Hi all,
Many thanks for the varied and wonderful answers. I am driving the relay
transistor (2N222A) through a 1K resistor from the PIC and the relay has
reverse BEMF protection.

                -----Original Message-----
                From:   Dale Botkin [mailto:dale@BOTKIN.ORG]
                Sent:   Friday, 10 March 2000 10:26
                To:     PICLIST@MITVMA.MIT.EDU
                Subject:        Re: Gotcha not

                On Fri, 10 Mar 2000, John Orhan wrote:

                8< snip...
                > valve ( this in fact turns on a relay that activates the
24vDC water valve),
                8< snip...
                > PIC is 16C84 and the whole thing sits under an acrylic
dome. Anyone hear of
                > heat or light affecting the PICs as it seems to be
latching up not turning
                > off the water valve. I don't think it would ever get more
than 40 or 50 on a
                > hot day   ( degrees C ). Any ideas??

                Do you have a reverse diode across the relay coil?  If not,
de-energizing
                the relay will cause back EMF as the magnetic field
collapses, which will
                spike the PIC pin and cause problems.  Pretty common
problem, from what I
                have heard.  Of course, one would think that would cause it
not to turn
                the water *on* the next time, but it would probably be
pretty
                unpredictable.

                Another thought -- how are you driving the relay?  I assume
you have a
                bipolar or a MOSFET to drive the coil?  You might be pulling
too much
                current thru the PIC pin if you're trying to drive it
dirrectly or thru a
                bipolar transistor.  That's why I prefer MOSFETs, too lazy
to compute
                base current. 8-)

                The PIC should be good to 125C under bias according to the
spec sheet...
                but remember to allow for the black PIC absorbing energy
from sunlight,
                which will raise the temp. of the PIC well above the 50C
ambient
                temperature.  Shouldn't get over 100C, though, I'd think.

                Dale
                ---
                The most exciting phrase to hear in science, the one that
heralds new
                discoveries, is not "Eureka!" (I found it!) but "That's
funny ..."
                                -- Isaac Asimov