"Don Y" wrote in message
...
On 9/3/2015 5:58 PM, Robert Green wrote:
"trader_4" wrote in message news:6612702d-

stuff snipped

Also,
not sure what a "PIC" is. If you used a general purpose CPU, it's only
part of the solution.

Programmable Interrupt Controller.

No. It's a (icky!) microcontroller series manufactured by Microchip.

http://www.google.com/search?q=Progr...upt+Controller.

Oops. It's both but in this case the OP probably did mean the
microcontroller by Microchip.

The original "PIC" was a (REALLY icky) manufactured by General
Instruments -- the original name for Microchip. They were originally
used in CATV tuner boxes -- as such, didn't really have to do much!
PIC == Programmable Interface Controller though I suspect that
acronym is lost over the years.

The devices have *slowly* improved in capability. Yet, are widely
used, today -- largely because they are cheap and Microchip is
very friendly to "hobbyists" making it easy for folks to play
with (and later, adopt) these components.

I think my furnace uses one. But. consider how simple the algorithm
there is:
- wait for thermostat to call for heat
- turn on exhaust blower
- turn on gas
- strike igniter
- check for ignition
- repeat a few times, MAXIMUM; if no joy, shutdown and vent the furnace
- keep gas running as long as call for heat and flame sensed

No fancy math, no user interface, etc.