Hi, I'm using an Arduino NANO 33 BLE to read the voltage across a pressure sensor (with the circuit advised by the manufacturer and a low-pass filter) from second to second on analog pin 7 to an SD card. Yesterday, the measurements were just fine, but today, when I tried reading it again, suddenly the voltages drop when the mass on the sensor increases (resistance across the sensor decreases). For a moment, I thought that I had burnt the pin, but when I turn on the Arduino when the sensor already has the mass on top of it, the first 10 seconds correspond to the correct value; then the voltage drops again and stays around that value.
I'm unable to identify the issue here. Any help would be appreciated!
The next best thing to following the posting instructions is to post an annotated schematic showing exactly how you have wired it, be sure to include all power, ground, and power sources. Hint: I do not read picture diagrams very well. At this point it sounds like a hardware problem.
Should I modify the circuit in any way? I required a negative voltage across the sensor (as shown in the link), so I attempted to create a virtual ground.
There is no "linear portion" to the output of the voltage divider circuit, but you can fit a calibration curve to it and get data as accurate as the sensor is capable of producing.
On the other hand, why not just try the first suggested circuit? You don't need a "virtual ground", but you DO need to choose a good quality rail to rail op amp and a stable negative bias voltage source.
@jremington
Thank you so much for this suggestion! I'll likely try this first since I already have some familiarity with how the sensor behaves in this configuration. I apologize for my limited experience in this area. Could you please recommend a rail-to-rail op-amp and provide more details about its application? Currently, I am using an LM324N op amp (octa).
Regarding the stable negative bias voltage, my initial thought was to acquire a Voltage Inverter IC. Would this be suitable for the intended purpose?
I genuinely appreciate all your assistance! Thank you so much.
Sorry, that means nothing. Op amps have official designations and data sheets. To communicate on this forum, please use the official designations.
For this application, the Microchip MCP6001 rail to rail op amp would probably be acceptable.
Voltage reference ICs are available in many forms and voltages.
Any modern rail to rail op amp (the LM324 is neither) should work in the manufacturer's suggested circuit.
The most serious problem with the LM324 is that the output voltage can never exceed (supply voltage - 1.5V), which is a disaster for the circuit you posted above.
I see, so if I change it to the rail to rail op amp, this one won't get damaged?
I'm trying to think of a simpler and safer way of getting the negative supply (around - 0.7V). I've looked around at the stores and I didn't find the voltage inverter. Do you have any suggestion? Should I opt for a op amp that allows negative voltage and do a voltage follower (buffer)?
Hi, first of all, thank you for your time! Yes, I've already checked it and it's around 9.2 V, so I didn't though it was the problem... I was using this one since I already had it, but my objective is to use a rechargeable Li-Po, I just didn't find any with more than 3.7V.
A completely different circuit. It will use a LM324 and be powered by a 9V battery.
I'm working on the details.
What is the resistance of the sensor when no load is applied?
Hi, @aakkds
How are you applying a testing mass to the sensor?
As a point contact or as a full sensor surface load?
You will have to decide so that you can calibrate your system.
A point contact will give a different resistance to a full surface applied force of the same magnitude.
Have you got metal or rigid plates either side of the sensor to even out the mass forces.
Can you please post some pictures of your project?
So we can see your component layout and how you are loading the sensors.
