DC controlled duty cycle of 555

Discussion in 'The Projects Forum' started by Luc1960 Apollo13, Mar 23, 2013.

  1. Luc1960 Apollo13

    Thread Starter New Member

    Feb 25, 2013

    I'm new to this forum, so I sincerely hope I didn't miss the answer to my question. This is the setting of the scene:

    I have a low cost, IR controlled, rgb LED dimmer that is capable of color change, fading, flickering, 6 colors to design your self, on/off. All works nice at a frequency of 100Hz and is PWM controlled between 10% and 90%.

    However, in my application, the LEDs are rotating at 100 rpm and photographed at night with long exposure times. Because of the slow frequency, colors such as yellow are clearly decomposed in red and green stripes.

    So, Without changing the original board which contains unidentified IC's, I would like to increase the frequency from 100Hz to 10kHz (or more or less, I have to see in reality) to get rid of the flicker.

    My idea was to pass the output of each of the three channels of the controller through cascaded low pass filters for each channel to get a DC voltage in function of the duty cycle.

    Then I would like to feed this DC voltage to a 555 timer to reproduce the duty cycle and I will also need some other circuitry to set a frequency which I would like to vary in the kHz range.

    Does someone have a suggestion how to implement dc controlled duty cycles with a 555 together with a controlled frequency range in the kHz.

    I do know about the way to do it with resistors and capacitors, but not with dc voltages.

    Kind regards,

  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Just put your "low pass filters" on the LEDs themselves. ie; a resistor in series with the LED and a large cap in parallel with the LED. Then the 100 Hz PWM frequency becomes a constant voltage and brightness at the LED.