Awesome idea and I need your help.

[Kurt_]

Basically, I need something that will measure a temperature (an electronic thermometer) and will apply voltage to some nichrome wire or other heating element if the temperature falls below a certain value.

Also, if possible it should be battery powered.

TI.com, maybe?

Any clue what I need?

[vskid]

Thermistors change resistance with temperature, that might help.

[jleemero]

Whatcha making anyway eh?

The only thing I could think of would be a digital thermometer hooked up to a microcontroller which is set up to do what you wish...

Then again a thermistor should actually work too... Simpler...

[Electric Rain]

Something like this (but cheaper)? Is this what you mean?

[jleemero]

Not quite...
Well sort of, but more like <a href="http://www.goldmine-elec-products.com/p ... 5">this</a>.
That's a thermistor.

There are two types of thermistors.

There is NTC and PTC.

PTC: Higher Temp, Higher Resistance (this is what you want)
NTC: Higher Temp, Lower Resistance

The link goes to a PTC type thermistor.
If you wire that up with your nichrome and a power supply, as the nichrome heats up, the temperature around it will rise and the thermistor's resistance will rise, meaning less power will be supplied to the nichrome wire, letting it cool down.

Ultra simplistic temperature control, albeit difficult to "calibrate"....

[timmeh87]

The on resistance of a thermistor is still like 25 ohms (at least the one that got linked). I dont think that dinky little part can handle the 10 or 15 amps that you need to run through nichrome wire to make it heat up enough to vacuum form (im assuming that that is the idea here). it would be dissipating like 250watts in the best case, creating tons of its own heat, and really defeating the purpose of it being a temp. sensor... it would turn off in like 10 seconds, regardless of how hot the wire got.

the cheapest and easiest thing i can think of is to just get one of those old-school thermostats with a bimetallic strip and a mercury switch in them. temperature makes the strip curl up until the switch engages. which is the opposite of what you want... so flip it upside down. haha.

or else, toasters have a similar device in them that make them pop at a certain temperature.

failing that, i have some low-temp sensors i got from digikey, youd need to interface them with a uC. i think they only go up to like 70C though

you could just buy one of those indoor/outdoor temperature sensor things, stick it in there, and manually turn it off when it gets too hot

or just use the oven and set it to a specific temperature.

check digikey, they have other electronic devices designed for ovens and stuff that fit your description pretty well.

[Kurt_]

Vacuum form? I think not.

I plan on making a self-heating coffee mug, unless the components would be huge and have to plug into a wall.

[codeman]

You're simply looking for a temperature monitor that says "on" or "off", right? Use and OP-AMP! Use a thermistor as part of a voltage divider, and look up how to set up an op-amp as a schmitt trigger. That way you can set a hysteresis loop, where it will turn "on" when the temp is below a certain value, and then once it passes a certain threshold it will turn "off".

[Krepticor]

the amperage required to heat set mug would be enourmous (like one or two amps at a higher voltage, like 110 high)

You could take the idea to a thermos level, since heat doesn't travel in a vaccum, you could develop something there, have at it

remember that P=VI and the p is power, I current, and V voltage

[Kurt_]

Let's stray from the subject of turning the heat on, and think about the actual heating element.

Nichrome wire, when shorted with a 9V, turns red and disappears. How is Nichrome stabilized for soldering irons? Lack of oxygen, or does AC current do something to it?

Next, if I wanted to apply a small amount of heat to the liquid (if the trigger mechanism works, it won't need to raise it more than 2-3 degrees) what would work? For portable power possibilities, should I go for more voltage, like a 9V, or more current, like a 2-4 AA's?

THE OP-AMP AS A SCHMITT TRIGGER
The OP-AMP can be wired as a Schmitt trigger. The diagram below shows this arrangement:


When the input of the Schmitt Trigger is LOW, the output is HIGH.
As the input rises, nothing happens to the output until the input is 3v3. This is the voltage on the "+" input due to the effect of the three 10k resistors. These 3 resistors form a voltage divider with two 10k resistors connected to the 5v supply and one 10k resistor connected to 0v.
When the "–" input is 3v3, the output of the OP-AMP goes LOW and it remains LOW until the input falls to less than 1v6. The 1v6 voltage on the "+" input is produced by the three 10k resistors. When the output is LOW, one 10k resistor is connected to the 5v supply and two resistors are connected to the 0v rail. This produces 1v6 on the "+" input.
The purpose of a Schmitt Trigger is to detect and respond to a signal that rises and falls a large amount - in other words it has "large excursions."
There are also signals that rise and fall very slowly - such as a photo transistor detecting daylight.
During the detection process, the output will rise and fall slightly during the morning light and the change from one level to the other will cause the project to turn on and off.
This is unwanted. The Schmitt trigger will produce an output when a definite condition is met and will not change until the daylight is reduced considerably.