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

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