Can You Help Me With This Circuit Plz

Discussion in 'General Electronics Chat' started by Plecc, Aug 26, 2010.

  1. Plecc

    Thread Starter New Member

    Aug 26, 2010
    22
    0
    Hi all,
    I would like to make a simple 10x LED circuit with a potentiometer to control the brightness.
    I want to use some 10mm blue LED's,
    Spec:
    Forward Voltage 3.2v
    Power Dissipation 70mW
    Peak Forward Current 100mA
    Continuous Forward Current 20mA

    I was thinking a circuit like this would be best:


    ^etc up to x10^
    |----(resistor?)------(+LED-)------|
    |----(resistor?)------(+LED-)------|
    |----(resistor?)------(+LED-)------|
    |(Potentiometer)| |---------------|
    | |
    (+) (-)
    (Power Supply 9-12V)​


    Sry i'm a total noob at this, never rly understood how to calculate resistance. (think i need 470 Ohm 1/2W for each LED)
    Can someone advise me what resistors, potentiometer and power supply to use? (3-12V would be convenient for me)
    Is this the best way to rig up this circuit?

    Thanks.
    Plecc.
     
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
  3. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
  4. Plecc

    Thread Starter New Member

    Aug 26, 2010
    22
    0
    Thanks for the quick feedback,
    Loads of info, i didn't know really where to start.
    Sooo much more complex than i thought to dim an array of LEDs.

    Thanks JaguarJoe for the link, i was going to go for that one but I'm finding the 7805 a bit tricky to get hold of.
    A friend of mine found one based around one of the 555 timers.

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

    I can get hold of all these bits really easily, would this be suitable for running 5 strings of two, each pair joined to a 1/2W 330Ohm resistor?

    Oh and the BC547 i have is 100mA rather than 200mA is this a problem?

    Thanks.
    Plecc
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Perhaps you can use a 2N2222 in stead of the BC547.
    This can handle upto 500 mA.
    Also use a 390 Ohms instead of the 330 Ohms to reduce the current a little bit.

    See the attached table for more transistor data (unzip and open in excel).

    Bertus
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    7805's are as common as bed bugs on an 8 mile rd. hooker.
    "All Electronics" has them for 50 cents each. They have 555's for 75 cents and 2N2222's at 2 for a buck. Just enter the part numbers in the search box and they will pop up.
    PWM is the slickest way to dim LED's.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    We don't know where you are.

    $0.75 for a 555 timer sounds steep. 2N2222's at $0.50 is really expensive. You should be able to find both of those for far less even at authorized distributors like Digikey, Mouser, AvnetExpress, etc.
     
  8. Plecc

    Thread Starter New Member

    Aug 26, 2010
    22
    0
    Hi all,

    burtus, i can get a BC549C which has a 200mA rating, I checked the spec sheet for this one and i looks very similar to the BC547.
    Would this be ok?

    Jaguarjoe, Yeah your correct 7805's are rly easy to get hold of even in the uk, but as it's only a small project i want to get all the bits from my local electronics shop, so i don't have to pay postage on individual components.

    http://www.maplin.co.uk/

    Very similar to Radio Shack in the US.
    The components are stupidly expensive from maplin, but its really convenient for small projects.
    I think all the bits for this project come to about 3 pound.

    Thanks.
    Plecc.
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    There's another way you could go. Get a cheap microprocessor that supports ADC (analog to digital conversion) as well as PWM like a Picaxe 08M. You'll need a potentiometer of almost any value, a 7805 to power it, and the transistor, LED's and resistors for them.
    You program the 'axe in very simple BASIC. All of the software you need is free from Picaxe, just buy a download cable from them. The guys on the Picaxe forum would gladly walk you through the steps to accomplish your goal. The nice thing about this is that you can program that chip to do just about anything you like.
     
  10. Plecc

    Thread Starter New Member

    Aug 26, 2010
    22
    0
    OK,
    so heres where I'm up to.
    I managed to get all the bits together to build these two dimmer circuits.

    http://www.reuk.co.uk/LED-Dimmer-Circuit.htm
    and
    http://pcbheaven.com/circuitpages/LED_PWM_Dimmer/

    Both use a 555 timer in a very similar setup.
    I built both circuits up on a project board and connected up a single string of 3x 5mm high power white LEDs and the power to a 9v cell.

    Both configurations lit up the string of LEDs but the potentiometer in both circuits had no effect on the brightness of the LEDs.

    There was possibly a very faint change in brightness but barely noticeable.

    From what i have read i think these circuits cause the LEDs to strobe at a really high frequency and the dimming is caused by lengthening the off period of the led to make it appear dimmer.
    Do i need to dial in the frequency somehow or is this just rubbish?

    Any info on what i could have got wrong would be great.

    For the pcbheaven.com one I'm using a BC547C W87 instead of the 2N2222 recommended and a 9v cell to power it.

    For the reuk.co.uk one I'm using a 47K pot instead of the recommended 50K, the big 470uF cap is a 35v one instead of 25v the transistor I'm using in this one is also BC547C W87. all being powered by a 9v cell.

    All resistors for both are 1/4W.

    Wish i wasn't such a noob.
    Plecc.
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Can you make a photo of the build circuit and post it here?
    Both sides of the PCB would be handy.
    That way we can see if you made a connection failure.

    Bertus
     
    Plecc likes this.
  12. Plecc

    Thread Starter New Member

    Aug 26, 2010
    22
    0
    Sorry such a slow reply, ive been away working.

    Thanks so much for all your help, i managed to get the circuit working.
    Turns out i had the transistor wired incorrectly, i followed an NPN guide on some guys site that turned out to be total rubbish.
    After checking the data sheet for the BC547C i realise where i was going wrong.

    Thanks again.
    Plecc.
     
Loading...