# Input Voltage

Hi There,

I am measuring the torsional strain in a pipe over time and hoping to display the strain in a real time graph. I am hoping to use an Arduino as an analogue to digital converter but have an issue with the input voltage.

I have an analogue linear output of +- 2.5V max of which I want to use as an input to an Arduino. However I have noticed that the input voltage on most of the Arduino boards are (limits):6-20V. Is there a way that this can be done without jeopardizing the input signal which is coming from a Vishay - P3500 strain indicator and will vary slightly (+-440microVolt/microStrain) according to induced strain in the pipe.

Thanks for the help,

Regards

Dave

I think you'll have to shift your voltage with an opamp.

Are you measuring the strain in both directions? If not you could rectify the signal with a Schottky diode but you'd lose the bottom +.4v or so as well.

Rob

http://masteringelectronicsdesign.com/how-to-derive-the-summing-amplifier-transfer-function/

your signal on one input, 2.5V on the other so output sits at 2.5V under steady state.

You haven't mentioned what kind of resolution you require. Do you need to resolve +/-1 microstrain? If so, it means you have to resolve 440 microvolts from a 5V range, or 1 part in 11364. Unfortunately the Arduino A/D converter is only 10 bits (best case) so can only resolve 1 part in 1024. You need at least a 14-bit A/D converter and careful attention to signal integrity to make good use of those bits and not overrun by noise (440 microvolts is well down into the muck of the noise surrounding a hobbyist-level product like an Arduino).

Assuming you DON'T need +/-1 microstrain resolution then here's a similar circuit to what you want, except its input range is -5V to +5V:

http://ruggedcircuits.com/html/circuit__25.html

You can adapt it for -2.5V-to-2.5V operation by replacing R3=33k, R4=91k, R2=82k, R1=93.1k. Its output voltage will be in the range [0.3V-4.7V] suitable for reading by the Arduino.

-- The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

Hi,

Thanks for the input everyone. I am hoping not to use any extra "self built" circuitry as Im very new to electronics and was hoping there was just a switch on the board to compensate or an extra part that Arduino sold:) Rugged circuits, now that you mention it I am looking to get as high a resolution as possible. I think an alternative could be to go straight from the strain indicator into an oscilloscope for the read out.

Thanks

Dave

Well, you could always add something like this for increased resolution. http://www.gravitech.us/i2c128anco.html I don't know if anyone makes an adjustable gain block in that kind of format with a settable offset to bring negative signals up into the positive realm.