Dimming LED Strip with 555timer

Discussion in 'The Projects Forum' started by fearnofish420, Feb 17, 2009.

  1. fearnofish420

    Thread Starter New Member

    Feb 17, 2009
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    I recently tried to build a PWM trying to dim a LED strip. Im not sure what I did wrong or what Im forgetting but all it did was pulse, slow or fast depending on the potentiometer setting. Does anyone have access to a Schematic that I could double check my work here.I am guessing I have something missing or bad part? I am looking to be able to dim the brightness of the led strip. Thank You To all with info!
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Your oscillator is running way too slowly. A PWM is simply a pulsing circuit which runs fast enough we can't tell it is pulsing. Can you share your schematic? If so, we can suggest better values for the timing components.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    A 555 makes PWM with two diodes connected to a pot. PWM is changing the duty-cycle of the on-time of its pulses, not just changing the frequency of a simple oscillator.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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  5. italo

    New Member

    Nov 20, 2005
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    LEDS do not tend to be very usual for light dimming at as current increases it follows an exponential curve some more then the others. PWM is one way to achieve that which does not make sense since it may interfere with radios ,PC ,TV PHONES a limiting potentiometer can achieve the same results. Actually a variable current source like lm317 is desirable but then again it is your project do as you wish.
     
  6. fearnofish420

    Thread Starter New Member

    Feb 17, 2009
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    Here is my schematic. Thanks to all who have any input?
     
    • pwm.doc
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  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Gee, I don't know what program you used to draw the schematic, but the 555_Virtual timer pins are mislabled.
    1 - GND
    2 - Trigger
    3 - Out
    4 - Reset
    5 - Control
    6 - Threshold
    7 - Discharge
    8 - Vcc

    The way you show it connected in the schematic, I'd be genuinely surprised if it did anything besides output smoke... :eek:

    See the attached; it's a variation on a circuit Bill Marsden posted months back. C2 on the CTL pin (5) is optional, but it can help stabilize the frequency in a noisy environment.

    R1 controls the ratio of on/off time. R2 limits maximum charge/discharge current through the diodes in case R1 is turned all the way to one or the other stops.
    C3 is to help with the transients created when the 555's output toggles states. You can use your 470uF cap; the value really isn't critical.

    This version uses an N-channel power MOSFET (IRF510) instead of a transistor to switch the load current. You could use a transistor if you wish, but as of yet you haven't told us what transistor you're using. R3 limits the current flowing into/out of the MOSFET's gate to prevent high frequency oscillations. R4 turns the MOSFET off in case the 555 timer is removed from the circuit or is damaged.

    [​IMG]
     
  8. fearnofish420

    Thread Starter New Member

    Feb 17, 2009
    8
    0
    If someone could look over this and see if it would work before i try to put everything together. I am assuming this is correct and would inreturn be able to dim a light strip. Thanks to all whom have helped. I have learned lots! If it helps I used electronic work bench to make this.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Well, if you're going to decrease the pot from 50k to 5k, then increase the timing cap to around 22nF to 47nF.
     
  10. fearnofish420

    Thread Starter New Member

    Feb 17, 2009
    8
    0
    I have made this circuit as you illustrated in the schematic. My only issue I now have is I didnt have the exact values for the 10nf capacitors. I had to use larger ones. The issue I am having is the light is not dimming down the the level I would like. Would the larger capacitors make a difference OR do I need to adjust the values of my resistors to achieve a dimmer level of light?
    When I dim the potentiometer I barely need to more the dial and the light drastically either brightens or dims. Why is this and how can it be changed? My other issue is the potentiometer seems to be split in half. If its turned all the way to the left or all the way to the right its equally dim. If its in the middle its Bright!From the center point it does not matter what way the dial is turned it dims equally as if turned the other direction? What causes this as well?
    I am also using a 50k Potentiometer if that possibly matters
    Thank You AGAIN
     
    Last edited: Mar 3, 2009
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