light dependant PWM

Hi there, I am looking for a way to control a PWM output with LDR(s). PWM is going to LEDs to dim them (decided this was probably better than altering the current since they will be in series) such that an increase (or decrease even) in light dims the LEDs. 555 circuit seems the best, and altering the duty cycle with two resistors. Looking at it I think that I would need two LDRs, one with an inverse light resistance relationship (!?!). A bit stumped on how to do this... And if anyone one has a better solution to the problem that would be great, thanks in advance for any help you may offer

 

Can you post a schematic of what you are proposing?

 

shamelessly copies from
http://www.afrotechmods.com/groovy/PWM_tutorial/PWM_tutorial.htm

and

http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

thanks

 

Here's a couple of examples where a similar thing is done with 2 555 timers (a 556). I think this is the control you are looking for.

http://www.scribd.com/doc/3612715/pwm-556
http://www.eleinmec.com/article.asp?28

 

nice, frequency is preset by the astable, then the high time can be controlled on the second monostable by the LDR,

thanks for the help