- Thu Dec 01, 2011 8:19 pm
#136491
Just to have something to look at and refer to here's an initial block diagram of the basic parts that I think you need.
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atedeschi wrote:I could not view the image. Here is my email if that will work better: atedesch@gmail.comYeah, I'm letting that post stay AFU so the SF admin can see it. I'll try again below.
atedeschi wrote:I know its been awhile but I have not given up and you advice has not gone on heard. This is what I have come up with and was wondering what you thought. Also is my gain correct for the amps? I know you had said something about a voltage regulator but your image is gone and I am not familiar with using those.There are a couple of things I see right off that need some changing. First the gain is not R1/R2 but rather 1 + the ratio of th resistors for a non-inverting op-amp configuration. See the wiki on this ...
Here is my first run at it:
https://picasaweb.google.com/1032740155 ... directlink
atedeschi wrote:Could you recommend a better op amp? I was thinking a display like this possible http://www.sparkfun.com/products/9678I had a chance this weekend to look at this a bit more and I think your LM358 might be OK afterall, so long as you're using the 9V (or at least 5V) supply you showed. While the LM358 can't output more than 1.5V below the + supply, it can go pretty close to the -supply rail ... close enough for your purposes. I do want to do a noise analysis but you can limit the bandwidth of the circuit to a small amount so I don't expect this to become an issue.
But I was going to just have it talk with the computer while testing and get the LED version working first. Then once that is all working make a duplicate with no leds and a screen.
rebelzach wrote:I'm wondering if you could send a lower voltage to the "aref" pin using a resistor from the 3.3v on the Arduino. Effectively recalibrating the ADC to read from a much smaller range. See here http://arduino.cc/en/Reference/AnalogRe ... rence.AREFThat was one of my earlier thoughts for the OP to ponder. While you could reduce the gain of the op-amp circuit this way, I think you'll still need some amplification to get the A/D resolution (and quantization error) to where the OP wanted it. That said I think he'll want to use some external precision voltage reference @ 2.8 - 3.0 V to get a stable, reliable reading.
atedeschi wrote:I thought with the external precision voltage the reading will change as the battery power becomes less.Just the opposite. The A/D outputs a count that is the ratio of the input voltage to the reference voltage. If the ref voltage is just the battery voltage, which changes as the battery discharges, then the A/D counts will change even when the input voltage from the sensors stays the same. You want a reference voltage that stays the same even as the battery changes.