A HVAC THERMOSTAT USING A PIC

[moldmaker]

DOES ANY ONE KNOW WHERE I CAN GET A SCHEMATIC
AND SOURCE CODE FOR A HVAC THERMOSTAT THAT USES
A PIC<p>THANKS<p>[ July 14, 2004: Message edited by: moldmaker ]</p>

[bodgy]

Well another rejected project - I have a circuit using the Sensirion SHT humidity/temperature sensor.<p>You can buy them from Parallax.<p>It uses a 16F873, has outputs to control, over temperature, under temperature or under humidity.<p>The heater output (undertemp) can be via a DC switch or an AC solid state relay.<p>I can let you have the diagrams and the text and code if you wish. I have the DC control board PCB's, takes upto 6 mosfets, but they wouldn't be free.<p>Colin

[moldmaker]

SOUNDS INTERISTING BUT I NEVER USED ANYTHING FROM
Parallax, SO I REALLY WOULDNT KNOW HOW TO
PROGRAM IT.<p>[ July 15, 2004: Message edited by: moldmaker ]</p>

[bodgy]

Only the Pic needs to be programmed - if this interests you then email me privately. Parallax just sell the sensor.<p>The photo's that I have of my unit use the straight SHT11 an engineering sample I got from Sensirion.<p>The SHT sensor is an ultra small SMD part, all Parallax have done is to premount them on to a PCB so that they can be plugged in to an 8pin DIL socket.<p>Also Peter Anderson sells them. <p>www.sensirion.com
www.parallax.com
www.phanderson.com<p>Colin<p>[ July 15, 2004: Message edited by: bodgy ]</p>

[Deal]

HVAC is a wide field. Most of the PC boards are proprietary and built around select and specific control sequences based on the HVAC unit's particular control requirements. My advice is you write your own, starting with the simplest flow chart, based on your control requirements. Colin's suggestions are a good start. Most control sequences are simple. Adapting and tuning and controlling the controls and sensors to your system is the crazy trick. Another advice is to lay it out exactly what you are trying to do. Lots of experienced capable people read this site and are interested in helping and learning.

[moldmaker]

all i really would like to do is
program the unit to a certain temperture
and have the furnace or air conditioner
stay at that setting then shut off when the temperture goes 4 or 5 degrees above the value
that i preset it too.(thats for heat the opposit
would be for cooling)<p>i was thinking about using a thermister<p>i am not really good at programming pics
or desinging my own circuits<p>but i do like colins circuit<p>[ July 16, 2004: Message edited by: moldmaker ]</p>

[bodgy]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by moldmaker:
or desinging my own circuits<p>[ July 16, 2004: Message edited by: moldmaker ]<hr></blockquote><p>Ahh desinging a circuit, that whiff of exploded tantalum, the chips running hot, that soupcon of ozone, the molten solder, that sceptered isle, that -- er sorry just couldn't resist.<p>Now by furnace do you mean something that is hot enough to melt metal, or what us english lot call a central heating boiler?<p>If the former, probably a thermocouple is better to use, though some thermistors can take and measure high temperatures.<p>A cheap version of a thermistor is a string of silicon signal diodes 1N4148 in series can work.
It doesn't work the same way as a thermistor but the result is very similar.<p>The range of temperatures that need to be measured are important as well as whether the measuring sensor is going to measure ambient temperature or as in a metal furnace or crucible actual temperature.<p>Colin<p>[ July 16, 2004: Message edited by: bodgy ]</p>

[moldmaker]

no its just a forced air furnace for a machine shop so i am not looking for precision
temperture controll<p>[ July 17, 2004: Message edited by: moldmaker ]</p>

[JKMADSCI]

I recently had great luck using a commercial thermostat to power a large relay. I used the relay to power on and off my large AC(28000 btu).
i purchased the relay on line (30 amps 240 volts)
The relay has a 5 volt coil powered by a wall adapter, through the thermostat.

[toejam]

why overcomplicate things? A comparator a zenar diode a pot and a temperature sensitive resistor will do the job just fine with the help of a few inpensive parts.

[moldmaker]

the problem is that my thermostat is shot
and i really dont know what the temperture
is i thought if i used a pic with a lcd display
at least i could set it without guessing
what the temperture is.
the thermomiter on the thermostat isnt very
reliable

[toejam]

how many degrees f is your temp. range?

[moldmaker]

the temperture range that i am looking for is
50-80 degrees F.
the reason i need to go so low
is in the winter time i could leave the shop for a week or so and the pipes wont freeze.