Salt-taster

[walid]

Hi
I built this circuit on a test board
http://www.redcircuits.com/Page68.htm

the sensing leads are simply two thin copper wires
i connect these two leads together, this equivalent to a very low resistive solution.
when operate the circuit, the red led (D3) is on and the other green and yellow are OFF
when trying to adjust the variable resistor R4, it has no effect at all
i check the connections many times, they are OK.
I understand the three comparators operation
and know that the two red leds D1 & D2 are a voltage references as the writer said
also i know that the first op amp (IC1A) is function like this:

and this:

I use a 10.5V power supply
I measure pin1 voltage = 9.1V where pin5 = 4.3v
i try to make pin1 voltage lower than pin5 but can't
please help me in fixing the broblem
thanks

[walid]

Now I have a Specific question which is:
comparing the folloing figure:

with this figure (salt0001):

Noting that the voltage at pin 1 (Vo) = (e2-e1)(R2/R1)
I note that e1 = 1.8 V
e2 must be Slightly lower than e1, say e2 = 1.7V
so Vo = (1.7-1.8)(220K/10K) = - 2.2 V
If the solution resistance is increased, then e2 will be lower, say, 1.5 V
so Vo = (1.5-1.8)(220K/10K) = - 6.6 V
In all cases the o/p Vo will = 0 V because we supply LM324 with a single power supply.

Either the equation (Vo) = (e2-e1)(R2/R1) is wrong and must be (Vo) = (e1-e2)(R2/R1)
Or the design is wrong?????

[walid]

Hi
I feel I am alone with this question.
But in any case, I built the circuit and worked excellently.
The question remains puzzled me is the importance of R1 (=470 ohm)
when i tried to remove it, the red led is illuminate
It remains as to whether or not there is a salt solution
can anyone explain to me the function of this R1
thanks 

[audioguru]

Hi Walid,
It takes a higher current to develop voltage across 470 ohms than it does if R1 is removed and instead 230k is there. Without R1 then the circuit would detect a very weak salt solution.

Without R1, the first opamp would amplify the voltage developed across the 220k resistor caused by the opamp's input current.

[walid]

Thank you guru
I understand you
The author said that the voltage drop on a red led is 1.6v
Is this standard value
I measure it , it was 2v and sometimes 2.2 volt
thanks

[audioguru]

Hi Walid,
An LED is a semiconductor. Semiconductors have a wide range of forward voltage, and it changes with current.
The red LED that I use has a range of forward voltage from 1.5V to 2.4V at 20mA. Mine were all made at the same time and have a forward voltage of 1.75V at 20mA.

[walid]

thank you guru it is a complete answer