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for R1 and R2 to make the addition variable, by twisting the potentiometer.
The addition circuit also requires a plus/minus 9V power supply for the op-amps. In
addition, a tap from the 5V supply used for the logic is used although this could be
done with the positive 9V side as well, provided the voltages are computed correctly.
Combining Conversions
So the above conversions define addition, subtraction, multiplication, and division of a
voltage. Each of these conversions can be thought of in isolation as shown above or
they can be combined to create composite conversions. We essentially have an algebra
of blocks we can use to achieve a wide variety of overall conversions.
Say you have a sensor that creates -100 to 100V and you want to read the value with a
0-5V A/D converter. You would need to scale down the original voltage to -2.5 to 2.5V
first and then offset the result by adding 2.5V to get the result into the desired range of
0-5V for your A/D converter. You can chain together the conversions for such an effect
which would look like this graphically:
Winkler, Arduino workshop, sensors, p.14
Conversion Impurities
The above conversions all introduce impurities in the resulting signal in the form of
noise, non-linearity, and other corruptions of the original input voltage. Care must be
taken to minimize the number of stages and also to order them for reduced error.
Testing and careful thought can typically reduce these impurities to a minimum but they
cannot be disregarded.
There is a general rule of thumb with regard to these introduced impurities. The more
you are changing the original voltage, the more impurities you will introduce. For
instance, an amplification of 100x would be generally noisier than one of 2x.
Power Supply Ideas
Several of these circuits require a plus/minus 9V supply for the Op Amps. This can
readily be accomplished using two standard 9V batteries. More sophisticated options
include standard power supplies, charge pumps and inverters and several other options.
The 9V battery is cheap, simple and it works well. Op Amp circuits tend to be pretty
efficient so the batteries should last quite some time.
Circuit Diagram of +-9V Battery Supply
Further Reading
The subject of sensor interfaces is vast. This article attempts to give some basics that
are easy, practical, and quick to implement. Here are two great references to consider:
* The Art of Electronics, by Horowitz and Hill
* Op Amp IC Circuits, by Forrest M. Mims III
Winkler, Arduino workshop, sensors, p.15
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