only. The latter is what car remotes use, and your usage probably falls under that as well. Among other requirements, you can transmit for no more than 5 seconds--unless they are signalling an emergency alarm condition. If you don't follow either part there is a very minimal power output that is generally not useful for wireless data transmission.

[0] https://www.law.cornell.edu/cfr/text/47/15.240 [1] https://www.law.cornell.edu/cfr/text/47/15.231

Even if part of the spectrum is unlicensed, you're still bound by power limits.

There are several limitations for 433Mhz.

If i remember correctly the main two are: - Power limits - Transmission length per interval of time (I believe max 300ms in any 10s window)

I think this land isn't owned so I can dump anything I want on it, right?

I see, so it was constantly broadcasting and then he’d encode some message in the signal to indicate if someone was up stairs, instead of only broadcasting when there was a message to send. That certainly would explain it.

You're probably right that they used a guide like that. It seems irresponsible to publish that blog post. 99% of Pi tinkerers will read that disclaimer in the third paragraph and either not understand that life critical devices _do_ can be interfered with or think their use-case is special/low-powered and won't do any harm.

So they are generating a signal by toggling the IO pin on and off. Not only will there be a huge amount of harmonics, wouldn't processor interrupts wreak havoc on the output signal and cause even more interference?

I'm assuming he also didn't account for harmonics. It'd be interesting to see how dirty his transmissions made the spectrum.

Why do you assume there was a microprocessor involved at all? Most PIR sensors come as modules that operate either with a normally open or normally closed trigger pin and a couple of pots for adjusting sensitivity and trigger duration.

It would be easy to build a transmitter to blast out a constant high pulse and then just cut it off using a transistor or mosfet hooked up to the trigger on the PIR.

Simple circuit with no microprocessor or coding required.

Like all the best projects.

I get 26 characters portrait, 40 landscape.

I almost exclusively read HN on mobile, occasionally emailing things to myself to do on a desktop/laptop later.

I’d wager it’s a pretty high percentage. HN works great on mobile aside from this. There’s no reasonable column width, just use quotes for quotes.

Hard-wrapping to some column width only ever really works when you know the physical width of the screen/paper. Otherwise, dynamic soft wrapping.

I hate preformatted text in HN comments (and I never even read HN on mobile).

I could probably find such a transmission using my personal equipment in a relatively short amount of time. That’s all well and good, but you’re a city council person and a constituent complains. Then another, and another. Call up mikestew and his crack team of Rogue Transmission Finders? No, you went to business school, you haven’t the first clue how this shit works. I mean on the level of “it uses radio waves? Is it like WiFi on my computer?” cluelessness. Garage door people, though a good first guess and good folk who know how to fix that giant spring without getting killed, are not RF engineers. AT&T? phhhhttt, what, they’re sending their crack RF engineer into the field to diagnose a frequency that is not in their allotted bands? And everyone’s cell phone still works, so take a guess at what defined “best effort”.

So my guess is the vast majority of the time was spent tracking down someone with equipment and the knowledge to use it. Or probably more accurately, just finding the person who even knows who to ask in the first place.

I have a feeling it was weeks before someone with a clue stepped in, and then a matter of minutes.

315 MHz is a perfectly lovely frequency for direction-finding. Hams have "fox hunt" exercises around 440 MHz all the time, and 315 just means the optimal antennae are slightly larger, but still easily portable.

I've only participated in a few fox-hunts, but they're incredibly fun. Typically our "search area" is the bounds of a city, or a set of roads maybe 3-5 miles on a side, and our time frame is an hour. I have yet to see one where the fox isn't found within the hour, typically about half that, and in many cases the fox only transmits for 30 seconds every 5 minutes or something, just to make it harder.

> Was it difficult to find the exact source, or not? Is there some reason that getting directionality on a constant 315 MHz signal is harder than one would think?

Physically, it's easier to find a UHF beacon (315 MHz is considered ultra high frequency) than it is to find a lower frequency beacon (lots of direction finding contests are done on VHF and HF). The higher frequency means a shorter wavelength and better phase (direction) resolution with a smaller aperture (distance between antennas).

