Help with sensor connections

[strokedmaro]

Hi again. I just bought the attached Flexiforce sensor and have a few questions. Im attempting to merry this to a PIC as a simple weight sensor. The problem is I've never messed with op amps before and have a basic understanding of their operation but how in the attached schematic do you get negative power (-5vdc/-9vdc)??? Could someone give me a brief explanation or even better make me a schematic of this connected to a PIC? Any info or links other than wikipedia welcome. THANKS!

 

[davidbear]

strokedmaro,

Looks like a cool sensor. I don't know anything about PICs, but maybe I can help with OpAmps. OpAmps do what they can to make the difference in voltage between the inverting (-) and non-inverting (+) inputs zero. Since the non-inverting pin is connected to ground, the OpAmp will produce enough voltage so that the voltage at the inverting input is 0V. This will occur when the current through Rf is equal and opposite the current going through the sensor Rs. So, since Vout = If x Rf and Vt = Is x Rs and If = -Is, we can say that Vout = -Is x Rf and Vout = -(Vt / Rs) x Rf = -Vt (Rf/Rs). So, you will feed this output voltage into your PIC AtoD pins and read the voltage in mV. By applying a negative voltage to your sensor, you assure yourself that Vout will be positive and be inversely proportional to Rs.

Hope this helps,

DavidBear