Download Turbidity Meter Progress Report I

James Berg
3 March 2005
TURBIDITY METER
PROGRESS REPORT 1
I have spent the last month attempting to figure out how to work two sensors, one
90° and one 180°, into the turbidity meter. This poses a much greater challenge then a
simple one sensor device because math is now involved, specifically division. While this
is not complex math for a person to do circuitry has to do it in a rather round about away,
I believe by doing many multiplications. There are two different ways to the divide the
voltage from the 90° sensor by the voltage from the 180° sensor, an analogue method and
a digital method.
The sensors themselves output an analogue signal, a voltage that varies
continuously with light intensity. Op amps set up in varying configurations can be used
to perform certain math functions with voltage. Specifically, given two voltages V90 and
V180, take the Ln(V90) & the Ln(V180) => Ln(V90) – Ln(V180) = Ln(V90/ V180) =>
exp(Ln(V90/ V180)) = V90/ V180. It is fairly easy to us a logarithmic amplifier to take the
Ln of a voltage but, despite the fact that I have been told that there are inverselogarithmic amplifiers, I have been unable to find one. It is true that this method would
use a lot of separate amplifiers but they are fairly cheep. The major disadvantage of the
analogue method is that it would require a fair amount of soldering, and it still does not
solve the problem of an output device.
The other option, and the one that looks most promising, is to convert the
analogue signal into a digital one and then run it through a device called a micro
controller. A micro controller is an integrated circuit that has a little bit of rewritable
memory, allowing the user to program specific functions into it, using C. This would
mean that the micro controller could not only be programmed to divide the voltage from
the 90° sensor by the voltage from the 180° sensor, but also to convert the final signal
into NTU's rather then Volts. Additionally the micro controller has an analogue to digital
converter built into it and can be set up to output directly to a digital readout. The major
disadvantage is that the analogue to digital conversion is limited. An analogue signal
varies continuously, the conversion process divides it up into a set number of possible
values; so for an 8-bit chip there are 254 possible digital output values evenly divided
between a high and low value. Some converters are preset with a high and low value, say
0V to 5V; other chips allow the user to set the high and low values. Because the digital
signal can only be one of 254 different values the resolution of the whole sensor
assembly would be limited. If your minimum turbidity is 0 NTU and your maximum is
254 NTU then your resolution is going to be limited to 1 NTU, the solution to this
problem is to get a higher bit chip, but that costs more.
Another issue with the micro controller is that the initial cost can be rather high,
anywhere between $80 and $150. The initial cost is so high because there is equipment
that is needed to program the micro controller, however once a micro controller has been
programmed it can be removed from the equipment and installed in any device you wish.
Subsequent micro controllers would only cost $10 or $12 as the programming equipment
can be reused. It is also possible that I could use equipment in another lab to program the
micro controller. Selecting a micro controller poses another problem, they come with a
dizzying array of options and I do not have the technical expertise to understand all of
them. A friend has recommended I talk to a professor (whose name escapes me at the
moment) who is knowledgeable about these devices; however I have not met with him
yet.
The final design using a micro controller would look
Component
Price ($)
something like this. Two light sensors at 90° and 180°; they
Light Sensor (2)
8
would each connect to an op amp, a very simple device that
Op Amp (4)
4
multiplies the input voltage by a specific, easily definable
value; if necessary at this stage the signals can be put
Micro Controller
12
Digital Readout
3
Total
27
through another op amp with a slightly different configuration to average the value over a
given time period, 2 or 3 sec (the 90° and 180° sensor signals are still separate); the two
signals are fed into a micro controller where they are first converted into a digital signal;
the 90° sensor value is then divided by the 180° sensor value, giving you Vdivided; Vdivided
is then put through a function that converts it from volts to NTU; the signal is then turned
into a signal appropriate for a digital readout; then the signal shows up as number on a
digital readout.
90° light
sensor
Op amp, multiplies
voltage by 10,
increasing it from
around 80 mV to
around 0.8 V
Op amp, averages
analogue signal
over a 3 second
time period
Micro controller, converts
signal from analogue to
digital, divides signals, and
turns signal from volts into
NTU, turns signal into one
appropriate for a digital
readout.
180° light
sensor
Op amp (multiply
voltage by 10,
increasing it from
around 160 mV to
around 1.6 V
Digital readout,
displays turbidity
value on a 4
character, 7-segment
digital readout
Op amp averages
analogue signal
over a 3 second
time period
The resolution of a micro controller may end up not being an issue depending on
the desired NTU range and the desired resolution. Unfortunately I do not have exact
specifications for desired final product, this is large oversight. Last semester the goal was
just to see if a cheep turbidity meter was even remotely feasible and the goals were
somewhat flexible. Some ranges I found for commercial turbidity meters were from 1199 NTU or 0-1000 NTU. Off the top of my head 1-199 NTU sounds like a reasonable
range but of course the necessary range varies depending on where you are. This
turbidity meter does not need to read exceptionally high values because at some point the
water becomes so dirty that one would only need to look at it and declare it undrinkable.
Again though I do not know what these values are, nor do I know what kind of resolution
is needed for a turbidity meter to be useful in determining the necessary level of
chemicals that should be added. Determining the required range and resolution, finding
an appropriate micro controller, and starting to program the micro controller are my
primary concerns for the next month.