I am using a PIC microcontroller in a relatively low-complexity application, but still valid enough to use a PIC microprocessor.  I want to create a small "intelligent transmitter" that interfaces to only one switch.  The main idea is to know how long the button was pressed, and on what transmitter.  I also plan to include a small serial communications port as well to allow the transmitter to interface with a "handheld receiver".

 

Here is the general scenario:

 

Transmitters are scattered throughout an open field,  the terrain is fair, ( flat, some hills, thick trees ).  The end result should transmit a signal "blindly" and hope the receiver catches it. However, the transmitter must be able to take extremes in temperature abuse, as they will be outside.  A respectable range for this application would allow a receiver to intercept signals in a .3+ mile radius.  This is not a major factor, repeaters can be emplaced to time-stamp and rebrodcast signals.

 

The end result will transmitter will transmit a very simple message that tells the receiver (which monitors all transmitters) how long the switch was pressed.  I though about using a RTC in the transmitter, but I have decided against it.  The receiver (which is a PIC, EEPROM, and a parallel port interface) will simply log the reception of the signal.  The signal must consist of an ID unique to each PIC and a data packet that contains the time.  The transmitter must stay small, so any error correction must be one way.

 

Size and battery power are priority design considerations but so is range.  Speed may ease some issues, but there is so little data being transmitted, it makes little difference.

 

The problem that is kicking my butt is contention.  If two transmitters are triggered simultaneously,  no good can come.  I though of using a "listen" feature, but this kills size and battery power.  I also thought of using a form of real time clock( so that a transmitter would act only during its "allotted time slot"), but an external clock would use too much power, and an internal routine does also.  I also thought of having the PIC send the signal multiple times at random intervals, to overcome the odds.  But the practicality of this approach diminishes quickly as the number of transmitters increases.  Any suggestions?

 

I also need more information on the "free bands" that such an application would be allowed on.  Transmitter power is obvoiusly in the mW range and is suppossed to have very short range.  I do not want the signals to bother someone a 1.5 miles away.  I am think of somewhere around 300MHz, but I have no real basis to choose on,  I only require a speed of ~ 300 bit/s, less would not hurt. 

 

Another possibility is sound.  Can a modulated, high frequency audio tone be heard at that range?  It could prove to be very interesting.  Also does anyone know of any "matched" transmitter and receiver ICs?  They need to operate on an "open" frequency, which has not yet been determined.  Any idea helps.

 

Anthony