There are a fair number of folks that are trying to use this platform without an internet connection and/or Particle cloud. While not impossible to do, the purpose of this platform is IoT (internet of things). By removing the “internet”, this eliminates a lot of the built in utilities/functionality that would make this project very feasible.
Should you have “internet” for all your photons, the “master” can listen for the on/off closure of the switch. It would send a Particle.Publish signal to the Particle cloud. Any number of your auxiliary photons would subscribe to the Particle cloud to obtain the signal from the “master”. All this infrastructure is provided and would essentially be very plug and play.
If you want to continue down the route of no internet / no cloud (oT), there is a significant amount of infrastructure that you would have to re-invent or re-create.
You could turn a Raspberry Pi 3 into an access point for your photons. You would have to setup the infrastructure on the Raspberry Pi to replace the Particle cloud infrastructure. Once that infrastructure is in place, the photon “master” could signal the Raspberry Pi which would then distribute that signal to your auxiliary photons.
The other three pieces is pulling together the server software on the Raspberry Pi to listen for the signals from the “master” and send it out to any listening photons. Firmware for the “master” and firmware for the photon slaves controlling lights, etc.
It does not matter if you use a Raspberry Pi or a Photon using SoftAP to be the coordinator. You would have to do the same development on the Photon to determine how it receives a signal and then how it needs to be sent to the remaining Photons. At the moment, I don’t have any experience with the SoftAP side of things, so I am at a loss to say how easy or hard this might be.
There are no rules to be broken or infringed. The design choices have implications on the path forward. Choosing not to use the Particle cloud has significant implications for this use case.