Ok that(edit:)last story doesn’t apply here. First off the guy on 87.9 Mhz was at the bottom the the FM band and in the audio band (channel 6) for TV. I have stations at 88.1 and we have to do interference studies and for Tv stations. Only low power now as the big guys are digital.
Each band has different allowable usages and requirements. You get over half a watt in the wrong spot in the FM band and you run a big risk of FCC fines. Now here is the loop hole
FCC Rule 15.231
http://sujan.hallikainen.org/FCC/FccRules/2007/15/231/
Specifically
(b) In addition to the provisions of Sec. 15.205, the field strength of
emissions from intentional radiators operated under this section shall not
exceed the following:
Fundamental frequency (MHz) Field strength of fundamental (microvolts/meter)
Field strength of spurious emissions (microvolts/meter)
40.66–40.70 2,250 225
70–130 1,250 125
130–174 ^11,250 to 3,750 ^1125 to 375
174–260 3,750 375
260–470 ^13,750 to 12,500 ^1375 to 1,250
Above 470 12,500 1,250
^1Linear interpolations.
You have to stay under those field strengths. also you have to be periodic. The system has to automatically shut off in under 5 seconds. No continuous transmission is allowed.
Thats legal as far as I see it but I’m running on little to no sleep right now. Stupid rain and satellite signal loss last night.
In short for a hobbyist to poll a PIR sensor you’ll be fine with the sparkfun stuff. For a manufacturer you will have to submit the completed product to be FCC certified and about $150k to $200k later you can get that nice FCC certified stamp on your case.
15.231 covers remote control cars (40 mhz, 70 mhz) and garage door openers and stuff like that. There are other bands that can be used for continuous and come under other rules like 2.4 Ghz Just stay way from FM and other known and common freqs and you’ll be good. If you can buy a module it is probably legal for standard use.
Licenses are required in some bands but I’m not up on all of those. I can barely keep up with FM regs for work.