Need help smoothing out a pwm signal

Discussion in 'The Projects Forum' started by branndonw, May 29, 2013.

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  1. branndonw

    Thread Starter New Member

    May 29, 2013
    Hey guys, Im pretty much a noob when it comes to electronics and need some help smoothing out a PWM signal. Im going to be modifying the headlamps circuit in my vehicle. The Body Control Module supplies a PWM ground to a solid state relay which then supplies voltage to the headlamps. When the vehicle is in Park, everything is off. When it is bright outside and the vehicle is placed into gear, Daytime Running Lamp mode is active and the headlamps run at 85% DC. When it gets dark, Headlamp mode is active and the headlamps run at 100% DC.

    What I want to do is to smooth that PWM ground so that I can use it to control a 12v mechanical relay in order to use my Park Lamps at 100% for DRL instead of the headlamps. If its necessary to use a solid state relay to convert that pwm ground to a pwm positive thats fine but I would prefer not to if at all possible to keep it as simple and inexpensive as possible.

    I've used a 100 ohm resistor and 100uf / 220uf / 470uf cap combos and successfully got the mechanical relay to switch and stay switched but the mechanical relay makes a faint humming sound that I am not comfortable with.

    Any help would be greatly appreciated. Thanks in advance.
  2. edwardholmes91

    Active Member

    Feb 25, 2013
    I suspect this thread will be locked soon because AAC does not discuss automotive modifications.

    However... can I just point out, as far as I'm aware you can't smooth PWM. PWM stands for Pulse Width Modulation. You change the effective output, in this case brightness of something by turning it on and off very quickly. The longer the pulse is on the brighter the lamp appears to be. By varying the duty cycle (percentage on time) you can vary the effective brightness. Therefore in my limited understanding of this technique, you can't smooth the output.
  3. branndonw

    Thread Starter New Member

    May 29, 2013
    I understand what pwm is and how it works. I was under the impression you could use a low pass filter (resistor and cap) that would basically absorb the pulses and give a steady output.
  4. Experimentonomen


    Feb 16, 2011
    Or an inductor and capacitor for power applications. Like a class d amplifier which is basically a buck converter.
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Yes you can get a DC from a PWM with an RC. Which values depends on the PWM frequency. You could simply try two RCs in series and play around with some values. But that will not give you a 100% output at 85% duty cycle. Maybe I didn't get that point?

    Ah and yes, this thread will be locked soon. Recommendation: Don't make mods to your vehicle if you are not familiar with car electronics.
  6. Georacer


    Nov 25, 2009
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