How to "invert" an LDR action?

[artsinbloodshed]

hey guys...
I've search for infos but nothing popped up yet so...
i'd like to know if someone has an idea how to invert the action of an LDR...
I mean adding a non mecanical system (just a electronic thing that remain simple if possible) to invert the action:
Like that: if light> conductance>new system>resistance (vs light>conductance)
Does anyone have a clue about it?
Thanks dudes

[Gurner]

If you used the LDR in the feedback resistor chain of an opamp, then that ought to work as you require...



LDR as R2 , when LDR gets more light, resistance goes down - as does the opamp signal output.

LDR as R1 , when LDR gets more light, resistance goes down -  the opamp signal output gets larger.

You'd need to pay close attention to the associated fixed resistance though...probably a bit of experimentation would be in order

[artsinbloodshed]

Thanks gurner!
I have some questions though...
My aim is just to have voltage resistance/conductance in the reverse way an LDR does..
When I see "opamp", the word "audio signal"  pops in my mind...and not voltage res/conductance...
I just want to be sure to understand...

[Gurner]

So you want a control voltage (not a signeal?) Firstly, Not sure how you can get an inverse resistance going on...eecept to use say a control voltage derived from the ldr, then invert it & use the inverted voltage to drive an LED...the end result woukd be a resistance inverse to the amount of light shining on the original LDR.

Your intended application would shed more light (pun intendeed)

[artifus]

http://www.aronnelson.com/gallery/main.php/v/chickpea/darklight2.jpg.html

[artsinbloodshed]

Thanks for all the hints guys!
The darklight seems to be the perfect fit!
Now I can die in peace...

[artsinbloodshed]

I replied too fast...
about the dark light: http://www.aronnelson.com/gallery/main.php/v/chickpea/darklight2.jpg.html
Just one more thing...I don't see clearly how to implement this schematics in a non polarized LDR to ground...
The ldr is meant to replace a 50k linear pot to ground.
How do I add this to my simple ldr to ground?
(and what is the value/nature of D1)

[DavenPaget]

OOPS . Sorry 

[deadastronaut]

d1  is an led....cover ldr and it lights up....

[artifus]

How do I add this to my simple ldr to ground?

the ldr of the darklight would be mounted externally for you to 'play' with your internal ldr to ground pointing at and controlled by the darklights led.

[DavenPaget]

How do I add this to my simple ldr to ground?

the ldr of the darklight would be mounted externally for you to 'play' with your internal ldr to ground pointing at and controlled by the darklights led.

What you mean is that normal connection to the LDR , and the LDR facing the LED D1 ?

[armstrom]

Rather than trying to figure out how to invert the action of an LDR why not just re-evaluate where to place the LDR in the circuit? LDRs are rarely used all by themselves. They usually work in concert with other passive components to achieve some sort of desired action. The most common being a voltage divider. Lets say you have an LDR as the "lower" resistor in a voltage divider. That means that as the light level increases the voltage output of the divider decreases (the LDR resistance to ground decreases meaning more voltage is dropped across the LDR, so less makes its way out of the divider). Now, lets say you wanted to "invert" this effect, simply make the LDR the "upper resistor in the divider and an increase in light will result in an increase in voltage. Of course, you will have to adjust other components to achieve the range you want and take care not to affect any other parts of the circuit... But there's no reason it can't be done.
-Matt

[artifus]

What you mean is that normal connection to the LDR , and the LDR facing the LED D1 ?

the darklight circuit would be an addition to your existing circuit so there would be a total of two ldrs in the project.

for example, if you had built an idiot wah or a colorsound wah and had used a ldr as vr to ground that you planned to mount externally to play with your foot but wanted to reverse the action of the ldr you would add the darklight circuit with the darklights led facing the wahs ldr mounted internally and mount the darklights ldr externally to play with your foot.

[artsinbloodshed]

hey guys!
I come to you one more time because I have a problem with the darklight II system.
i've implemented it 2 times...
-first time it was ok (not really tweakable though) but after a month (without) using the pedal, it's not working anymore.
-2nd time the system is not functioning at all:
the trans 3904 is burning hot and the b10k pot had even some sparks in it!
no variation can be heard on the wah effect on both pedals...I checked the circuit 5 times and the prob doesn't come from it...
any hints?

[deadastronaut]

check this out on the right hand side of the lightwah schematic, this darklight it has adjustable sensitivity..

make sure you have that 4,7k there ok...it'll work fine, ive used it lots of times ......wth no smoking etc...

http://www.diystompboxes.com/smfforum/index.php?topic=88582.msg820716#msg820716

[artsinbloodshed]

Do I have to keep the 2 leds top right?

[deadastronaut]

not really, but they react to the ldr....i was using one led for the box..one for the ldr.....i just like leds..