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LED dimmer circuit

Discussion in 'LEDs and Optoelectronics' started by candlepin, Feb 9, 2009.

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  1. candlepin

    candlepin

    2
    0
    Feb 9, 2009
    Hi folks. I'm new to the forum and relatively new to diy electronics.

    My current project involves trying to make a battery-powered LED dimmer circuit. I''ll be using a 9v battery to power a single LED. In order to make the dimming linear (preferable) it appears I'll have to make a circuit like the one on the bottom of this page:

    http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

    I was hoping someone might be able to help me figure out the exact components I'll need in order to make a circuit like this work for 1 LED.

    Any help would be appreciated. Thanks!
     
  2. candlepin

    candlepin

    2
    0
    Feb 9, 2009
    Here is a basic outline of the schematic I think would work. What I need help determining are the values of the capacitors, resistors, transistor, diodes, and potentiometer. I'd like the circuit to be variable about 5-95%. I'll be including a switch in this circuit as well.

    [​IMG]
     
  3. alexm865

    alexm865

    3
    0
    Feb 12, 2009
    Hi candlepin,

    You might want to look into using an LM338 or LM138 Voltage Regulator instead of the 555 Timer. It's good for 5A and can effortlessly adjust your 9V down to 1.2V using a variable resistor.

    It's quite simple to use and all you need is a resistor, and a variable resistor to get it to work.

    The Datasheet is here and has many example circuits:

    http://www.national.com/ds/LM/LM138.pdf

    If you look under "Typical applications" you should find a circuit for a "1.2Vā€“25V Adjustable Regulator". You will need to reduce the value of the R2 variable resistor to get the sort of fine tuning you want for your led dimmer, I'd say try 400ohms for a start, and add a diode in series with your LED to drop the 1.2V output down to 0.5V. You don't really need to add the coupling capacitors for this type of application.

    That should give you an adjustable voltage on your LED from about 5volts down to 0.5volts using only a diode, LM338, a resistor, and a variable resistor.

    These voltage regulators are quite simple to use. I've mainly used them for controlling the speed of my computer fans to quieten them down and they work perfectly!

    The 555 timer would work fine too but I think that the LM338 voltage regulator would be easier to use in this case.

    Hope this helps

    -Alex
     
  4. alexm865

    alexm865

    3
    0
    Feb 12, 2009
    Hi candlepin,

    I kinda realised my last post missed the point a bit.

    If you want to use the 555 timer, the values stated on that web page you referenced can be used for 9V as well, i.e:

    R1=1k
    R2=1k
    P1=50k
    C1=0.1uF
    C2=0.01uF
    **NOTE: Decreasing C1 and C2 will increase the switching frequency
    C3=can be any large capacitance, you can pick 470uF if you like. It's just there to take a bit of the ripple current away from the source.
    R3=is to limit the current through the LED, it really depends on the LED that you choose but for a 3V LED you probably want somewhere around 300ohms.

    I ran a few simulations through Saber for 12V and 9V and running the circuit at 9V doesn't appear to change the frequency or duty cycle much (the change is less than 1%).

    Also, try using a high gain NPN transistor such as a Darlington. They work off very little base current and have extremely high gains. A good one to use is the BD681, they can handle about a 1 amp (more than enough for an LED) and have a very high gain, and best of all they are really cheap!

    Hope this helps,

    -Alex
     
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