Ask HN: Most interesting tech you built for just yourself?

If the police come, I'll use the Constanza "Was that wrong?" defence.

https://www.youtube.com/watch?v=-RvNS7JfcMM

Anyway, cool to see you hack this, maybe try to tweak the power levels a bit so the neighbors don't have a reason to talk about it.

I wouldn't play at all with non-approved RF frequencies personally.

Edit: “§ 97.403 Safety of life and protection of property. No provision of these rules prevents the use by an amateur station of any means of radio communication at its disposal to provide essential communication needs in connection with the immediate safety of human life and immediate protection of property when normal communication systems are not available.”

-- The maximum power output of a personal FM transmitter allowed by the FCC is 250 microvolts per meter at a distance of 3 meters. The range of the transmitter depends on various factors such as terrain, obstructions, and interference.

Assuming ideal conditions, such as no obstructions or interference, the range of the transmitter can be calculated using the inverse square law. This law states that the strength of a signal decreases with the square of the distance from the source.

At a distance of 3 meters, the signal strength would be 250 microvolts per meter. At a distance of 6 meters, the signal strength would be 62.5 microvolts per meter (250/4). At a distance of 9 meters, the signal strength would be 27.8 microvolts per meter (250/9).

Typical car and household stereos have a sensitivity of about 2 microvolts per meter. Using this sensitivity value, we can calculate the range of the transmitter for these devices.

For a car stereo, the transmitter would have a range of about 26 meters (square root of 250/2). For a household stereo, the transmitter would have a range of about 63 meters (square root of 250/0.5).

However, in reality, the actual range of the transmitter may be shorter due to various factors such as interference and obstructions.

For the record I'm sure OP meant his has a range of 20 metres

As someone who's worked in telecom for a number of years, the fines for broadcasting I've seen issued to individuals are insane. Not just radio but wireless amplification too. Different European country though.

Always check the legislation in the country of operation. Emergency frequencies are held sacred by the powers that be.

I suspect the OPs transmitter is hard to detect even at close (< 100 meters > 30 meters) range. Anything more powerful and you would definitely attract attention.

The built-in collection either didn't have songs that she liked (from the '40s), or they weren't clustered together, or were mixed up with other songs.

I could load a flash disk with her playlist to plug into that player, but it wouldn't know what to play at what time (calm songs in the early and late hours, peppier numbers on other days, festival specific numbers on some days). This was a big deal. I can even change the playlist from elsewhere (a script automatically mirrors the playlist that I maintain on a server)

Bluetooth streaming is possible with the device, but not an option for my MIL ... that would require her to learn to use a cellphone.

In germany its also legal to use plenty of bands in the RF Spectrcum with up to 750Watts (and potential more).

We are also allowed to do CB Funk in Channels 1-40 without anything and up to 80/85 (forgot details) when you register with Germany.

So your statement reads more like you are not allowed to do anything in germany. Its hard to believe to disturb stable systems just because someone is doing a little bit of FM on some known frequencies.

I guess I shouldn't be. Even letting them know you were fully aware you were breaking the law, most people would see its intended purpose -- to bring a little peace and comfort to a very old woman -- and have their own compassion kick in.

YMMV but I'm guessing you'd hear something along the lines of "Oh,... well,... (shuffles feet) ... just turn it off, then". Many of us have elderly people in our lives we wish we could provide some comfort to and most of us know we're headed there (if we're lucky to live that long). You know, assuming your 20m radius FM transmitter didn't, say, cause some cataclysmic event/knock emergency services offline for several city blocks, etc.

Put another way, while some police actually will pull you over and write you a ticket for going a couple of miles (km) per hour over the speed limit, most won't waste the brain power/physical energy/thermal paper to bother enforcing it.

  > Yes, *technically* it is illegal.

The law is an interesting beast. I know nothing about the law in India as it relates to FM band transmitters but I suspect that the law predates the common availability of adapters that one might use in ones car to add an input to a stereo that lacks such a highly technical circular hole for such purposes[0]. Once these devices gained wide adoption due to both their utility and -- more generally -- the fact that operating one is usually so benign that they can be difficult to discover let alone actually cause enough interference to warrant them to be seized.

The intention of the law was to prevent someone from operating a pirate radio station/give exclusivity to a single license-holder for that frequency. Since these devices don't violate the spirit of the law, the governing body finds it easier to carve out an informal exemption rather than explicitly write one in. It can also be tricky to correct a law that has a very valid reason for existing but may have cases where total enforcement isn't realistic[1].

The law may not have caught up to the reality on the ground and the legislatures answer to it is "enforce it when the interference is enough that someone notices." One might imagine a world where something akin to TV Detector-like Vans[2] drape the country-side in a dragnet to catch all of those pirate FM-input-devices but that usually only happens if there's a substantial amount of tax revenue to be gained ... to pay for the vans.

[0] I had one of these in the 90s (in the US, where it's not illegal if designed correctly) that connected my Discman to my ridiculously sad factory radio which lacked both external input and even a cassette deck.

[1] I do very little with regard to radio communication (if that isn't obvious) but I'd imagine most lawmakers do even less, so now you have to bring in experts to figure out "what's an acceptable amount of interference in this specific use case" and "how should a device like this be restricted." Not that government isn't famous for wildly wasting money or anything but I'd imagine the thinking is that it's not worth the effort to correct.

[2] https://en.wikipedia.org/wiki/TV_detector_van

https://www.fcc.gov/media/radio/low-power-radio-general-info...

Looks like India doesn't permit it, but is looking at doing so (at least, for some purposes):

https://trai.gov.in/notifications/press-release/trai-release...

I haven't found the exact law in India, but looks like maybe it's legal for personal usage of FM provided power of transmitter is under 500 mW?

FM transmitting for private use is completely legal for UKW frequencies between 87.5 and 105 MHz and transmission powers lower than 50nW [1]. You can buy perfectly FM transmitters for your car, etch

[1] (German) https://www.autozeitung.de/fm-transmitter-bluetooth-nachrues...

I bought a house in my very early 20s. Roommates of mine finished the third floor with no permits. Went to sell the house ten years later and the location it was in required a U&O inspection. My realtor told me to lie, apply for a permit and pretend I just did the work.

Instead I called the local building inspector and said, “Hello my name is xxx and I’m calling to confess.” He cracked up laughing, came to the house immediately to look at everything and told me I was fine.

https://www.amazon.com/s?k=fm+transmitter&ref=nb_sb_noss

This is what he means, not some bigger pirate radio situation

Just kidding, I'm in the U.S. But more than a few times I have suddenly got Mexican musical content on my radio in the car when passing close to another car.

Heh. I drive a 2002 Chevy Suburban (don't laugh, I have a strong aversion to spending money on new vehicles) and it lacks an AUX input, so to this day I use one of those low-power FM transmitter adapters to pipe my phone's audio output to the vehicle stereo. They are amazingly handy little gadgets.

edit: Halfords still stocks these! https://www.halfords.com/technology/mobile-phone-accessories...

This is in line with Peelian principles of policing. It was explained to me by a policeman and it was fascinating. He had worked in the UK, Australia and New Zealand.

In his view, Australia was considerably harsher in its policing and NZ was at the other end of the spectrum, UK in the middle.

https://en.m.wikipedia.org/wiki/Peelian_principles

[0]: https://de.wikipedia.org/wiki/Rettungsgasse?wprov=sfla1