However, many (most) hams don't have equipment that covers 315 MHz. UHF isn't popular because it is shorter range and more expensive equipment (you can build HF equipment with much simpler components, and the design is less complex). Plus, 315 MHz isn't in one of the ham bands, so ham direction finding equipment probably won't be designed for it even if it is UHF.

A wide band SDR would include 315 MHz. While there are many younger hams that enjoy cheap SDRs like the RTL-SDR, many of the older, grumpier hams see it as dangerous and letting undisciplined "hackers" into their hobby. To build a direction finder with an SDR you either need to have an expensive multi-channel SDR (I have a Lime SDR that would work), or you need to construct your own mixer to feed a single channel SDR, or you need to wire up two RTL-SDRs with a coherent clock signal, which puts this out of the range of your "common hacker." Here are two ways you could do this on the cheap: https://www.rtl-sdr.com/building-a-passive-radar-system-with... https://www.rtl-sdr.com/building-a-direction-finding-system-...

So while most armchair HN readers would love to tell you about how easy this is, most people don't have the equipment sitting around to do direction finding on a 315 MHz signal. The best most people could do would be to "wardrive" around the neighborhood and look for where the signal strength peaks, but this will really only narrow it down to a few houses and then you need to start knocking on doors. A good direction finding setup would pinpoint the exact house.

Source: I'm a ham (recently made extra), but no, I won't tell you my callsign to prove it.

The article says it took a few weeks. I don't think the worth of this article boils down to whether it was a "hard" problem or not, just that an unexpected device caused unexpected problems.

This isn't a modern fad, it's literally the point of storytelling.

This is a situation where the law and the actual practice is so diverged that we've basically just enshrined selective prosecution by the FCC.

Small volume manufacturers just try to design well and roll the dice because (almost?) no one is spending four figures on an accredited lab to do emissions testing on a device that sells a few dozen or couple hundred units per year.

Some try to find a way to indicate that it's used only in transportation, that it's industrial/commercial test equipment, that it's a peripheral device or sub-assembly and not a finished end-user device, others just realize that the FCC isn't going to bust down anyone's door because a device not emitting harmful RF doesn't have a testing certificate on file.

> tends to get mirrored onto many frequencies for some reason I'm not familiar with.

Play with this for a few minutes: http://www.jezzamon.com/fourier/index.html

And watch any of these: https://www.youtube.com/results?search_query=vibrating+reed+...

Okay, so think of a radio receiver as being sensitive to a particular frequency, because it has a filter in front of it. A vibrating reed of a specific length, if you will. (This is almost exactly true for certain types of crystal and SAW filters, actually.)

But when you flip a digital output from 0 to 1 or vice-versa, it changes state very quickly -- the output is essentially a square wave. (Physical limits mean it does have a finite slew rate...) Which means it contains components of every frequency. Think of coming up to one of those vibrating-reed meters and smacking it -- a single impulse excites ALL the reeds.

Smack it regularly (change the pin state at a specific frequency) and the reeds that're way off frequency will just sort of bounce around but not be particularly excited. You're creating noise, but it's just noise. But the reeds that're odd multiples of the smacking frequency will vibrate too -- these are the harmonics. They'll get really excited, almost as excited as the fundamental. (The evens essentially cancel out.)

Here's what it looks like on the radio spectrum: https://www.youtube.com/watch?v=eC36AqL5mw8

Incidentally, this is why so many chips now boast "slew-rate limited" outputs: to intentionally slow down those transitions, to reduce the amount of noise that has to be filtered and shielded in order for the device to pass RF emissions testing.

We'd all be impressed and terrified if your telemetry "messed up" a burrito, whether it was in the microwave oven or out.

Even though he inadvertently caused consternation with people with keyfobs and rc garage doors, he did quit after it was brought to his attention. I'm much much more lenient on someone who's tinkering and causes that accidentally.

And if you read into the article, the person who did it was special needs (mentally or physically). They're also likely on a fixed income. Jail would be unconscionable, as would fining someone which lack of payment would lead to jail.

Hopefully the result here is "person quits, person reads up on proper RF practice, and does things better next time"