I made a 12v timer today, using a programmable thermostat.

Shown here: http://www.reuk.co.uk/Convert-Thermo...mer-Switch.htm

I checked some circuit boards I had laying around, till I found a resistor close to the same resistance as the t-stat's temp sensor at room temp. Found one that keeps the temp display at 89 degrees constantly,(thinks it is,anyhow) then desoldered, and put it in the t-stat.

then, set the t-stat for cooling, with the target at the max of 95 degrees, so it would always be off, then set 2 specific programs ....one for all days of the week, one for weekends, to switch to 59 degrees at 5:45. It then "kicks on", and switches a relay that provides 12v power to one of those small 12v defrosting heaters.....150 watt, for 15 minutes, or until I turn the heater off

$20 for the programmable t-stat(must be battery operated!!)

$15 for the defroster

$3 for the relay

Never having to scrape my windows after work, priceless!!

 

Very nice. I can appreciate a good electronics project since I build that kind of stuff too. Great Idea!

 

Here is a link to various charts showing what temperature = what resistance.
you may have to play around but these will give you a starting point at least.

http://www.greystoneenergy.com/Engli....html#techinfo

 

What am I missing here?

A programmable stat can be set to heat only, cool only or auto switchover.

You can take any programmable stat, even a heat only programmable stat and set your program to come on/off whenever you wish. This way if the vehicle or OSA temp is 72 and your stat was set to 68 it just wouldn't force the heat mode unncessarily. Yes, it does need to be battery operated and therein lies another problem. You will need to recharge the batteries frequently. Plus a lot of stats lose their memory settings when the batt is pulled or weak. Bummer.

I can see the need for this, and I think you have a nice idea. If yours does not work out the way you thought and need help then PM me and let me know, I'll work with you best I can (I am in America with American products) and we'll get something going that everyone can share.

Over here stats are normally 24VAV and vehicles 12VDC I guess this is the reason for the remote start function, either cool down or heat up the vehicle. Dunno, never had the need.

 

First, I'm in Ohio

This T-stat can be set for heat, or cool. You MUST have a constant temp to be able to use it as a timer only. The TStat always thinks it's 89 degrees(or whatever),so, a setting to cool to 95 degrees leaves it ALWAYS off, EXCEPT for the 15 minutes prior to getting off work, when I request a cooler 63 degrees(or whatever). Otherwise, you'll be dealing with constantly changing temps that can't be predicted, or controlled. If it's 10 degrees below zero, how can I set it to a lower temp for heat....(or higher for cool), so it will remain OFF? They don't have that big of a variable on temp settings.

You could find the resistor needed to make it always show 35degrees, set it for HEAT, at a desired temp of 30, then have the 15 minute period set up for 40, which would trigger the relay for the same 15 minute period.

This TStat IS 24v, but doesn't matter....all it's doing is transferring a 12v "input" that "should be" 24v, to a relay that does the work. And, the batteries are only powering the LCD display, NOT the relay's trigger. If the batteries don't last all winter, I'll personally eat them, and post a video of it on youtube!

 

HEE HEE, I'll be waiting for that video! But pullease, I'd far rather see you eat worms then batteries which will harm you. I would in HVAC and do automation, programming and blah blah blah

It the stat is set on heat only, programmed for that 15 minute cycle you are speaking of, then you could set the heating setpoint to 80 degree's if you wish and the heat will come on at the appointed time as long as the temp is below 80 degree's.

If it is about 80 you certainly won't need the heat. Sorry about thinking you were in the UK, I just took it for granted when I saw the URL was in the UK.

Don't show your rig to a cop or have it laying on the seat.

 

If it's 80 degrees, why would I want to preheat my Jeep?
You're in Ca.? Have you ever been in below zero temps? Do you have any idea what the normal temp. setting range is for a thermostat? THAT, is the problem.

If you work in HVAC and programming, you're making this MUCH more difficult than it is. It's just a simple circuit, hooked to a relay, and made possible by a simple resistor to remove the impossible task of programming temps that are outside the range of a thermostat.

 

I just used 80 as a starting point because of the high temp you used with the reistor.

The point I was trying to make is that you don't need to modify the stat to make it do what you want. Unless I am really missing something here. I was just trying to show you or anyone how you can use your idea a bit easier.

Yes, I have been in lower temps. I also can get programmable stats that can handle the amperage (meaning you don't need the relay) but they cost a good deal more.

The normal range for stats like domestic stats for AC and heat run from about 50 to 85 degree's. There are many other ranges out there, but these are the normal household ones.

To follow through with your idea of a 15 minute timer for heat, you can simply set the stat demand higher than the surrounding temp. and then have the program change to setting 15 minutes later to a point that it will shut off. The colder it gets the longer you may need to run the heat so you simply extend the demand time to say 1/2 hour. You can also run a fan across the heater to warm the windshield up and cover a larger area.

Whatever, you have a good idea there but a battery operated stat will most likely not last a week in operation. The batteries are there mainly to hold the program in memory. (meaning the electronic ones)

Sorry if this came across as if I was raining on your parade. I did not mean it that way.

 

50 to 85? OOOOOKay, You can't set a heat OR cooling point, when it's below freezing, or worse yet, below ZERO. As you said, normal range for a t-stat....50 to 85....unfortunately, that t-stat is sitting in my Jeep at work all night, in FRIGID temps. It it's 10 BELOW zero, what'cha gonna' set the heat setting at to keep it normally off? It's OUT OF THE TSTATS RANGE!

And, why get a more expensive t-stat that can handle the amps? So I wouldn't need that $3 relay?

My batteries won't last a week?

I give up....if you're in HVAC and Automation, then I'm a self taught brain surgeon.