Help with 555 led dimmer circuit

Discussion in 'Digital Circuit Design' started by Akrasiel, Nov 17, 2016.

  1. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Hi all! I'm Luca and i'm from Italy. I'm not expert in electronics but in my spare time i like to create and study (in the limit of my ability) simply electronic projects useful for everyday life.

    Some days ago i was asked from a friend to make a simply circuit to dimmer a led string. I discovered that the simplest way to do it is by using a 555 as PWM generator and a MOSFET. In order to go step by step i decided to start realizing a simple single led dimming circuit (to which i could add a mosfet afterwards). I already got a SA555 timer at home and so.. i started prototyping it on a stripboard.

    Actually the result is that i always got an High output on pin 3, and even rotating the 50k trimmer i cannot modulate led brightness (the led is always ON).
    I attached the schematic i'm using.. I would be pleased if somebody could tell me if the scheme is valid or not.
    In the meanwhile i'm trying to get more 555 timers to test (maybe the one i got was broken).

    Thanks in advance.
     
  2. Alec_t

    Expert

    Sep 17, 2013
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    The schematic is ok, so if you aren't getting the wanted result you probably have a wiring error.
    Welcome to AAC!
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You need a dvm/voltmeter to test the circuit, sounds like the capacitor is not charging up on pin2, via the diode and preset resistor.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    My first step when I have a 555 problem is to substitute a much larger timing capacitor to bring the frequency into the visual range, i.e. below 5Hz or so or even slower. A very slow flash allows you watch voltages as the circuit works. Once I get it flashing, I put the small cap back in place.
     
  5. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Thanks a lot for replies. I verified several times connections and it seems everything is wired exactly as the scheme.
    Would like to follo @wayneh idea to try a different cap in order to test the 555. What value of capacitor do you think is better to use?
     
  6. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Connected a 10uF capacitor in parallel with the 100nF (the one from pin 2,6 to ground) and nothing changed, even rotating the trimmer the led has always exactly the same brightness.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Try also replacing R1 with a 10K to further slow things down. You should have seen flashing, but with an even slower cycle you might be able to watch the voltages on the various pins.

    Better yet, show us a picture of your setup. It's very easy to keep overlooking the same mistake that another set of eyes can spot right away. We've all done it.
     
  8. Alec_t

    Expert

    Sep 17, 2013
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    Are you sure you have the correct pinout for the trimmer? One connection must be to the wiper.
     
  9. #12

    Expert

    Nov 30, 2010
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    Change the 1k resistor to 22k, change the 100 nf capacitor to 100 uf, and leave the trimmer pot out.
    That will give you 2.4 seconds per cycle.

    The time formula is 1.1 x R x C
    Many combinations of resistor and capacitor are possible to slow down the timer enough for a person to see the changes in light.
     
  10. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Guys... you're simply awesome. I wanna really thank you for the support you're donating..

    I added two photos of the setup i did.. I also added some details to the photos in order to understand quickly all the wiring.
     
  11. #12

    Expert

    Nov 30, 2010
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    I don't see anything wrong except it's going to blink faster than a human can see.
     
  12. ian field

    AAC Fanatic!

    Oct 27, 2012
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    That's one way of doing it, and looks like it should work. You can also drive the pair of diodes directly from pin 3 - that eliminates the 1k resistor and leaves pin 7 as an uncommitted open collector output that's in phase with pin 3.
     
  13. Wendy

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    Mar 24, 2008
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  14. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Wendy your "guide" was very useful for me, but at the same time a doubt raised: should i use a different circuit to obtain what i'm looking for? (maybe the one with the lm393)

    The need is to control the brightness of a very long led strip (i calculated about 6-7A), starting from 0 till to next the 99%.


    (Anyway I'm pretty sure that my problem comes from the 555, i ordered many and i will receive them within some days.)
     
    Last edited: Nov 19, 2016
  15. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Its easy enough to test the one you have with a bare bones astable circuit, something to do while you wait.
     
  16. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    Hi all, i'm back. After discovering that my SA555 was not working at all i waited for the new ones to come. The day has come.
    I tested the original circuit (the one in my first post) and everything went fine. However as expected (even replacing the 50K potentiometer with a 100k one) i can't have a full dimming control (from 0% to 100%).

    Thanks to Wendy i found that is possible to reach this result combining a 555 timer with an op-amp used as comparator. I built a new prototype using both his scheme and another one i made (i attached both... my scheme is the hand drawn).

    Anyway... I'm not satisfied with the result: yes, it's true, now i can go from no light to about full brightness.... but all the brightness change is concentrated in the first half of potentiometer. Even with a small rotation i get half of the brightness and it's very hard to set a desired low brightness setup.

    As you can see from the scheme i'm actually powering the circuit from 12v; i'm using a led lamp (12v 3.5w) as load; i used an UA741CN as op-amp and as mosfet a 2SK2700 (i'm waiting for new models to come...). The 2SK2700 has a gate threshold voltage of about 2-4v.

    Hope to get some good advice from your knowledge and experience. I thank you all in advance.
     
  17. dannyf

    Well-Known Member

    Sep 13, 2015
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    that's an issue with the 555 itself -> it cannot go to 0% DC. No solution here.

    two solutions:

    1) use padding;
    2) use non-linear pot. for example, use an audio pot here.

    BTW, a simple opamp can do what you are trying to do.

    another solution is to use an mcu: lots of algorithm on gamma correction.
     
  18. Akrasiel

    Thread Starter New Member

    Nov 17, 2016
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    I found some scheme of a LM393 used half to generate a "sawtooth wave" and half as comparator but would it solve the "problem"?
     
  19. Bernard

    Expert

    Aug 7, 2008
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    LED's are quite non linear, small change at low current = large change in brightness, small change at high current = no visable change in brightness. As suggested use an audio non linear pot. with fast change end controlling high brightness.
     
  20. EM Fields

    Active Member

    Jun 8, 2016
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    Try this:
    555 fixed freq variable duty cycle.png
     
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