I have an Opta Wifi powered with a 12VDC power supply. And I have a temperature probe powered with 3.3VDC via an LM1117. There are two resistors R2 and R4 in the circuit that make the linear conversion for the temp probe which is then measured by the microprocessor (see attached line diagram showing the set-up minus the 12VDC power supply and LM1117, all grounds are common to each other).
With older Arduinos like the Uno and Nano, I didn't have problems reading the voltage on TEMP-1. For example, with a temp of 64F, the voltage reading between TEMP-1 and Ground (negative) is about 1.754 VDC. This is checked with a multimeter.
However, when I connect TEMP-1 to the Opta I1, I2 or any of the Ix pins, the volts drop significantly as well as a big drop in resistance readings across R2 and R4. In the example above, 1.754VDC drops to 0.815 VDC. And the resistance across R2 drops to 6K. The Opta actually reads the correct voltage of 0.815VDC and also verified by a multimeter.
Are there resistors on Ix on the Opta causing this drop? I've validated this with other Optas, Uno and Nano boards.
I set the ADC resolution to "analogReadResolution(12);". But I don't think this is a software issue.
I'm trying to migrate away from Nano and Uno to Optas across our facility. Any help in the matter would be great.
The analogue inputs on Unos/Nanos have extremely high input resistances and present little loading to your circuit.
According to the Opta datasheet the Analogue Inputs have a low (8.9kΩ) input resistance.
This will be due to an attenuator network that is inside the Opta, so that it can measure a 0-10V range.
This low input resistance loads your circuit, and gives you the lower voltage that you are experiencing.
You could get around this problem by using an op-amp as a buffer amplifier between your circuit and the Opta analogue input.
There may be other ways of interfacing the temperature sensor that don't need an op-amp.
You haven't told us what this mystery temperature sensor is, could we have a make/model number or a link to it's datasheet?
Why do you have the resistors R2 and R4 in the first place?
Thanks for the information. Learned something new about input impedance on the analog pin. I've never considered it before, the Uno/Nanos made things easy before. I'll take a little time to digest and understand for the next project.
The temp probe was made by a company called Climatronics. P/N: 100093. Spec sheets below,
I think trying to rig up a small circuit so the temp probe interfaces with the Opta is overkill. I was able to fit an Arduino 33 IOT device that we had in our shop to interface and it's working just fine. The 33 IOT has the IOT Cloud connectivity that we are looking for albeit not the nice form factor of the Opta. The Optas will be used elsewhere.
This exercise is making me second-guess the other Optas we have on site, for example reading pressure sensors throughout our irrigation and water delivery systems. In the PSI sensor applications, we drive them with external 5VDC power supplies, but run the reference to the Ix pins on the Optas (grounds are common). We also run a reference from the 5VDC power supply to an Ix pin to help with calibration in the coding, and this seems to work. But I'll go back and check voltages with a multimeter with reference wires connected and unconnected to the Optas to see if there are large variations. If so, then we may need to rethink the Optas or revisit the external Op-Amps you mention. We just love the ruggedness, IOT Connectivity and form factors of these Otpas though!
