Hello,
I am a student and need some help. I've tried to reproduce the Inverting Op Amp model on my circuit with arduino, and I don't know if the voltage is right according to the model (images bellow). I have a FSR with a voltage divider (I am only using Sig and VCC pin) connected with a LM324N and 22k-ohms resistor. I am only receiving 13 on the A0 port (I think its 0.06V).
I dont know if I can get 5V. I want to get this linearity.
Thanks.
Matheus
You can only use the inverting configuration with plus and minus 5V supplies.
You need to use a non-inverting configuration
To add to what Jim said, it is not clear how you are powering the FSR, but you appear to be following other designs where the FSR is powered with a negative voltage. -Vc to the FSR will output a negative Vin to the op-amp, which will work with a single supply configuration.
But if you are powering the FSR from say +5V from the arduino, then the output of the FSR is a positive voltage as well. In that case the op-amp cannot go negative with out a negative power rail , so it swings to close to ground, which looks like probably what you are seeing.
Hi, @matheus_harduim
You can get inverting to work, if you bias the non-inverting input above gnd with a single ended supply.
I can only see you using one resistor in your circuit plus the FSR.
Where is the feedback resistors needed to control the gain of your op-amp?
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
Why do you need to invert, you could do that in code in your Arduino?
Thanks.. Tom.. 
The app note has plenty of example circuits:
FSR Integration Guide - Interlink Electronics.pdf (795.0 KB)
Thanks for your response Tom,
I think its better I send the circuit that I'd want to find and the components I have, so you can send me the instructions (I really dont know if everything is right with the circuit that I am using. I only follow that schema image). Its no necessary a negative Vout.
I was expecting to find something like this
I tried hard to do this circuit but all time I failed 
Why don't you use the one in the app note?
Hi, @matheus_harduim
You need to gnd reference one end of the FRS to get a potential divider system for the amplifier to work, however as pointed out the inverting amplifier will not work in your case.
OR by the way you configure the FRS in your divider circuit.
Thanks.. Tom...
Do you have a working circuit or do you still need help?
OK, study your circuit.
Forget the op-amp for now.
Set your Vcc to +5V. Connect the signal to an ADC pin on the Arduino. Apply some force and observe the readings. Apply more force and observe the readings.
Next use a voltage divider to cut your Vcc to ~2.5V. Then repeat. By now you should have a better feel for how your FSR works.
Next build some basic non-inverting op-amp circuits without the FSR using the various voltage dividers to input signals from say 0.5 to 5V to the op-amp. See how the output varies for each voltage for different gains. Record your gains and inputs and outputs in a spreadsheet so you can study them.
Next study op-amp circuits a little more until you understand that without a negative Vss rail on the op-amp you cannot output a negative signal from an inverting op-amp.
Learn also that your arduino cannot measure a negative input anyway.
Finally learn how to supply a negative Vcc to your FSR and see that now the op-amp in your original inverting configuration can output a positive signal which the arduino can also measure.
"Cut and paste" projects without learning the basics and familiarizing yourself with your components will lead nowhere. You will make much more progress by investing the time into the simple experiments outlined above than you will "trying different things you found on the internet".
This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.
