Just thinking out loud. Is it in any way possible to monitor 1 thermistor with 2 devices?
Say I have a solar hot water system with a roof sensor (thermistor) running to a controller in the mechanical room. What if I wanted to tap that line and have an Imp report to me via the internet the readings on the thermistor without interfering with the controller already in place?
I’m thinking it can’t be done without messing-up the calibration … but how many features does the controller have? Some controllers retransmit the temp to an external terminal strip. Or, if you can get a schematic of the controller and tap into the electronics somewhere after the thermistor signals?
Give us the name and model number of the controller.
The best way would be to put an op-amp set up as a unity gain buffer between the thermistor output (ie the mid-point between the bias resistor and the thermistor) to buffer the voltage in order to convert the high impedance signal into a low impedance one. The amount of current that flows into an op-amp’s input (leakage current) is small enough that this won’t affect the existing system.
Things you’ll have to do, though:
Work out which thermistor pin is ground (or bias). It’s likely to be grounded on one side.
Check the voltage of the other pin is in range. Not knowing the controller, this may be hard to work out. You’ll have to power the op-amp with a voltage high enough to pass the maximum range of thermistor output voltage. If it’s too high for the imp, that’s fine - you can use a resistor divider on the output of the op-amp to divide it down for the imp’s consumption and this won’t affect the thermistor at all.
Determine the thermistor’s resistance, and the bias resistance. Just disconnect the thermistor totally, try and get it to around 25C, and then measure with a meter. To work out the bias resistance, you could put a known resistance across the controller’s thermistor input and then do the math.
Hope that the system biases the thermistor all the time. It’s likely to, especially with a long wire run, because otherwise the capacitance would mean it’d have to wait ages (relatively) for the readings to stabilise after enabling bias.
See fig 3 here for the buffer op-amp circuit. Very simple!
I am sorry for my long delay in replying. I want to thank you two, Hugo and Mlseim, your input was is very valuable and I appreciate your time.
I’m going to work on tapping existing thermistor lines in a week or two and I will let you know how it goes.
I’m using Sunearth TR0301 U controllers.
A very special thanks to you Hugo for making the Imp. I have been testing conversions of existing solar hot water systems to a more stable stagnation type, and having an imp in the attic sending me temps on piping while I sit in the mechanical room recording pressures has been a game changer for me. The imp is amazing.
@mlseim, thanks for the reply. Apologies for the delay. Been playing contractor and paperwork manager more than I would like. Would rather stay home and do product development for sure.
Thanks for correcting me on the Steca 301 using pt1000 sensors. It was pretty dumb of me to assume they were thermistors. And yes, we did use the hidden menu functions on the Steca 301. Good find. Now that our solar hot water systems use steamback, we encourage them to shut down early in overheating and go stagnant. We needed the hidden menu to lock the pump out until collectors cooled down, after stagnation. Though, I would rather have an Imp watching temperature and locking the Steca out via relay someday, and by default the pump too…which is another topic…
Seems you are right on about contacting the designers of the Steca 301 and asking about auxiliary output or possible use of a dual RTD.
My current issue can be divided into two groups. One group is previous installations with only one RTD on the collector, and limited chance to cost-effectively run a new wire from the roof to the mechanical room. The second group would be future installations, where I’m running a two-pipe plumbing line to the attic/roof and it’s no problem to add 1, 2, or even three additional wire pairs for individual sensors.
I guess the root of my question concerns the former, where only one RTD is in place, or meaning a monitoring retrofit. I passed on Hugo’s awesome explanation of the buffer amplifier to a couple friends, but I might not get help from them any time soon.
I see a few things that might help but am only now in initial research stage. Things like temperature transmitters and signal amplifiers. I’m starting to wonder if best might just be to retransmit the analog output with something.
Anyway, I’m sending a message out now to the designers of the 301 asking how they would most cheaply do this. If aux output is possible, maybe they will say. If not maybe the dual RTD is an option. If not, I might reply to this post again with more questions…
–edit-- I forgot to mention that I contacted Steca and asked about tapping the signal at the controller, dual rtd’s and the buffer amplifier…and they cringed. They wanted nothing to do with it.
I’m revisiting this topic today. Have a few hours to work on it.
In summary, I want to use an electric imp to tap and read a pt1000 sensor connected to a Steca 301 solar hot water controller.
@Hugo and @mlseim helped tremendously already. In fact I’m having trouble digesting the great info.
My question is this, and it may be wicked simple, and if so I apologize: I think I’m seeing chips (like from Toshiba) which might be a packaged solution for splitting the signal and reading the pt1000 sensors with 2 devices. Does this sound crazy? I’m having trouble identifying the chip lingo and the !#$%-load of options.
Thanks, @Chrischi. Yes, apologies for being vague. I’m not really sure what questions I should be asking. I’m pretty sure you didn’t misunderstand.
Put another way, I guess my question is this. Could I use a chip like the MCP602 from Microchip to split one RTD signal and send to 2 devices (solar controller and imp)? I’m having trouble interpreting the specs on chips like these, but there seem to be a lot of options from various manufacturers, and they are cheap.
The devices you indicate are not per se in a unity gain buffer configuration, but this is easy to do as @Hugo indicates. You might want to refer to figure 3 of the wiki link he provided earlier for the appropriate wiring.
(Edit: Ah, just noted that @Hugo had already pointed to figure 3 … that’s what you’re going to want to do).
@Chrischi ahh true but the RTD sensors require amplification and some noise reduction which those boards should do for you. I am getting in a dual board soon so I will play with them some and let everyone know how they perform.
This scenario is not possible unless I install 2 sensors to get the same readout (1), which seems rather redundant. This would also be impossible in many cases, as there is not room for that many sensors in the radiators.
I need a device to receive the initial temperature (1) and then present the same temperature (or resistance) to two devices (2) and (3).
The end devices must not know they don’t get the result from a standard 10k temperature sensor (1). This is in order for compatibility with existing products: ie.: would not do if specialised LCD displays or fan-adapters were needed to make it work.
If we talked about a proxy-server, I would say it worked in transparent mode. Which means true plug and play where end devices are unaware of the in-the-middle device.