Re: LDR detector

Dear Kenn ;
I have not recive your shematic in gmail yet that you post last time
Would you please send it to harirart@xxxxxxxxx
and if possible ziped it to reduce the size
Thank you very much
a.Jafry

Ken Moffett wrote:
"abajan" <abdolahj@xxxxxxxxx> wrote in news:1151040436.998097.212090
@r2g2000cwb.googlegroups.com:

Dear ken
tahnk you
you mean connect a capasitor to op-amp input and ldr junction ?
And would you plaese say me which op amp is good for this ?
thanks
Ken Moffett wrote:
Jim Thompson <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote in
news:8poj92pscdkpokcfh4tla0sdtait1ffsu2@xxxxxxx:

On Thu, 22 Jun 2006 00:20:33 GMT, Ken Moffett <KMoffet@xxxxxxxxx>
wrote:

"abajan" <abdolahj@xxxxxxxxx> wrote in news:1150872573.829638.176200
@y41g2000cwy.googlegroups.com:


Adelec Bakkal نوشته است:
You did not state the purpose of the circuit. Anyway you can try
this:

Combine the LDR in series with a fixed-value resistor to make a
simple
voltage divider which can act like a voltage s
ource that varies
with
light. This makes the first part of it. Let's call the voltage at
the
LDR V_Light. Now you have converted the resistance of the LDR into
voltage which can be compared to a reference.

Now use a potentiometer to create a programmable voltage reference.
You
will use this to set the level that will trigger the
comparator.Let's
call this reference voltage V_Ref.

The point is to have the comparator compare two voltages. And now
you
have them, V_Light and V_Ref. All you have to do now is to connect
V_Ref to the 'reference pin' on the comparator, you can connect it
to
the 'inverting input', next is to connect V_Light to the 'V_in', or
just the 'non-inverting input'.



Going this way, the comparator will output a logic 1 if V_light is
greater than V_ref, and a logic 0 if V_light < V_ref, and the other
way
around if you interchange V_Light and V_Ref (choose according to
application). The behavior of the circuit depends on the trigger
level
you have set with the potentiometer, and the value of the resistor
you
have put in series with the LDR to form a voltage divider. You set
the
potentiometer according to the transition level between 'dark' and
'bright', this goes with trial-and-errors untill you get the
satisfactory behavior.

The comparator needs a pull-up resistor. Connect the output of the
comparator to the positive supply through a 3K resistor. Any close
value should do.

Good Luck!


Dear friend ;

Thank you very much for your answer ;
I used your idea and got good result
Now I want advanced this circuit as I explain
Imagine we want this circuit work in darkness and brightness
automatically !
I mean when we change to sun light for example it can auto calibrate
ref voltage .
I explain more :
In my project I use a laser beam that incidence to LDR and then I
want detect it in op-amp output but
it must work in room environment with normal light and sun light
automatically without any adjust of potentiometer .
in fact it must determine thresholds of voltage in both
environment
.is it possible or no ?
I think I need a auto adjust voltage changer that can make threshold
voltage
May you help me ?
Thank you and I'm sorry for my poor English
A.jafry



A. Jafry,

I think I understand what you are trying to do. You are trying to
distinguish when a laser beam touches your LDR in either sun or low
light conditions. You are looking for the "change" form a background
light condition. Is this correct?

Ken

Modulate the laser, then AC-couple (and/or DC-restore) the receiver.

...Jim Thompson

Jim,

That's the way I was thinking, but without the modulation of the beam.

LDR and resistor as a voltage divider. Capacitively couple the juntion
of the LDR and resistor to a high gain opamp. Only rapid voltage changes
would be coupled to the amp. This would also assume a relatively stable
or slowly changing light environment, with changes in the laser hitting
the LDR as the major event. I did this once with an array of 8 IR
phototransistors (and IR LRDs) all capacitor coupled to one opamp. I was
trying to detect the daily activity of 3mm to 5mm blind cave a fish in a
1cm x 1cm x 8cm Plexiglas fish tank. Worked very well. The modulated
beam, though a little more complex, would be better. I guess it would
depend if A. Jafry is able to modulate his laser.

Ken



A. Jafry.

Since this is not a binary compatable newsgroup. I'll send you a sketch of
the schematic to : abdolahj@xxxxxxxxx
To maintain the detail in the sketch it is a large file (1Mbtye). Let me
know if this a problem.

In the schematic your LDR would replace the Q8 phototransistor. You will
have to experiment with values for C8, R8, and maybe the 100K and 10K
resistors on pin 3 of the opamp.

If you have specific questions email me at <moffett@xxxxxxxxxxxxxx>

.

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