Gabriel Gonzalez wrote:

> Low cost and reliability are the main issues here.

  OK, well if you are prepared to accept low reliability and low cost,
that's fine and no great problem, because reliability sure doesn't come
*cheap*.

  This sounds like one of those popular low-reliability house alarms!

  If you want reliability, you will require all units to have both
transmitters and receivers, so that the central station regularly
"polls" the sensors for their reports, thus avoiding any collisions of
the various sensors' transmissions.

  The "rough and ready" (*low* reliability) method is to have each
sensor simply transmit its report repeatedly, at a period which is coded
by its identification number; i.e., all periods are slightly different.
>From time to time, transmissions will collide, but the receiver will
simply have to wait for the next report.

  The *clever* part is to have the receiver "remember" the different
intervals, so that it "realises" when a collision is occurring and
ignores the lack of data, but takes particular note of an absent report
(or in fact, three absent reports in succession) when no collision(s)
should have occurred.
--
  Cheers,
        Paul B.