Newbie Fader help

Discussion in 'General Electronics Chat' started by drcarnine, Sep 11, 2013.

  1. drcarnine

    Thread Starter New Member

    Sep 11, 2013
    2
    0
    I am trying to fade on a set of 15 LEDs that are on a circuit board with resistors limiting the current to 20ma each. The board runs on 3.6 volts. I have a pir circuit that drives the LEDs. Currently it uses the 3.6 volts from a AA battery pack. I can remove the battery pack and use a 6 volt external lead acid 4.5Ah battery that is charged each day with a solar panel. I would like to slowly fade the circuit up when the pir circuit triggers it. I would like to try to take about 30 seconds to come from fully off to a full 3.6 volts. I can put that circuit into the space where the internal AA batteries were if I can make it small enough. I have read many of the past posts and most use 9 volts and also fade off. I just need it to fade on and go off instantly if possible. Fading off would be okay if I can get the full 3.6 volts to the LED board. I tried several resistor, capacitor, transistor circuits I found online but they do not let the full 3.6 volts come through. I was using a 1.2k resistor to the collector of a PN2222A transistor and to the positive side of a 1000uf capacitor, and the transistor never did fully conduct and the LEDs never got bright, so I need something else. I would like to just be able to put this circuit between the pir circuit and the LED board--6 volts in and 3.6 volts out after about 30 seconds, even longer would be better. Any help would be appreciated. Thank you, Dennis
     
    Last edited: Sep 11, 2013
  2. drcarnine

    Thread Starter New Member

    Sep 11, 2013
    2
    0
    LEDs, 555s, Flashers, and Light Chasers

    I can use 12 volts to power my pir circuit. I tried to look at the above post chapter 12. I don't know enough to fully figure it all out. I do not understand what u3a and u3b are. I think figure 12.2D will work for me but I need some help on what all of the components are. Also, I only want the LEDs to fade up and stay there until the pir turns off the power. I do not want it to oscillate. So therefore, I do not know which part of the circuit I need to use. Thanks again
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,236
    Is there anything besides LEDs on the LED board or do you need to run other circuits at 3.6 volts?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    The "U" in 12.2.D is a LM393 comparator. I think that circuit is a good option for you. Your low voltage requires some careful choices.

    Note that you should not be thinking in terms of controlling voltage. You need to control current. LED brightness is directly proportional to current, both of which change from "off" to "burnt out" over a very small voltage range, maybe 0.5V or less. So plan to run your LEDs off the system voltage (I think you said 6V?) with the proper current limiting.
     
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