Mike Harrison wrote: > On Wed, 24 May 2006 09:19:03 -0700 (PDT), you wrote: > > >> Whats your estimated hardware costs for the WiFi portion? >> >> Olin Lathrop wrote: alan smith wrote: >> >>> Client has asked....possibility of doing a WiFi with a PIC. Well...the >>> project in discussion is for a large greenhouse facility and monitoring >>> temps and other stuff. >>> > > Wifi would probably overkill for this - a more appropriate way may be low cost sensor/transmitters > on one of the unlicensed UHF bands sending short packets every few minutes to one or more receivers, > depending on distance - the receiver could either be a PC or a data concentrator that bulks up the > data to send via wifi/GSM/GPRS etc. > > I agree. These tiny transceivers operate in the 433Mhz band, and are quite reliable within 1/4mile. A PIC would provide packeting for each data transfer (start char + data + CRC + term char) and another PIC would unwrap the data and stuff it into a host PC through a serial port. This would be very inexpensive, just a few dollars per station. You could make it even cheaper by sending the packet only at random intervals; for example, if the intent was to poll 16 stations every minute, you could packet data for the last 3 intervals, then send it at RANDOM intervals once a minute. Its always possible that two stations would send on top of each other, but since the data is triplicated, nothing will be lost. The receiving station would simply receive packets and install them into a database, and toss duplicated data. To make this work, the packet length has to be short and the interval must have a lot of dead air. Even more reliable if the interval is longer, such as 15 minutes. A simple encryption scheme can be devised, such as substitution cypher, bit rotation, etc. I hope I didn't get too far off the subject. In general, I am not enthused about PC wireless schemes. I and some associates looked into these for a police data dump station, and the results were NOT heartwarming; not one method was free from hacking. Personally, I think that IEEE803 etc was a plot hatched by the NSA/IRS/FBI cabal to easily read everybody's computers by simply parking in the front driveway and hacking in. It is INCREDIBLY easy to hack. --Bob -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist