Could the analogical input and output of the minijack on smartphones be used to mesure temperature via a type K thermocouple?

Question

I'm an advanced industrial engineering student and I'm currently looking for some "final project" material. I have this idea of a product where you can plug in a type K thermocouple at the end of a meter long wire to your smartphone via the minijack port, as to mesure mainly kitchen-like temperatures (you know, how hot is the oven, is the meat ready, that kind of stuff) but it may also be used for other things. The concept would work initially for cooking and baking. It would need the output part of the minijack to get a steady and known electrical potential, and then to read the potential at the input part, where the difference found is proportional to the temperature measured. Can this be done? Can one get into writing and reading certain values of potential at the minijack port?

PS: I've got some basic knowledge of programming and null experience whatsoever with smartphone programming. If you can help me, please add links, references and material where I should look into and introduce myself further to the matter.

Answer 1

The only input possibility is for the thermocouple to act as a microphone or audio source. There are several reasons why this wouldn't work.

1. There will most likely be a DC blocking capacitor on the audio input. This will block your thermocouple signal.
2. There will be a bias voltage of a volt or two on the microphone input to enable FET microphone capsules to operate. This will swamp any thermocouple voltage.
3. You have no way to effect cold-junction compensation in the phone.
4. You won't have compensating connectors so you'll have another bunch of thermocouple junctions at the plug and socket.
5. You'll get mV out of the thermocouple and the ADC resolution would be useless.
6. If it were possible it would have been done already.

You could create a Bluetooth module with thermocouple input but who's going to buy it when regular analog or digital oven thermometers are so readily available and cheap?

Answer 2

This probably won't work the way you describe. However, what you could do is use a generic thermocouple amplifier to generate a dc voltage from the thermocouple and then use a voltage-to-frequency converter to generate a frequency proportional to temperature. Input that signal to your phone/laptop. Run an FFT on the phone to determine the frequency and then convert back to temperature.

The problem I see with your plan is that the audio input of a phone is unlikely to handle dc voltages very well. Usually the dc component of an audio signal is blocked.

Answer 3

Microphone plugs expect AC and the circuitry is AC-coupled, that prevents any possibility of it ever working. Thermocouples output a DC voltage, and very small, voltage at that which needs to be carefully amplified. Even if you had just a DC-coupled ADC, just connecting a thermocouple to it is unlikely to work due to the very low level signal. There needs to be signal conditioning.

Thermocouples also only measured the temperature difference between its ends. So to measure absolute temperature you need a temperature sensor on the "receiving" end or a way to set the temperature on that end to something known (like an ice-bath).