# Thermometer with LED

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1. Oct 28, 2014

### nboston

Hi, I'm a freshman in college and a beginner in electronics. Does anyone know of a way to use a micro analog thermometer to change the color of a small LED light once it hits a certain temperature. I am looking to make this on an extremely small scale.

2. Oct 28, 2014

### Staff: Mentor

What is an extremely small scale? Centimeter? Millimeter?

How large can the transition region be? Just getting some color change depending on temperature is very easy, switching from "pure red" to "pure green" (as an example) within 1K will need more logic (probably something like a comparator).
Note that LEDs have a fixed color, changing the apparent color will need two different LEDs and some control over their current.

3. Oct 28, 2014

### sophiecentaur

I seem to remember that you could buy LED packages with two colours in them. You can produce an infinitely variable set of intermediate colours between the Red and Green of the diodes taken singly by varying the voltage supplied, iirc. Actually, I now remember I have two such indicators on the regulator for a Rutland 12v Wind Generator.

I should think that it would not be hard to drive a two colour LED, using a single transistor as an interface with the output of the thermometer. But you would need to specify what the thermometer actually outputs.

4. Oct 28, 2014

### Staff: Mentor

And probably 3 or 4 pins, which just means there are two different LEDs inside.
That would give a slow transition over a large temperature range.
I guess it is just a variable resistor.

5. Oct 28, 2014

### sophiecentaur

One package is aesthetically more pleasing ('colour mixing' built in and looks 'modern') and uses just one hole in a panel.

Could well be - but we'd need to know.

6. Oct 28, 2014

### 256bits

Use pulse width modulation to drive either of the LED's or both, and one can get a whole range of colors. Vary the voltage for the intensity.

7. Oct 28, 2014

### 256bits

A circuit utilizing a schmitt trigger might be what is needed, but still it, is a comparator circuit.

8. Oct 29, 2014

### sophiecentaur

Unfortunately, as we don't know what this 'thermometer' consists of we cannot usefully speculate about suitable circuits - only the possibilities for displays.
The OP seems to have run away in panic at our responses.