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The kettle, spark core, relay

Hi! I'm still working on my kettle project. I want to obtain the kettle temp from the existing thermometer. I'm able to do this, but much like trying to look at an electron my circuits all reduce the voltage going to the kettle control board. This means the kettle's microcontroller (SN8P2722 SONiX) acts as if the kettle is cooler than it really is.

5v control board.

I don't think there is an easy way to program this microcontroller (if there is I'd love to know) -- I suspect the bad temp data will lead to overheating and hot water all over my kitchen.

enter image description here

Is there any way to passively obtain the voltage? I've been looking into the way that voltmeters work. Is this the right idea? is there a breakout board for this?

I really wanted to have the existing buttons still work as they always have. I've done this with power. I can press it remotely or on the kettle and I can tell of the kettle is on or off both on the control panel and remotely.

enter image description here

I can press all the other buttons remotely but I can't tell if it's working without knowing the kettle temp.

Maybe I should try to replace the entire control board? What would be easier? I'm something of a beginner.

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futurebird
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    Please post the schematic for the voltage-measuring (temperature measuring) part of your system. The input impedance of the analogRead should be pretty high. – Nick Gammon Jul 13 '15 at 20:51
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    I'm the last person to question somebody who's making a tea-related project, but are you ultimately attempting to control your kettle over WiFi? Do you *really* need to be able to turn your kettle on from your computer or phone so that it's just boiling as you walk into the kitchen and thus eliminate all that unnecessary hanging around while OH MY WORD THIS IDEA IS GENIUS. – CharlieHanson Jul 13 '15 at 22:25
  • `This means the kettle's microcontroller acts as if the kettle is cooler than it really is.` So what? Just "press" the button again to turn it off when you determine empirically that you have reached the right temperature. – Nick Gammon Jul 13 '15 at 22:43
  • `I can press all the other buttons remotely but I can't tell if it's working without knowing the kettle temp.` Why wouldn't it work? You have pressed the button, you detected the "on" LED lit up. What could go wrong now? – Nick Gammon Jul 13 '15 at 22:46
  • Where are you attaching the wire going into the analog pin on the arduino? There might be some amplification/buffering going on on the board. Try tracing where the signal goes, and then connect the wire to the trace that's going strait into the microcontroller. Also, do you have any idea what kind of sensor they use for temperature measurement? – Gerben Jul 14 '15 at 12:40
  • 1. The goal is to have hot water waiting for me when I wake up or get home to save time. The other goal is just to learn how the kettle works and learn about micros. 2. " So what? Just "press" the button again to turn it off" This is the kind of kettle that heats water and keeps it at a set temperature for an hour. Many teas need water that is hot but not boiling. Monitoring the kettle manually defeats the purpose and would mean my project is worse than the kettle I started with. – futurebird Jul 14 '15 at 14:16
  • 3. I will see if I can figure out the "voltage-measuring (temperature measuring) part of the system" I'm unclear if you want to know about the sensor, or if you want more information on the 5v board and display? 4. "Where are you attaching the wire going into the analog pin on the arduino?" I'm tapping the output to the 5v board coming from the HV board (2nd to last image) 5. "Also, do you have any idea what kind of sensor they use for temperature measurement?" No, it's built into the kettle base. I'll see if I can find out though. I do have and extra kettle that I can rip apart. – futurebird Jul 14 '15 at 14:16

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A JFET op amp such as the TL071 has an input impedance in the hundreds of gigaohms or higher. Using one to buffer the voltage should allow you to sample it without much interference to the existing circuit.

Ignacio Vazquez-Abrams
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  • so the goal is to make this: https://en.wikipedia.org/wiki/Precision_rectifier – futurebird Jul 13 '15 at 16:52
  • Why do you need to rectify the signal? – Ignacio Vazquez-Abrams Jul 13 '15 at 16:53
  • "Total Supply Voltage (Min) (+5V=5, +/-5V=10)" the voltage is 0-5v. This is less than the min? I can look for one with a different set of specs. – futurebird Jul 13 '15 at 16:54
  • I just need to read about this more. I'm really confused. – futurebird Jul 13 '15 at 16:55
  • A circuit diagram would help muchly. However: the analog voltages are most likely DC. If they are within Arduino ADC range then reading most such would affect them minimally. If a buffer is needed then a circuit as simple as in Fig 3 in [Ignacio's buffere reference](https://en.wikipedia.org/wiki/Buffer_amplifier#Voltage_buffer_examples) should be good enoug in may cases - but a TL071 may not be the best choice, depending on voltage to be measured and available supply voltages. | Using a voltmeter (DMM) will allow you to measure voltage and advise us how it varies with temperature. ... – Russell McMahon Jul 13 '15 at 17:33
  • ... BUT it would be very useful if you can provide even a partial hand drwawn cct of what you are measuring. If you do not know you are wandering around in the dark and expecting us to discern what is in the darkness by remote control. (We may be able to hazard a good guess but your help will help. | What is range of voltage to be measured? AC or (presumably) DC. . How is it produced (temperature measurement technology). | This should be "quite easy" once a little light is shone. – Russell McMahon Jul 13 '15 at 17:37
  • The range is 0-5v. But I need to reduce it to 0-3v or my chip will fry. – futurebird Jul 14 '15 at 14:17