LDR controlled dimmer w/Offset and Span adjustment

[qbone]

Hello guys.
Would it be possible to make an LDR controlled dimmer for LEDs with adjustable offset and span (gain) ?
I got an LDR that goes from daylight (55ohm) to total darkness (2Mohm+), more specifivly its a NORP12.
So I want it to work like this: at daylight output of the circut is 4.5V, and in total darkness output is 500mV. These two values need to be adjustable.

I have been fiddling around with this alot lately. Beem trying with a transistor circut and opamp summer. Also tried with a regular inverting opamp where I just adjusted the virtual ground, but whatever I do, offset and span keep affecting eachother so it gets pretty difficult to adjust.
Any help would be awesome. Even if its just to point in a direction where I can read up on something like this.

 

[ericgibbs]

Hello guys.
Would it be possible to make an LDR controlled dimmer for LEDs with adjustable offset and span (gain) ?
I got an LDR that goes from daylight (55ohm) to total darkness (2Mohm+), more specifivly its a NORP12.
So I want it to work like this: at daylight output of the circut is 4.5V, and in total darkness output is 500mV. These two values need to be adjustable.

I have been fiddling around with this alot lately. Beem trying with a transistor circut and opamp summer. Also tried with a regular inverting opamp where I just adjusted the virtual ground, but whatever I do, offset and span keep affecting eachother so it gets pretty difficult to adjust.
Any help would be awesome. Even if its just to point in a direction where I can read up on something like this.

hi,
The usual way to dim led's is by using PWM to vary the ON/OFF periods of the LED.

Its possible to control the led brightness over limited range by controlling the led current, but PWM gives better control.

The 555 timer IC is often used in simple PWM applications.

EDIT:
these links may be useful.
https://www.google.com/search?q=555...t&rls=com.yahoo:en-GB:official&client=firefox

https://www.google.com/images?oe=ut...tle&resnum=3&ved=0CDgQsAQwAg&biw=1301&bih=823

 

[qbone]

Thanks alot, it makes good sense to use PWM, but I am not sure I can use it. The circut im working on doesnt have the dimmer directly connected to the LEDs, but to a variable Buck Converter made from the LT1074. The output from this circut I need goes through a transistor on the input of my buck converter.
Actually this whole circut Im working on is to upgrade my current really simple circut, which you can see here: