Amazing work with an ATtiny814, only 8KB. Love it.
This device though doesn't seem to support mesh connectivity because it doesn't have this short range limitation in the first place. It uses a LoRa chip with a range of a few kilometers. The bandwidth is tiny though, for reasons that are both technological and legal. In particular your are asked to respect a duty cycle of 1% (or even 0.1%, depending on the exact frequency you're using). That's 36seconds every hour. On top of that add some cities offer LoRaWAN gateways (between LoRa devices and the internet) and the limits are even more drastic like 10 messages per day, 51 bits being the maximum payload length.
LoRa was designed for async metering of IoT devices basically. This application is pushing it to its limits I guess.
I'm not an expert, I have a couple LoRa chips but never used them, however here are some back of the napkin calculations:
Assuming a spread factor of 12 (very long rage, very low bandwith) and a 1% duty cycle, you can send about 40 messages per hour if they are short like "yo what's up". 50 chars -> 20 messages/hour. 100 chars -> 10 messages/hour.
However, it uses GMRS bands, not LoRA, so all the FCC restrictions apply.
You can then use GMRS. GMRS is all the same FRS channels plus several more. GMRS can also transmit at up to 50mw on some of the non-FRS channels.
To be using GMRS in compliance you have to use an FCC Part 95E certified device. These Baofeng / Btech devices are usually not GMRS certified. So you need a HAM license to use them. . . But HAM licenses doesn’t cover GMRS frequencies. So there is no technically compliant way to use these devices and check all the boxes. Even if you have both HAM and GMRS you are using a non Part 95E certified device. You’re likely fine as long as you’re not harassing people or causing interference. Generally the FCC is pretty reasonable. They send a letter saying knock it off before they knock on your door. But if you continue to harass people or use high power that causes interference then you will get a hefty fine.
At the very least get your GMRS license. But I encourage you to get your HAM license. I have found that often HAM nerds are into a lot of other stuff I like and my local club has been a welcome place to make friends and build fun stuff.
In any case, I'm pretty sure this device is illegal to use for short text messages. It doesn't appear to comply with several of the restrictions on digital emissions in 47 CFR 95.1787(a), namely it appears to have a removable antenna. Removable antennas are fine for regular GMRS use, but not when the device can send digital emissions.
https://www.ecfr.gov/current/title-47/part-95/subpart-E#p-95...
Also I'd be shocked if it enforced the time limits for digital transmissions in software. This leads me to believe it's not actually type-certified for use which then calls into question anything else it does. Caveat emptor.
But discussion of that is irrelevant because the regulation is no more than one every 30 seconds and each one can't be longer than 1 second in duration. This necessarily limits the length of messages you can send or requires more efficient modulation and/or weaker error correction at the tradeoff of worse weak-signal performance.
It does enforce time limits. If I send a message or something that uses digital communication (like gps coordinates), it won't let me send another one immediately after.
Tepix•2h ago
clbrmbr•1h ago
You’d need some scheme for synchronization if you want to reduce power consumption.
kingkawn•1h ago
arghwhat•50m ago
Low-power requires you to turn the receiver off for extended periods of time, but what you can do there is limited by how interactive the device needs to be, and how much the power the transmitter is willing to waste on retries/longer preambles.
For proper low-power (e.g., devices with ≥ 1 year battery life on small batteries), you're likely to need sleep periods of minutes to hours, or only waking up on physical interaction.
tonyarkles•49m ago
tonyarkles•51m ago
The tradeoff is:
- The end nodes can spend the vast majority of the time in deep sleep without the radios turned on.
- The base station has access to a bigger power source (usually line voltage) and doesn't care about turning its receiver off.
- You can't, however, send data to the end nodes at arbitrary points in time. You have to wait for them to send to you and you have to reply back to them before they go back to sleep.
In a peer-to-peer system like the one in the article you don't get to make this tradeoff.