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FirePIC PIC 16F84A Based Gas Fireplace Timer

Background

I live in Maryland, and our house has a heat pump. Maryland is borderline for heat pumps as far as I'm concerned. It can be 20 degrees around here and then the heat pump just blows cool air and runs forever.

In 2001 we installed a gas fireplace in our living room. It came with a remote with a thermostat, but I was never happy with it. Sometimes you'd notice the fire hadn't been on in a while, I guess because the remote signal hadn't made it to the fireplace. It never seemed to stick to the temperature either. I got to thinking that what I'd really like is to be able to run the fire for a short time on a regular basis, just to take the chill off.

The original fireplace PIC (photo bottom of page) had 4 switches in addition to the main Off\On:

On Time lets you set 5, 7.5, 10 or 15 minute cycles.
Cycle Time lets you set 30 or 60 minute cycles.
Run Time allows run times of 2 or 4 hours.
Always On manually turns the fire on full time.

I normally use 10 minute On Time and 30 minute Cycle Time. The fireplace runs for 10 minutes, is off for 20 minutes, runs for 10 minutes, is off for 20 minutes etc. It will run for the total Run Time and shut off for good.

If you set a Run Time of 15 minutes and a Cycle Time of 60 minutes, it will run for 15 minutes, be off for 45 minutes etc,

We added an enclosed porch to the house in 2003. It has heat ducts, but they're so far from the furnace they really don't do a whole lot so we added another fireplace out there. I ran it off a breadboarded circuit for the first year or two, but I really wanted to build another FirePIC.

This is the newer version of the FirePIC. With the breadboard circuit I had noticed that the first 10 minute run didn't really warm up the room enough. Most times I'd end up resetting to get a second run. This newer version has a Preheat switch that causes the fireplace to run for 10 minutes before going into its regular cycle. So if the On Time is set for 10 minutes and Preheat is on it will run for 20 minutes the first time.


Files

Schematic

fire22.asm for PIC OR fire22.hex already compiled


Miscellaneous Notes

The original FirePIC used a 16C54 PIC. I used a 16F84A for the new one bacause I had some laying around. Got to do away with the pullup resistors on Port B with that too.

The main switch provides power but also cuts the grounds to the IRLL014N MOSFETs too as a safety precaution. No way anything can turn on when the switch is off. What we normally do is run it about 5 PM and let it run for 4 hours so it turns itself off at 9 PM. When I'm passing by later in the evening I flip the switch off.

The case is a bit large. Something I happened to have around. But it also gets hard figuring out where to put all the switches and a bigger case makes it easier. I like the tilted top for on top of a fireplace too.

Batteries are 4 penlight cells which seem to last a season (October to around April). I just used stick-on Velcro tabs to hold the batteries in the bottom of the case.

The MOSFETs are surface mount. If you want to use a TO-220 case get IRLZ14s. That's what I used in the original. They're nice logic level MOSFETs. Easy to use with PICs.

The first FirePIC had a circuit board and all the switch wires were soldered to it. Kind of a rat's nest. The second one I made a circuit board that held everything.Circuit board photo.

The fireplaces we have are Desa Compact Classics with the remote ready option. The gas valve on the fireplaces have terninals to which you can attach a remote, thermostat etc. Basically, when you short the two terminals together the fireplace comes on. You connect to the top and bottom terminals in the following photo gasvalve.jpg.

You can actually see the original FirePIC to the upper left of the fireplace in the one photo on my Web site. Since that photo was taken I added a 120V switch on the front of the cabinet to turn the fireplace fan on. We turn on the fan as soon as we start the fireplace. I find that if you depend on the thermostatically controlled fan enough heat builds up to dry out and crack wood surrounds. I always advise customers I build fireplaces for to turn on the fan manually.

Jeff Thorssell - Remove the brackets in the email address to email: jthorsse[@]mchsi[.]com

My Trim Capentry