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I have a solar thermal system on my house and that system is run by a differential temperature controller having two thermistors. The controller does not have any kind of readout on it, thus I have no way of telling what the two temperatures are at any given moment.

My question is: is there any way to wire a digital display into the controller to read those temperatures? The two sensors are 10,000 ohm two-wire thermistors. The sensor inputs on the controller are 0-5VDC and the thermistors are of the NTC type. Are there any LED displays made that run on a 120volt power source and accept a 0-5VDC input that can be scaled as needed? Any assistance is greatly appreciated. Thank you.

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  • \$\begingroup\$ I'm assuming you're referring to 120V AC when talking about a power source for the display? \$\endgroup\$ – Toby Lawrence Sep 23 '12 at 18:19
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Many digital panel meter modules will work at 5 V power supply, and probably won't take much power, so that you might draw that from the thermostat.

But these are no good. They will measure voltage, but don't take the non-linear characteristic nor the offset into account. You'll need a microcontroller to convert the voltage input to a temperature reading which then can be displayed.

Most microcontrollers have ADCs (Analog-to-Digital Converters) to read the analog voltage of the thermistor, and convert it to a digital value the controller can work with. You'll need to know the characteristic of the thermistor and its series resistor to know which voltage agrees with which temperature. You can use a mathematical formula for that, or a lookup table.

If you use a microcontroller with an on-chip LCD driver you can directly connect a display like this one, and you won't need anything else; just the microcontroller and the display. TI's MSP430 controllers are very low power, and so is the display, so you'll be able to run it of the thermostat's power supply.

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You are asking about a "display", but seem to want much more than that. You apparently need these building blocks:

  1. A power supply. This can be as simple as a small "wall wart" that produces 5 VDC from your 120 volt house power.

  2. A analog circuit that receives the thermistor signals. This must not load these signals else it will confuse your existing solar thermal system. At the very least, this needs to be a buffer with high impedance input that produces a low enough impedance output signal to present to a A/D. You may possibly want to add some gain and offset so that the valid thermistor voltage range covers more of your A/D range.

  3. A A/D converter that reads the buffered signals and produces digital values from them. 10 bits is probably enough, 12 bits most likely enough.

  4. Some kind of digital logic that takes the A/D results and does the conversion to temperature in whatever units you want to see, like °F or °C.

  5. Conversion from the binary values in the right units to decimal.

  6. Digital logic that drives a appropriate display with the decimal values from step 5.

  7. The display. LCD is probably appropriate, but LED might also be, depending on tradeoffs you haven't told us about.

Items 3-6 can be found all integrated into a microcontroller. Most micros have at least 10 bit A/Ds nowaday, and a reasonable number 12 bit. Some can even drive "bare glass" LCD displays directly. For example, Microchip PICs with "9" near the end of their part number can usually do this.

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I believe your temperature sensor should give a current or voltage signal output and that output signal should be connected as input to your microcontroller for its temperature to display.

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  • \$\begingroup\$ Please proof-read your posts for spelling and grammar. Stackexchange is a place for quality questions and answers. \$\endgroup\$ – Bort Jul 11 '18 at 12:44

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