Where would the cost lie? Lets assume I can get the board milled for free , now how difficult/ time consuming and expensive would the bluetooth module be . Keep in mind I would need between 10-20 of them .
bluetooth uses RF as a transmission medium.
it can change it's operating channel (frequency) to find one that is not being used much.
transmissions bust be addressed to specific receiving nodes so all nodes receiving the transmission know which one it is destined for.
as it's a 2 way system each node will need a transmitter and a receiver. not just any transmitter either. it will need to be able to power down while the receiver is in use and power up (within a few microseconds) when it needs to transmit.
so while it would theoretically be possible to build radio transmitters and receivers with the desired properties if you want anything like high speed communications you will have to buy the radio part.
try looking for "RF transceivers". you would need one that operates at the same frequencies as other bluetooth devices.
as well as the radio you will need some sort of microcontroller to handle interfacing with the bluetooth stack.
i seriously doubt you will be able to buy the components requires to communicate with other bluetooth devices for under $40 and you would not yet have started trying to reverse engineer the software side of the blutooth stack.
Also I chose bluetooth because it seemed the easiest at the time - doesn't it autochannel so no interference?
less interference but as there are still only a limited number of channels there will still be times you are sharing a channel with others.
If you can provide some more details on making RF have the same transmission properties(no interference and multiple devices per dongle) , I would really appreciate that .
this is not an interference issue. it's an addressing issue.
just transmit an address byte at the start of every data packet. if a receiving node sees a packet come in have it ignore any that are not addressed to it.
interference will still happen. you will need some sort of checksum to make sure you got the whole packet.
Side note : Bluetooth is RF , isnt it? Like TrickyNekro said , its 2.4Ghz. Right?
bluetooth communicates over RF but it is more complicated than that.
bluetooth has all the addressing and pairing side of the protocol as well.