Thermocouples (need amp, op amp, diff line drivers, use low AREF?)

I got a wad of K thermocouples and wanted to use them either directly or through some very inexpensive LM324 quad op-amps which I also have on hand. I know AREF can be used to change the window of the 10bit ADC, but suspednly a simple code error (which I’m prone to), can damage the chip. The tCs are calibrated to output 1mV at 1deg K. That’s easy math to code, but dealing with low voltage is prone to noise and low resolution.

My goal is to have temp sensing of room temps, cooler temps, and hot water temps stored in the nano’s register and sent via wire to others and eventually into MS Access/Excel via a virtual USB-serial com port. I wish about 1/3 degree F resolution in end result.

I know Sparkfun has custom calibrated amps but that puts these things at $8-9 per sensing unit which is above my budget.

Should I use the LM324s? Was the quad option a waste? Seeing as each probe is strewn thru two rooms of house there is 10+ feet of wire between tCs and from tC to nano.

  • Should I twist the signal around the ground wire?
  • Do I need differential line drivers (not On hand)?

moderator removed cross post

Google "lm324 thermocouple amplifier circuit"

Multiple rooms, and temps from freezing to boiling.
Isn't it easier to buy a bunch of waterproof DS18B20 sensors (ebay).
And wire them all to a single Arduino (one pin for many sensors).
Leo..

The only reason to use a thermocouple is if your temperature range is outside what the DS18B20 can read. (It shuts down at exactly -55C, just like the datasheet says.) Or maybe you have some other special requirement like response time or something.

It's certainly fun to learn and experiment but you should put the Apple 1 away and use an iPad for real work.

mattlogue:
I got a wad of K thermocouples and wanted to use them either directly or through some very inexpensive LM324 quad op-amps which I also have on hand. I know AREF can be used to change the window of the 10bit ADC, but suspednly a simple code error (which I'm prone to), can damage the chip. The tCs are calibrated to output 1mV at 1deg K. That's easy math to code, but dealing with low voltage is prone to noise and low resolution.

My goal is to have temp sensing of room temps, cooler temps, and hot water temps stored in the nano's register and sent via wire to others and eventually into MS Access/Excel via a virtual USB-serial com port. I wish about 1/3 degree F resolution in end result.

I know Sparkfun has custom calibrated amps but that puts these things at $8-9 per sensing unit which is above my budget.

Should I use the LM324s? Was the quad option a waste? Seeing as each probe is strewn thru two rooms of house there is 10+ feet of wire between tCs and from tC to nano.

  • Should I twist the signal around the ground wire?
  • Do I need differential line drivers (not On hand)?

Hi,
I think you need to do some research on thermocouples, they are very low signal devices that require very low noise amplifiers, in fact you need what is called an "Instrumentation Amplifier".
You have to have control over stability and noise.
Believe me $8.00 for a complete amplifier and I2C interface is cheap.
Using thermocouples as normal room and hot water type temperatures is not what they are designed for.

There are many other temperature sensors that will do the job and be less of a problem to design and maintain.
Can you please tell us your electronics, programming, Arduino, hardware experience?
Tom... :slight_smile:

WaWa and MorganS:

https://www.maximintegrated.com/en/products/analog/sensors-and-sensor-interface/DS18B20.html
Hot shit! Not that I need to measure temp of shit, but... 1 signal line for all, $3 each, and 12bits? Nice! Gonna just pitch my tCs and keep the fiberglass wire sleeves... those are nice.

TomGeorge:

I have dabbled in electronics a bit, know a deal of VBA programming and had several aborative attempts in C++ about a decade ago. I'm depressed and have add, so it hits my ability to concentrate. I can still confuse the polarity of a damned LED when i'm feeling off.

I was overzealous. Let me tell you, believe me... fake news, Wrong... oops, sorry,

Anyhow, I got a bunch for xmas and wanted to measure temps for rooms to decide [are rooms all cold? is air stratified, as in needs mixxing]. I leave thermostat on FAN to mix air and then forget to turn it back to heat, a lot! As in right now. I also wanted them to warn if water out of spigot was too hot by audio alarm. Otherwise I'd put them in kitchen, but then I'd use the stainless steel probes instead. They certainly respond fast as I have a thermometer unit.

I plan to have a whole house automation system that I build and code from ground up. Given my disability, I budget 2 years and $4,000 to complete. I am less about turning on and off lights and more about monitoring doors, utilities, air quality, etc.

I spent the evening wiring interfaces off my magnetic lock system so the board can unlock, detect key in, pex button presses, see if plates are touching, door is shut, bolt is thrown. Now I need to get them to talk to eachother.

Little things can cheer me up, like I got my arduinio to talk to the clicker, just had to find a Q that worked.

** $8 for a compensated Amp and I2C is nice. I like the last part especially as I will have a lot and I2Cs address-ability is enticing. I figure what you may be refering to in lieu of these K tCs are the 3 pin transister looking package things. AllElectronics has them.

Sounds like a great project. Also sounds like a good way to go at it - only do little projects that have a quick outcome. There's no need to wire the whole house right now when you can get a temperature sensor or door lock working right away.

I2C is great except the line lengths are extremely limited. I have had trouble with wires as short as 10cm. With effort, you can stretch it to 6m. The one-wire sensors can work on much longer wires. You will probably start seeing some interference as you go past 6m. You should always check the checksum to be sure you got valid data from each sensor.

Beyond that distance, as you get to wiring between rooms, the cost of the wire suddenly becomes greater than the cost of wireless radio units. Have a look at the JeeNode project that does this kind of house automation totally wirelessly with Arduino-compatible boards.

Thermocouples rely on the Seebeck effect to produce a voltage dependent on the temperature difference between two junctions of dissimilar metals.

Historically, you would have put one of the junctions in a mixture of ice / water to give a 0°C reference to measure temperatures using a thermocouple.
Modern circuits do away with the need for an ice bath by using cold junction compensation techniques. These use an additional temperature sensor to measure the ambient temperature

If you are measuring room temperature with a thermocouple then you are relying on another sensor as well as the thermocouple.
Just ditch the thermocouple and rely on the _other sensor _if you mainly want to measure room temperature.