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I have some PT1000 RTD Temperature sensors between -10deg and +120deg C. in an existing application. but the controller reading this is undocumented and now obsolete.

I want to be able to use the existing sensor in situ, but switch the sensor from the existing controller, so i can read it from my own microcontroller every 5 or 10 minutes.

the existing controller will tolerate the sensor being disconnected for up to 30 seconds before it alerts..and resets the alert once its reconnected.

so I could just use a relay to switch the pair of wires going to the sensor. but the relay probably wont last long.

so is there a better method i can use to achieve the same effect.

for example, Either An electronic (rather than electro-mechanical) method of switching the sensor between the 2 controllers.

or alternatively A method by where the sensor could be read by my microcontroller, but the reading then 'spoofed' to the existing controller in some way.

the sensor in question is a PT1000 RTD based sensor which changes resistance by +3.91 Ohm for each increase of 1 Deg C.

thanks Rich

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  • \$\begingroup\$ You can buy RTD splitters/repeaters. Eg: itaia.si sells some. \$\endgroup\$ Commented Mar 23, 2018 at 13:30
  • \$\begingroup\$ I didn't know that, and i have been looking, Thanks ! I have found some others now too. \$\endgroup\$ Commented Mar 23, 2018 at 13:42

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If you can figure out how the existing controller reads the sensor, then maybe you can just watch the voltage, and the existing controller would never know.

I know you said the existing controller is undocumented, but there aren't that many ways to read a resistance. You have a reasonable chance to figure out what it is doing. Use a 1.5 kΩ potentiometer, or even a switchable resistance box. Most likely, the circuit either uses the RTD as one resistor of a divider, or it puts a fixed current through it, and measures the voltage. There are other more exotic ways that might be difficult to work with, but you have a good chance to be able to figure this out.

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  • \$\begingroup\$ I'm fine reading the value from the sensor, can do that no problem, once i connect to it. The question is about the best way of switching the sensor between the existing device, and my own microcontroler. (and possibility of spoofing my own device readings to the old device) \$\endgroup\$ Commented Mar 23, 2018 at 13:26
  • \$\begingroup\$ Ok, re-read you comment, i see what you are saying. Piggyback, but i am concerned anything i connect to the sensor will distort the reading the existing device sees if I do that. \$\endgroup\$ Commented Mar 23, 2018 at 13:32
  • \$\begingroup\$ @rich: As long as what you connect has high impedance, the existing circuit shouldn't be able to tell the difference. "High" impedance relative to 1000 Ohms isn't really that hard to do. If the lines will be long, buffer them close to the source. That will cut down on additional noise on the signal. \$\endgroup\$ Commented Mar 23, 2018 at 15:46

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