fading with PWM

Discussion in 'General Electronics Chat' started by MikeA, Jan 24, 2013.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
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    17
    I'm trying to build a circuit to control accent lightning made up for multiple LED strips. The total draw of all the LEDs would be approaching 5 amps @ 12v, and I'd like to control them all from the same point, and just wire the LEDs in parallel.

    The behaviour I'm trying to achieve is to have the LEDs stay on all the time at a low brightness (~20% duty), then when a signal from a motion sensor comes in, turn the brightness to maximum.

    I think this could be done with a high current transistor being driven by a 555 set to 20% duty and a trigger from the motion sensor tied to the transistor also. And that would be simplest. But I'm also trying to get the effect of fading on/off. When the LEDs transition from 20% to 100% duty cycle, I'd like to make it a smooth, rather than an abrupt increase in brightness.

    What kind of circuit would be able to accomplish this elegantly?:confused:
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Dimming an LED from 100% to 50% results in a very small amount of dimming. 100% to 20% is just noticeable because our vision's response to brightness is logarithmic.

    A 555 circuit can produce a duty cycle from 95% to about 5% SMOOTHLY which is fair amount of dimming.

    I have some LEDs controlled by PWM. When the duty cycle is maximum then the LEDs are as bright as they are with DC. But when the duty cycle is 0.1% or less then the LEDs are dimmed but are still visible. I added a filter capacitor to kill the PWM at low duty cycles then then LEDs turn off.
     
  3. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    Getting the brightness right I'd think would be the last step in my endeavour. What I'm having trouble is finding a circuit that will allow me to have a trigger, that would tell the circuit to fade on or fade off the LCD array using PWM. :confused:
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Trigger?
    Do you want to trigger a circuit then the circuit adjusts the PWM from the 555?
    Usually the PWM from a 555 is manually adjusted with a potentiometer but the range of PWM from a 555 is not enough for complete dimming.
    You need a PWM circuit with a wide range of duty cycle that is voltage-controlled.
    Then you need a slowly ramping circuit.
    Then you need a circuit that triggers the ramping circuit.

    LCD array?
    Do you have LEDs or LCDs?
     
  5. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    Correct, I will have a motion sensor that will trigger this circuit.

    So when motion is detected, I'd like to initiate the fading of the LED array.

    If the current state is off, I'd like to fade on the LEDs automatically. Say to go from 20% duty to 100% duty over 2-3 seconds. Then if the state is on, do the opposite, fade off the LEDs.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Here is a long thread about a slow LED fader.
     
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