[GPS_Standard] RE: Xtal Oven coming up to temperature from cold?

[Dick]

Nice to see you on the 'List' Zack and thanks again for originally pointing
me in this direction <grin> ... I have ordered some of the IC's (another day
or two to get here) but in the meantime I'm building up the various 
supporting' circuitry !! 
 
To manually do things is, for me anyway, a no-no !! .. I like to automate
things (guess I'm basically lazy <smile>)  - I run a ham-radio BBS on radio
and Internet - I have written small programs/scripts for Linux to do many
chores which I originally did manually .. So the die is set - I like to
automate - I get a real big kick doing it that way <smile> .. 
 
With regard my Pin-1 reset pin on the PIC, I have decided that I can take
advantage of my 24 volts starting off at 18 volts with a cold oven and
approaching 24 volts when the oven has stabilised .. 
 
I intend to use a 555 timer chip but not in the conventional timer
configuration - pin-2 of the 555 will switch the 555 on when it senses that
Vcc is 1/3rd or less and pin-2 of the 555 will switch the 555 off when it
senses the Vcc is 2/3rds or more ... I could use an op-amp as a comparator
but I decided to use the 555 ... 
 
I'll use pin-6 of the 555 to monitor the 'high' 24 volts and make the 555's
pin-3 (output pin) go low .. The 555's pin-2 will watch when the 24 volts is
around 18 or so 'low' volts .. So when I switch on, pin-2 of the 555 finds
the 24 volt line at around 18 volts and switches the 555 IC on .. An
open-collector NPN transistor added to pin-3 of the 555 will keep the PIC's
pin-1 (reset) low ... When the 24 volt line gets to say 22-23 volts, pin-6
of the 555 will cause the 555 to switch off and the PIC's reset pin (pin-1)
will go high and the PIC can start doing it's designated job !! .. 
 
I will add a led to pin-3 of the 555, label it as 'oven' (or something
appropriate) and be able to see where things are on the front panel .. The
circuit is basically the 555, a few caps, a few resistors and two trimpots
to adjust the 'low' and 'high' voltage levels  .. 
 
 
73, Dick ZS6RO
Http://www.zs6ro.co.za