PWM fun for you clever people =)

Hello all, I need to create a circuit that uses PWM.
I am going to use a 555 timer to create the duty cycle which i would like to range from something like Time high 5% to 95%, so to make an LED go from very dim to nearly full.
However i notice in the equations for time high and time low that you cannot get time high to be less than 50%

Time high = 0.7 (Ra + Rb) C
Time low = 0.7 Rb C

So, time high will always be greater than (or roughly equal to) time low, because with the time high equation you are always adding Ra AND Rb, but time low is just Rb on its own, and since you can't have a negative resistance for Ra, i don't see how it could be lower.

In the diagram on this website
LED Dimmer Circuit - Lighting
I am guessing the way that their duty cycle of 5 - 95% is achieved is something to do with those two parallel diodes. If someone could explain to me how this does this, i would be vastly grateful. Thanks =)

hi,
The diodes enable the adjustment of the Discharge and Charge times of the timing capacitor by varying the potentiometer setting.
The diodes allow conduction in the forward biased state and block conduction in the reverse biased state.

Hey thanks for replying.
I'm sure it makes sense but i think i need it spelled out for me as I'm a bit of a newbie and I don't really get how it works too much. Also for my project I was thinking would it be possible to have an LDR as the potentiometer as i want my project to react to light levels.

Cheers =)

SERVO TESTER Circuit

use second schematic, set up with resistors(pots) first, then after you know your values you can switch to LDR's, also you may need to balance ldr with parallel resistance(if too high), or adjust values of R2 + C1 to fit with R1(LDR)