Rain sensor with snow.

[malc-c]

Thermostat/heater circuit diagram attached The thermistor is a NTC type and obtained from Maplin. The resistance quoted is at 25C. At around zero (freezing) it is 50K for this one. The variable resistor sets the temperature at which the heater switches on. The 270K resistor provides some hysteresis so that it switches cleanly and doesn't switch on and off too frequently. Without hysteresis, the transistor in series with the heater resistor, would turn on gradually and dissipate too much heat and fail.

Gina,

what was the wattage of the 330R heater resistor

[Gina]

The 33 ohm resistor I'm using has a 10 Watt rating. It won't get so hot that it could melt something.

[BigDave]

It doesn't look like we'll be getting any snow this year (famous last words, MetOffice report today that we might by end of week!), but when we had lots over the last couple of yrs, I began considering building some form of heater. Trouble for me is that whatever solution I came up with would have to be wireless.

I was initially thinking, solar panel to charge a 12v 7Ah battery, and then probably something along the lines of Gina's 741 opamp design,

My fear is that leaving a valve regulated lead-acid battery out in the cold (albeit within an insulated plastic enclosure) would reduce it's life somewhat. Additionally at precisely the times when I would want maximum current available, the cold would inhibit the chemical reaction within the battery and potentially reduce it's output voltage. Add to that the need to dust the snow off the solar panel, and I think my idea has lost all of it's supposed "merit"!

Has anyone come up with a workable wireless solution at all. If so, I would be MOST interested.

Thanks all...

Edit: typo

[AllyCat]

Hi Dave,

Car batteries seem to work well enough in snow conditions but I think the issues are more fundamental. Earlier in the thread, heaters of about 3 watts or more were suggested, so your 12v, 7Ah battery will only last about 24 hours. In snow conditions I would expect the air temperature to keep the heater on most/all of the time.

The peak solar energy in Winter is about 200 watts/sq metre or perhaps 30 watts averaged over 24 hours. With a typical 10% efficiency you'd need at least a one square metre PV panel to keep the battery charged. But if you're going to assume that there will be continuous sunlight during "snow" days, then probably a direct solar heat collection/storage system would be easier and cheaper.

However, calculating from another viewpoint, 10 cms of snow is equivalent to about 1cm of water/ice, the collecting area of a Fine Offset sensor is about 55 sq cms, so there could be about 55 ccs of ice which need to be melted. If I remember my schoolboy physics that would require about 55 x 4.2 x 80 = 18,000 joules (watt.seconds) or 5 watt.hours. So it might be possible if you can ensure that most of the battery energy is used to directly melt the snow.

If I had to find a solution I would try a directly-heated "snow collector" above the rain sensor (perhaps a wire grid or a metal funnel) fixed onto a precise weighing device. The weight sensor would switch on the heating only when the weight of the collector (including any water that could remain on it due to surface tension) exceeds a "normal" value (i.e. when any snow or ice is present).

But IMHO snow is so infrequent here that it's not worth the effort, and the method would probably not be applicable to regions where there is more snow. I suspect the battery would need to have sufficient capacity for a full "snow season" and then a reasonably-sized PV panel could recharge it during the summer months.

Cheers, Alan.

[BigDave]

Wow! Thanks for the detailed explanation!

That answers that one then!

Perhaps I will find a way to run power after all eh!