Thanks for the clarifications.

Question 1:
To prevent the buzzer from buzzing at about 1.5-3 v, can I use a resistor that would be infinite when the voltage is under 5V and then would be zero when the voltage is above? I was thinking about a Zener diode?
Would this one work for me then?
1N4733A 1W Zener Diode - RadioShack.com

Also, I tested the flocculent sediment sensor tonight.
The first surprise was that the buzzer went off when the sensor touches the water. Water attenuates IR very fast, but I did not expect it with an IR LED placed at 2 cm away from the phototransistor.

I tweaked the trimpot of the IR LED and increased the IR until the buzzer stopped buzzing.
When I touched the flocculent sediment, the buzzer went off. So, it works in the lake on campus... but the lake I shall survey next Thursday has murky water. So I guess more tweaking will be needed.
To the worst, I maybe will have to move the sensors closer and this demands some substantial work. I hope that when the IR diode is maxed out (See question 2 below), the murky water will not cut the IR beam off.

In the future, I will have two potentiometers that I can tweak without opening the box where the electronics is. It looks like there will always be a need for some tweaking.


Question 2:
Last question, what would be the minimal resistance I should put to protect the IR LED? Right now, it is directly connected to the trimpot... and as you know, these guys are very sensitive (1 turn does it all).


THANKS AGAIN SO MUCH!
Hydro.


chuddleston: I tried this, but what happens is that that, when I do not hear the buzzer, when it goes off, it emits a very low sound (kinda the same sound intensity as my background buzzer before I tried to lower it).

ex-hydro.

BTW: The current design works and has generated lots of valuable BTW: The current design works and has generated lots of valuable precise and accurate data. So, we know it works better than anything else that was attempted before. imagine a layer of mud in clear water. The mud has a pretty clean interface because it is denser than water. As long as it is not windy, the interface is like vinegar sitting on the bottom of a beaker with oil on top.. So, we know it works better than anything else that was attempted before. imagine a layer of mud in clear water. The mud has a pretty clean interface because it is denser than water. As long as it is not windy, the interface is like vinegar sitting on the bottom of a beaker with oil on top.

hi,
Sorry to hear that the circuit is not performing as expected.

I can only repeat my earlier post, this design will not IMHO give the valuable precise and accurate data that has been claimed.

Can you say who used this circuit to capture the claimed data.?

To prevent the buzzer from buzzing at about 1.5-3 v, can I use a resistor that would be infinite when the voltage is under 5V and then would be zero when the voltage is above?

What do you mean?

Assuming you're using the LM311 there shouldn't be any voltage across the buzzer when it's off, the 1M resistor shouldn't pass enough current to power it. If it's a problem, put a 100k resistor in parallel with the buzzer.

The first surprise was that the buzzer went off when the sensor touches the water.

Were the electrical connections to the back of the sensor fully water proofed?

If not, water could have asilly got in and short circuited the sensor making the circuit think that it was illuminated.

Hello Hero99:

- Well, I have a voltage going through the buzzer when the phototransistor has quite low resistance. I am pretty sure the wiring is right. I keep the suggestion of having a resistance in parallel. I am not currently using the LM311 but the LM741.

- The water attenuates IR pretty quickly. All the wires are embedded in caulk. There is no way water can make its way pass the tips of the LED and phototransistor which are the only components in contact with water.

Hydro.

Originally Posted by hydrocynus

ericgibbs:
Here is the paper
Myers, P., and C.L. Schelske. 2000. An inexpensive, optical (infrared) detector to locate the sediment/water interface in lakes with unconsolidated sediments. J Paleolimnol 23:201–205.
That paper got fairly cited.

Now, if you guys think that the design is no good, we can work together on a better design and publish the work. I think that this sensor is still pretty good if you compare to whatever was available before.

If you want the paper IM me with an email address and I shall send it to you as an attachment.

Hydro.

Hi again Hydro,

I had worked on a device similar to what you are talking about here
about two or three months ago. It did not have to work under water,
but the distance between IR LED and Photo transistor could vary a little
bit (and an additional requirement was a pass code had to be entered
to get the device to work). What i found was that it was best to
incorporate an auto calibrate mode of operation that eliminated having
to constantly tweek pots and try to get things right like that or have
to adjust the distance or other things.
An auto calibrate mode however can really only be incorporated in
a design that includes a micro controller, however the inclusion of
such makes life a whole lot easier.
Im not sure if you would want to go with a micro controller or not,
but i thought i would mention it because then you can calibrate
for any environmental situation by simply pressing a 'calibrate'
button and moving the sensor first to the water level you want to
detect as 'clear', and then maybe a few seconds later to the level
you want to detect as the 'flocculent' level. The box would then
be calibrated.
Other features such as auto adjust IR level and the like can also
be incorporated for environmental situations that vary considerably.
I think this would make a really nice instrument if you dont mind a
bit more complexity.
Another possibility would be two sensors where the device looks
for a certain differential density and acts on that to calibrate.
That would mean moving the box to the level to detect and pressing
"calibrate".
Im sure other members here could elaborate more on ideas like this too
if you are interested in creating a really nice piece of equipment.
Cost isnt very high either... two dollars US for a micro controller for
example.

One question we should have asked a long time ago too:
What kind of resolution do you need for this device...that is, do you have to
be accurate to within one foot, one meter, one inch, one cm, one mm, etc. ?

Originally Posted by hydrocynus

Hello Hero99:

- Well, I have a voltage going through the buzzer when the phototransistor has quite low resistance. I am pretty sure the wiring is right. I keep the suggestion of having a resistance in parallel. I am not currently using the LM311 but the LM741.
Hydro.

hi hydro,
The 741 opa as mentioned is a very old device and has a poor specification.
One of its short comings is its limited output voltage swing, its not from 0V thru Vbatt, but more like +1.2V thru Vbatt -2V.

That +1.2V maybe just enough to keep the buzzer sounding, you could try adding a small diode in series with the output of the LM741 and the buzzer, that would drop the 1.2V down to about 0.4ish.

Diode Anode to the LM741 and Cathode to the buzzer.

OK.

Originally Posted by MrAl

Hi Eric,

I had suggested an LM393 or LM339 or even a LM358 long time ago, but
he seemed to not understand the significance of this kind of suggestion.

More recently i suggested a uC because that would make one heck of an
instrument, provided of course the slightly higher complexity is acceptable.
Of course the added complexity only comes in the form of an algorithm, so
maybe that's not really more complexity anyway.

hi Al,
I would agree that some form of calibration is required else he will be forever chasing his tail, tweaking the pots to get a reading and never have a true datum reference.

It would be an ideal project for an mcu.

Hydro, in response to your question about the minimum resistor value, it depends on which IR LED you ended up getting. Going back through the thread I couldn't figure out which one you ended up getting, so here's how you figure it out:
minimum resistance = (9V-Vled)/Iled
where
Iled = maximum current through LED (from LED spec sheet) and
Vled = voltage drop across LED (from LED spec sheet)

So if you got the matched pair from radio shack (2V drop at 40mA (edit: these values from the website appear to be inaccurate)) the minimum resistance would be 175 ohms. Though if you pulse it on and off you can probably cut that in half.

I just checked the Radio Shack package for the IR pair. For the LED the V-fwd is 1.3V to 1.7V. The maximum "continuous" I-fwd is 150mA.

ken

Originally Posted by hydrocynus

I am not currently using the LM311 but the LM741.

Then you'll be forever plagued with problems.

The 741 is an old op-amp, not a comparator, use a comparator such as the LM311.