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I'm building a temperature logger with four PT100 sensors. I've already decided to use a MAX31865 converter (just the IC), 3-wire PT100s and the device will be a Raspberry PI HAT - the PI will talk to MAX31865 over SPI and process the data. I'm considering three options for connecting the sensors:

  1. Use four MAX31865s. This should be reliable, but feels wasteful; they are $10 each and I have very little practice with SMT soldering. I'd rather solder one than four.

  2. Use one MAX31865 and low-RON multiplexers to switch between sensors. I've seen people do this, but I'm not sure multiplexer resistance is uniform enough to get a precise reading from a 3-wire sensor?

  3. Use one MAX31865 and twelve reed relays to switch between sensors with low resistance. This is hopefully easy to solder and I can drive the relays with four PI GPIOs (each switching three relays) through a transistor (the relays have 5 V "coils" IIRC).

What's the simplest solution?

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  • \$\begingroup\$ please edit your post ... add links to the datasheets for the sensor, the converter and the Pi hat \$\endgroup\$
    – jsotola
    Commented Sep 12, 2023 at 21:45
  • \$\begingroup\$ Which option would you choose and why? asks for an opinion ... such questions are not allowed here \$\endgroup\$
    – jsotola
    Commented Sep 12, 2023 at 21:46
  • \$\begingroup\$ @jsotola To be fair, I think given three methodologies to accomplish something, the question could be inferred to be "which is the simplest?" That said, I think more detail is needed to qualify that. \$\endgroup\$
    – JYelton
    Commented Sep 12, 2023 at 21:54
  • \$\begingroup\$ Added links and made the question clearer. \$\endgroup\$
    – Marek
    Commented Sep 12, 2023 at 22:06

2 Answers 2

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The "simplest" solution is to use 4 MAX31865 modules (available at not much more than the cost of the bare chip, but which include connectors and all the support components). Most terminate on a 0.1" pitch, and are easy to integrate. I've done something similar in the past for quick and easy termination of multiple thermocouples.

In terms of switching rtd sensors it can be done with relays, but whether this will suit depends on the accuracy/repeatabilty you are seeking. You should look at your requirements, and work out what that equates to in terms of variance of resistance. This then needs to be compared against the resistance variability of any multiplexing scheme.

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  • \$\begingroup\$ Thanks! Is the module you're referring to something like the Adafruit board? This would make my HAT quite tall, but perhaps can be done. Is it fair to assume that the resistance of identical reed switches when "on" is very close? \$\endgroup\$
    – Marek
    Commented Sep 13, 2023 at 8:44
  • \$\begingroup\$ From memory typical reed switches have around 70mR-100mR resistance, wire to wire, with about 10mR-50mR in the contact itself. The key challenge is that at very low currents and voltages, contacts don't always "wet" properly. Have a look at this article for details en.wikipedia.org/wiki/Wetting_current . My own experience is that to minimize this effect you need to use gold plated contacts. \$\endgroup\$
    – colintd
    Commented Sep 13, 2023 at 10:01
  • \$\begingroup\$ I would go with the 4 modules to avoid the problem. If height is an issue, just mount then next to each rather than stacked. \$\endgroup\$
    – colintd
    Commented Sep 13, 2023 at 10:05
  • \$\begingroup\$ Here's more details of reed switches standexelectronics.com/wp-content/uploads/… \$\endgroup\$
    – colintd
    Commented Sep 13, 2023 at 10:10
  • \$\begingroup\$ Thanks for your input. In the meantime, I've also found LTC2986, which supports multiple RTDs and wiring schemes, is easier to integrate and cheaper than four MAX31865s. I'll try this and if it doesn't fit, I'll go with your suggestion. \$\endgroup\$
    – Marek
    Commented Sep 13, 2023 at 16:19
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Use a MAX4782 dual MUX and remove track on 31865, like in this thread from the Arduino forum:

https://forum.arduino.cc/t/max31865-pt100/612663/28

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