Temperature controlled Led build

[Tekno]

Being familiar with the cathode tube and leds that feature the music reactive feature i was wondering if this can be similarly achieved with temperature. For example if i had 3 or 4 strips of leds red, orange, yellow, blue.

At start orange would be on, when temperature reaches say 120c red would come on as well, 150 yellow and 200c blue.

I assume a thermocouple would be needed to gauge temperature but beyond that i am unsure as to how to acheive this.

[eccerr0r]

Re: Temperature controlled Led build

There are many ways to do this of course...

You'll need to get accurate resistors and a good voltage reference, and multiple op-amps. First you'll need an op amp stage for voltage gain as thermocouples do not generate that many volts, depending on what thermocouple you have you'll try to get enough gain to cover as much of the output swing of the op amp (and worry about noise...). Then you'll need several comparator stages which can be built with op amps or comparators. The comparators have the reference voltages set with the resistor dividers that match the voltages you get from the first stage. This is a "flash" ADC topology just like a lot of the digital VU meters. An LM3914 all-in-one flash ADC could be used too, similar to other designs...

I think the first stage, dealing with the thermocouple (or if you don't have to go very high, say 100C is enough a thermistor is somewhat easier to work with as it will have a higher voltage swing) is the trick. At least thermocouples are fairly linear unlike thermistor voltage dividers which have a very strange temperature to voltage output relation - which is not a big deal if you made your own turn on reference voltages...

Hope this is a good start on figuring out how to do such...

[Agent24]

Re: Temperature controlled Led build

https://www.sparkfun.com/products/306 could be quite useful

[clearchris]

Re: Temperature controlled Led build

Arduino + maxim type k to serial output chip is how Id do it.

[mariushm]

Re: Temperature controlled Led build

If you want to stick to below about 125c, you can use very cheap integrated temperature sensors such as MCP9700, which tell you the temperature with about +/- 2c accuracy by outputting a certain voltage for a specific temperature.
You can then adjust that voltage using an opamp or read the voltage into a microcontroller and do something with the information.

MCP9700: http://uk.farnell.com/microchip/mcp9...-92/dp/1332166

For higher temperatures, there are ICs like the one Agent24 mentions which are basically reading the K probe and have a built in correction table (output of a K probe is not linear from -something C to +something C, it has to be adjusted from place to place). The chip also allows you to adjust output to some specific voltage range (for example 2v for 0c and 5v for 500c or something like that).
The downside is that it's very expensive but at the same time it's very accurate. Sparkfun's price is actually quite reasonable for that IC.

There are other ICs that are cheaper but don't have the same precision or accuracy or are designed for other temperature probes which may not work at as high temperatures as the K probe. Lots of sensors and chips to explore here: http://uk.farnell.com/sensors_temperature