I have hacked this kettle so that I can turn it on and off remotely via wifi and that is working perfectly.

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Added this to get a better signal.

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This is my micro controlling the 5v buttons on the kettle.

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I keep running into a wall when it comes to reading the temperature of the water. The kettle does this automatically, but I want to transfer this information to my micro.

If I hook my multimeter to the ground and and yellow wire that connects to the lower voltage board (5v) that had the buttons and display I can use the voltage reading to predict the temp. The reading ranges from 0-5v, but my micro is a 3.3v micro so a direct connection isn't a great idea. I've tried using breakout boards designed to read voltage, but if I connect anything it throws the reading off.

The multi-meter is fine.

How can I get my micro to read the voltage as if it were a multi-meter? I even tried one of those little seven segment voltmeters but that threw off the reading just like the optocoupler and leds and other methods that I tried.

Here is how I have it set up:

  • Have you tried a voltage buffer followed by an attenuator yet? – Ignacio Vazquez-Abrams Dec 20 '15 at 0:11
  • no, but I'll research that next. – futurebird Dec 20 '15 at 0:16
  • @IgnacioVazquez-Abrams - I couldn't quite work out if it was a duplicate or a follow-up question. I believed it to be the latter. However, on re-reading it, it does appear to be a duplicate. It would be better if the original question was edited or appended to... – Greenonline Dec 30 '15 at 11:42
  • I am voting to close this... but I think that it would be a shame to lose the extra information... That information should be added to your original question. :-) – Greenonline Dec 30 '15 at 11:47

Voltage divider

You can possibly use a simple voltage divider. Try connecting two 100K ohm resistors in series between the voltage you want to measure and ground.

Yellow->100K ohm resistor->100K ohm resistor->ground

Now connect your multi-meter to the middle connection between the two resistors.

Depending on the circuit that happens to be driving the yellow wire, you should hopefully see a voltage that varies between ~ 0-2.5 volts. If so, you can connect this voltage to your 3.3v micro analog input.

Note that the value of the resistors is not critical - you just want them to be about the same and pretty large.

Op Amp

If the yellow wire output is so weak that even a voltage divider with very large resistors is pulling the voltage down significantly, then you can use a high impedance op-amp with 1x gain to buffer the signal.

So, you'd connect the yellow wire to the + input of the op amp, and connect ground to the - input of the op amp. The output voltage of the op amp would not track the input voltage (the yellow wire) and you can use a voltage divider on the output as above.

Use digital temp sensor

This option does not directly answer you question, but might be the best solution.

You can get something like a DS18B20 digital temperature sensor for a buck or two and connect it digitally to your MCU and avoid the problem altogether. This chip is happy at 3.3 volts, so you'd just need to put it physically someplace where it is sensing the same temperature as the yellow wire. This will likely be more accurate too!

  • When I do this (I tried this weeks ago) it changes the signal going to the micro in the kettle and the temp displayed by the kettle is no longer correct. – futurebird Dec 20 '15 at 16:13
  • What size resistors did you use? – bigjosh Dec 20 '15 at 19:36
  • I tried a few combinations. 10k, 100k, 1M then I tried 200ohm just to see what would happen. It changes the voltage, but it also messes up the temp readings on the teapot. – futurebird Dec 21 '15 at 0:04
  • So with 2x 100k ohm resistors in series connecting yellow wire to ground (nothing else) does the temp readings on the pot get messed up? Or only after you connect your ADC pin? – bigjosh Dec 21 '15 at 0:06
  • It's messed up before I connect it to the micro. – futurebird Dec 21 '15 at 0:18

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