Simple Led PWM driver

Discussion in 'General Electronics Chat' started by LostInSpace, Jan 20, 2011.

  1. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    I look for a simple PWM LED driver

    function

    2 bit control = 1 BIT On/off 1 BIT for 25%/50% PWM

    only 1 White LED so possible for use BC 547 as driver transistor

    it shut work on 5V

    Frequnce 10k+ Hz

    My LED is work on 3.4V

    my plan is to have 4.3v over the LED and the rest 0.7v over the BC 547

    Do some have a idea for a simple setup

    I shut build 20+ off the for garden light.

    :rolleyes:
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It seems likely that you are going to apply too much voltage to the LED this way, possibly resulting in too much current passing and burning things out.

    Could you please post a schematic?
     
  3. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    I know do more current in the LED it why i want to use PWM 25% & 50 %

    I just how to design it simple with on/off so it can be controled by microcontroller

    some say why not make the microcontroller do the PWM it have hard work by control network with the other microcontroller in the network
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Why 25% and 50%?

    I believe 50% and 100% would be easier to apply, and save a bit of money on LEDs.

    As for the method, an 8 piin microcontroller can do this, but if you don't know any programming languages. There is also a $50 "Startup Cost" for programmer and ICs.

    A discrete solution would be a 555 timer, on off being power to the circuit, and 50% or 100% putting the 555 at 50% duty cycle into the output or out.

    Trying for more levels would take a few more ICs.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I've helped several folks design PWM circuits from the ground up with fair success.

    I've written an article on LEDs in general to get people started.

    LEDs, 555s, Flashers, and Light Chasers

    Chapter 5 - The 555 and PWM

    What you want to do sounds pretty easy. How many LEDs and what kind of current (and power supply voltage) are you talking?

    What I think your asking for is one 555 and a transistor, but I'll wait for more info before moving on.
     
  6. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    2 LED in par the transistor shut be BC547

    Work on 5 Volt Max 50% Current The there will be 4.3 Volt over the LED in 50% off the time with freq on 10k+ the LED will not burn

    25% option is for Safe power

    1 PIN turn on/off from MCU
    1 PIN to change betwin 25% and 50% PWM
    :)
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Here are my assumptions:

    2 white LEDs, Vf =3.4V

    Power supply is 5V.

    Control input 0 = 25%, 1 = 50%

    LED current is 20ma. (50% means 10ma average, and 25% is 5ma average).

    NPN transistor (why? Little early to dictate parts.)

    10KHz modulation frequency, this really does not make sense as it doesn't matter, but OK.

    This makes no sense what so ever. I suspect how you think these things work don't work that way.

    Why not 100% of the LED?

    Did you read the link I gave you? Especially chapters 1 and 2, the basics. The above quote suggests you are extremely new at this (which is not a problem).

    To quote Mark Twain, "It ain't the things people know that gets them into trouble, but the things they know that ain't so.".

    Be right back, drawing...
     
    Last edited: Jan 22, 2011
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you are using an MCU, it could do the entire project with one output pin and a transistor. :confused:
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    OK, first effort. I'm basically rejecting this one because of parts count, it can be done simpler.

    [​IMG]

    The more I think about it the more I think 50%/100% is a better way to go. What do you think the advantage is in throwing away ½ the LEDs brightness? It can be adjusted down to any current via the resistor, and makes the design much simpler overall.

    I agree.
     
  10. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    I work on a MCU network base on PICAXE

    all a run on 3 wire 1 base (master) and x slave

    all slave main option is to listening for main and do what it wants and don't have time for this type off function

    basic slave is remote I/O board on 200 meter netvork

    your circuit give me some idea that i will try :)
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I'll come up with a simple 555 circuit. It really will be easier to do 50%/100%, so I'll start with it. Are you trying to keep the modulation for the LED for some reason?
     
  12. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    Yes all system run on battery, power by solar cell, the master know from 1 of the slave how much power back in the system,

    system will have 15+ MCU when it finish
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    This will do 100%/50%. You can adjust the max LED brightness using the LED resistors. The 50%/25% will be more complex, but not as bad as the first circuit I drew.

    [​IMG]
     
  14. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    I change it a little bit

    Key A Switch come from MCU PIN1 with Power (+5v)
    Key B Switch Come from MCU PIN2 change 50%/100% PWM

    and move R8 from Vcc to chip source ( it work fine)

    add the R9 so Q1 close when Power remove from 555

    I want to see if it possible to remove R4 & R5
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Actually it doesn't work fine. You are using a switch to turn the circuit off. In mine it was a true digital input, the 555 goes into standby. There is a substantial difference there, as I was designing for digital controls.

    However, if it works...

    Do you still want the 25%/50% circuit?
     
  16. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    The 2 On/off key only simulate input from a MCU


    Yes 25% / 50% is next

    in 25% at 10khz you can put more volt over the LED and it have 75% time to cool down

    and in 50% it have 50% to cool down


    I remember for old time where B&O remote control on the IR LED use 10% PWM and it have higher volt over the LED as normally remote control



    if there is 4volt over the LED with 25% PWM there current will be higher than normally
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    I don't know where you got your information, but it is flat wrong. LEDs are not voltage controlled devices, ever. They are current controlled, with a constant voltage drop that does not change. Different color LEDs have different voltage drops. If you overdrive them you shorten the life of the LED, period. The flip side is they have such a long life that even halving their lifespan is no big deal.

    There is no circumstance where LED dropping voltage changes, only the driving current.

    You really need to read this...

    LEDs, 555s, Flashers, and Light Chasers

    PWM will not extend the life of a LED. It will allow you to vary the brightness of a LED linearly. That is all it is for.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    OK last schematic. CR1 really screws with the math, so some fine tuning would be required. Making CR1 a germanium or Schottky diode would help though...

    [​IMG]

    All of these are based from My Cookbook I have started.
     
    Last edited: Jan 25, 2011
  19. LostInSpace

    Thread Starter New Member

    Mar 16, 2010
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    Thanks many time i build in test board tomorrow

    together with the picaxe MCU to check function :)
     
  20. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Wait a minute - each light will have two LEDs and a PICAxe processor in it?

    Which PICAxe model are you going to be using in the lights, or am I missing something?
     
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