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I found instructions on how to use a PT1000 sensor with an Arduino online. I want to implement this with a Raspberry Pi Pico, which should work the same way. However, I want to use 3.3 V instead of 5 V.

Now, my question is: How would I adjust the R2 and R4 to measure the temperature range of 0 °C to approximately 200 °C? (The PT1000 should have 1 kΩ at 0 °C and about 1760 Ω at 200 °C.)

Can anyone tell me some equations on how to calculate approximate output voltages for various values of the relevant resistors (R2, R4, and PT1000 (for the given temperature range))?

Here is a scheme of the circuit: enter image description here

I know the basic equations to calculate voltages at R1, R2, R3, and PT1000, but I am not sure how to calculate the voltage at the output of the LM358P, depending on the values for R1,... , R4 or whether and how to include the operational amplifier in these calculations.

PS: I had to use the PT100 tag since I lack the reputation to create a PT1000 tag.

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    \$\begingroup\$ The LM358 op-amp will only be able to produce a maximum output of around 1.9 to 2.0 volts when powered from 3.3 volts. Have you thought about using a simulator and a rail-to-rail op-amp? \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2023 at 14:56
  • \$\begingroup\$ @Andy aka If necessary, I could also use 5 V but I would have to make sure the analog input to the Pico does not exceed 3.3 V. If there was a way to scale the output voltage to 0 to max 3.3 V with a 5 V supply, this would also be an option... \$\endgroup\$
    – Manuel Popp
    Commented Feb 23, 2023 at 15:42
  • \$\begingroup\$ @Andyaka Just had to google rail-to-rail op-amp... so this would be basically a higher-quality version of the LM358 that almost reaches the input voltage of 3.3 V. I could try to get such a component. You think some rail-to-rail op could clip the 0...200 °C resistance range of the PT1000, producing a 0...3V3 output signal? \$\endgroup\$
    – Manuel Popp
    Commented Feb 23, 2023 at 15:57
  • \$\begingroup\$ It's best done in a simulator by yourself. I'm not being lazy; I truly believe you using a sim is a good idea. They are free and accurate for this sort of thing. \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2023 at 16:39
  • \$\begingroup\$ So far, I didn't manage to do this in a simulator... perhaps I have to keep searching for more simulators (I haven't used one for ~ 10 years...) \$\endgroup\$
    – Manuel Popp
    Commented Feb 23, 2023 at 21:49

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As stated in the comments, it's the easiest to simulate this circuit.

Here I've used TINA-TI (which you can download for free).

enter image description here

enter image description here

As you can see, this circuit is more or less unusable from around 1.4 kΩ (~110 °C).

Adjusting some values will net better results.

enter image description here

enter image description here

But this is still far from good, as it needlessly limits the working range from 0.6 V to 0.9 V. You're better off choosing an opamp with a rail-to-rail output. And then plug it into a simulator.

To do this kind of simulation in TINA:

Analysis → DC Analysis → DC Transfer Characteristic

Then select the component as input (R4 in this case), and specify start and end values.

enter image description here

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  • \$\begingroup\$ Thanks a lot. I tried CircuitLab online but it asked me to pay after a few clicks and in LTSpice, I didn't find the LM358 (I tried a simulation with other op amps but got unrealistic results). In other software I tested, I did not even get op amps even remotely similar to LM358 (if they had op amps at all). There are tons of free software but they are often incomplete or hard to figure out, imo. So thanks again, I'll check this TINA-TI out. \$\endgroup\$
    – Manuel Popp
    Commented Feb 24, 2023 at 16:31

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