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

I know. It is illegal in most parts of the world. I'm taking over a commercial FM channel that my MIL won't listen to, and the transmitter has about a 20m radius.

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

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

I hope they don't know how to use Google and if they do that that is a pseudonym you're posting under here or you might be in bigger trouble than you started with ;)

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.

The FCC allows personal FM transmitters to operate with a maximum power output of 250 microvolts per meter at a distance of 3 meters. Other countries are more permissive, so this is not a problem. As long as you don't interfere with anyone and emit in a band that's not used in the area, it's perfectly fine.

I'll tell them it was me to protect Spartacus.

I was curious what kind of range that might have, so I put what you said into chatgpt and asked what the range of a typical car or home stereo would be, and it gave me this (not sure if it's correct). FWIW, much less than 20 miles, haha.

-- 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.

Makes sense. I’ve seen Bluetooth fm dongles for cars to this effect. The dongle connects to your phone on Bluetooth and the old car can listen to your Spotify via the radio.

> FWIW, much less than 20 miles

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

sbcl: (* (sqrt (/ 250 2)) 3) => 33.54 meters, the factor that amplifies distance is the square root of the factor of signal strength.

Yeah no one will bother with such a low-powered device. I used an in-car bluetooth-to-FM tranmitter bought from Amazon India for years. They've been sold openly since forever. Like this one: https://www.amazon.in/Portronics-AUTO-10-Bluetooth-Car/dp/B0...

... I've always been amazed how often "Was that wrong?" works.

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.

I’ve had much better luck with 100% honesty. Just say you set up a 20m transmitter to improve the life of a 93 year old woman.

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.

Well put. Exactly my thoughts. And given my MIL's attitude towards any visitor, the cops will be plied with food and chai till they burst. They will forget what they came for :)

Ha ha, I must have pulled up along side you the other day.

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.

I love these things, it's so simple and it's often a better UX than any car bluetooth provides. You also used to be able to buy aux adapters that went in your cassette deck!

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

> I've always been amazed how often "Was that wrong?" works.

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