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I have made a camera vision setup with highpower LED's which are controlled with a simple NE555 timer and a mosfet. The LED system is lighting up an area where a camera is looking, but unfortunately i am experiencing ripples/stripes on the video stream due to the nature of the PWM dimmer. Is there a way to reduce these stripes or even eliminate them?

The camera: http://charmedlabs.com/default/?page_id=211

NE555 circuit:

enter image description here

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  • \$\begingroup\$ TRy changing the 100N on pin 6&2 to ground to 10N or other values. Higher and different frequencies may help. \$\endgroup\$
    – Russell McMahon
    Feb 9, 2015 at 12:28
  • \$\begingroup\$ Try frequencies twice, three times or more than the frequency of the capturing camera. \$\endgroup\$
    – Triak
    Feb 9, 2015 at 13:06

1 Answer 1

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If you used an inductor (with maybe a capacitor) filter in line with the LEDs you would get much lower ripple voltage on the LEDs. At the moment it sounds like the LEDs are being modulated directly by the PWM frequency and this will generate "beats" with the video frame and line rate. This produces ripples in the same way as aliasing can occur when sampling a frequency that is too high. You could also try increasing the PWM frequency or maybe locking it to the video signal somehow.

Here's a circuit that does this: -

enter image description here

By the way the circuit link doesn't work for me.

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  • \$\begingroup\$ Whats mostly important is somehow killing the ripples or beats as i am not able to do much on the camera side. Is it possible to adjust the current on this application circuit as i only see permanent resistors. Cheers. \$\endgroup\$
    – JavaCake
    Feb 9, 2015 at 11:27
  • \$\begingroup\$ Yes by changing the current feedback (Rsense) This cct regulates the current == brightness. Your 555timer circuit could be changed todo a similar thing by placing an inductor in series with the LED's (and providing an additional freewheel diode) \$\endgroup\$
    – user16222
    Feb 9, 2015 at 11:40
  • \$\begingroup\$ @JavaCake I still can't view your circuit for some reason. The circuit I've shown is an example of using an inductor/capacitor to reduce ripple. I expect you can use it but you'll have to read the data sheet. I also expect that increasing your PWM frequency will help a lot too. \$\endgroup\$
    – Andy aka
    Feb 9, 2015 at 11:42
  • \$\begingroup\$ @Andyaka the OP is showing up fine here. You may have some browser or proxy specific block: i.stack.imgur.com/k9Vkw.gif \$\endgroup\$
    – user16222
    Feb 9, 2015 at 11:56
  • \$\begingroup\$ @JonRB thanks for that - looks like my end is at fault! \$\endgroup\$
    – Andy aka
    Feb 9, 2015 at 12:01

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