light controlled PWM

[charliek]

Hi everyone, I want to design a circuit to control the brightness of an led, depending on ambient light levels. I also want to make the upper and lower limits adjustable via two independent variable resistors.

I have looked at some 555 circuits, but am not sure how I could change the diode / variable resistor array without getting complicated... so can anyone thing of a slightly simpler approach?

Thank you.

Charlie.

 

[charliek]

ok, so for my idea, I thought to replace the pot which controls PWM with two complimentary fets / transistors, which in turn could be controlled by the ldr / photo transistor? I could then control the upper and lower limits of the photo detector to control the PWM range??

 

[Mr RB]

Hi Charliek and welcome to the forum.

If you are new to 555 timers I would suggest starting with the simplest setup and refining from there. If you take a standard 555 astable the discharge period remains fixed (through one resistor into pin 7). The pot adjusts the cap charge period, so it controls the frequency and the duty cycle together. If you built that on a plugin breadboard you should get a working result just by replacing the pot with the LDR.

Once that is working I'm sure it won't be too hard to add a couple of trimpots and tweak some part values to add range adjustment.

 

[charliek]

Thanks, I will have to experiment. I was hoping to go for a fixed frequency, using double diodes and pot for maximum range, but I will try the simpler method and see if the effect is desirable.

 

[Mr RB]

The 555 circuit with 2 diodes and a pot requires that the pot have 3 pins (ie is two adjustable resistors). That makes it very hard to replace the pot with an LDR (which is ONE adjustable resistor).

If you want a fixed frequency you can use two 555 chips (or one 556 chip) and make the first timer an astable running at the right frequency. Then the second timer makes a monostable, where the HI period is adjustable and set by a variable resistor. That setup is a fixed frequency, adjustable PWM circuit. Then you can replace the variable resistor in the PWM monostable with an LDR.

You can google "555 circuits astable and monostable" for many examples of both circuits.

 

[alec_t]

Do you really need PWM just to control the brightness of a LED? Is it a high current type?

Edit: Perhaps adapt this method?
https://www.electro-tech-online.com/threads/amperage-of-8-aa-cells-connected-in-series.528/

 

[bryan]

Using a 74HC14. Frequency is 1/RxC . Some LED drivers have a specific frequency requirement

10k pot and a 470nf will give you a frequency of around 225hz.

https://www.dprg.org/tutorials/2003-10a/motorcontrol.pdf

 

[crutschow]

Here's an example of a voltage-controlled dual 555 (or one 556) PWM circuit. The modulation voltage could be generated by a light variable resistor in series with a DC voltage.