THE MONTREAL WEATHER RECORDER by Jacques Brodeur VE2EMM March 11,02

             Construction and operation information

WHAT IT DOES
   It displays a graph of more than 3 days of atmospheric pressure
and outside temperature. The graph shows the approching of low
pressure (bad weather) and high pressure (nice weather). It also
shows the MIN and MAX temperature of the last 3 days, you will
know when there is a change of air mass (abrupt change of
temperature).

HOW IT WORKS
   The output of the DS18S20 high precision digital thermometer
goes directly to the PIC. It has a resolution of 0.5 C.
   The pressure sensor is used at the top of its range, meaning
that the zero of the scale is at 3.8 Volts. IC3 restores
the range between 0 < 5 Volts. The MAX868 is a charge pump voltage
quadrupler generates the -18 Volts required by the LCD display.
The A/D conversions, the scaling and driving of the LCD are done in
the PIC.

ASSEMBLY
   Install all the components as per the PCB top silk drawing. Be
careful at the order of the wires to the LCD, they are somewhat
twisted on the PCB. The MAX868 -18V supply is very small. Use a
piece of scotch tape to hold it in place, using a very small
soldering iron tip and small solder, solder the corners pins, apply
the solder on the PCB pads, it will climb on the IC's pins by
itself.
   Cover the pins of the DS18S20 thermometer with epoxie to
waterproof it.

CALIBRATION
   The temperature sensor's output is a digital signal and requires
no calibration.
   To ajust the pressure, unsolder the sensor end of R1, place
3.845 volts on it, adjust VR2 for 0.500 volt mesured on pin 2
of the PIC. Now place 4.205 volts on R1, adjust VR1 for 4.500 Volts
measured on pin 2 of the PIC. Redo those adjustments until both
readings are perfect. Resolder the end of R1. Use your most
accurate voltmeter. Your weather recorder is now ready to enjoy.
   Set the clock accuracy by measuring the frequency of the xtal
at pin 10 of the PIC. It should be 3.194304 mHz. If you do not have
a frequency counter, touch up C10 and check the time 3 days later,
redo until you are satisfied of the results.

USING THE RECORDER
   First set the day of the week and the time then the graph will
builtup one point at each 1/2 hour, after 3 days the pressure and
temperature graphs will fill the screen.
   To adjust the pressure to the local height, find the present
pressure from a local source, and touch up VR1 for a correct
reading.


- A DECREASE IN BAROMETRIC PRESSURE ANNONCES BAD WEATHER, RAIN,
  WIND,STORM, HEAT.
- A RISE OF BAROMETRIC PRESSURE ANNOUNCES NICER, DRY, COLDER
  WEATHER.

1- A slow, regular and moderate fall in pressure indicates that a
   low pressure area is passing at a good distance. There should
   not be any marked change in weather.

2- A sudden decrease, even if it is small, announces a nearby
   disturbance; normally meaning wind, short showers. If large
   in 5 to 6 hours, signals a comming storm.

3- A large, slow and maintained decrease announces a long period
   of bad weather. This bad weather will be more pronounced if
   the barometer started higher and is going lower.

4- A sudden rise of the pressure, when the pressure is about
   average or above average and the weather is nice, always
   announces the aproch of a low and the barometer will soon
   start to come down.

5- A rapid rise when the pressure is low, annonces a short period
   of nice weather; but, if the rise is large and prolonged, we
   can count on a many days of good weather.

A sharp change in temperature indicates a change in air mass and
wind direction.