how to power more led's

Discussion in 'The Projects Forum' started by markosillypig, Oct 9, 2009.

  1. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    hi all
    i.m using this led chaser circuit on a project for my garden but i want it to power a bank of 10 led's on each leg of the sequencer so that is 10 led'ssimultaneously 8 outputs for the sequence i asume the led output can be used as a trigger
    so what do i have to do to finish the project

    thanks
    marko
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    An easy solution would be to use a ULN2804 Darlington driver IC. It has 8 Darlington drivers with 10.5k base current limiting resistors in it already, just fine for CMOS.

    Each channel in the ULN2804 can sink up to around 350mA current continuously. You will lose about 1v across the output.

    Digikey stocks ULN2804s:
    http://search.digikey.com/scripts/D..._link=hp_go_button&KeyWords=ULN2804&x=22&y=18

    You have not mentioned what LEDs you are using, nor their typical Vf @ current specs.
    [eta]
    If you are using a 9v battery for power, it won't last very long.
     
    Last edited: Oct 9, 2009
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Just in case anyone is interested, CD4015 datasheet.

    As Wookie suggested, a simple CMOS/LED driver might be a good idea.

    [​IMG]
     
  4. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    would a 2n7000 do the job or something a bit bigger ??

    or have you got a scamatic for a cmos driver ?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A 2N7000 is a single MOSFET. You would need eight of them, plus eight resistors from the outputs of the 4015 to their gates to keep them from "ringing" when turned on/off.

    A ULN2804 is a single 18-pin IC that contains 8 Darlington drivers and base resistors. One part replacing 16 individual parts.

    You need to give us some more information:
    1) What is the power supply that you are planning on using?
    2) What is the typical Vf @ current rating for the LEDs that you are planning on using?
     
  6. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    it is a 9volt 2 amp driver
    the fv of the led's are 3.6v
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ok, Vf is 3.6v, but at what current? 20mA? 30mA? 70mA?
     
  8. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    20ma per led
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, so see the attached. It's basically what you need to do.

    Not all of your existing schematic is shown, just some of the 4015 interconnects, and the connections to the ULN2804.

    Only two outputs of the ULN2804 are shown; the others need the same kind of connection.

    You'll need to supply the LED current limiting resistors right from your 9v supply, not via the 1N4148 diode.

    With a 100mA current sink load, the ULN2804 will drop about 0.8v. You say your LEDs are rated for a typical Vf of 3.6v @ 20mA, so two in series is 7.2v
    Rlimit >= (Vsupply - (VfLED + VdropDriver)) / 20mA
    Rlimit >= (9v - (7.2v + 0.8v) / 0.02 A
    Rlimit >= 1/.02 = 50 Ohms. 51 Ohms is the closest standard value.
    Rlimit power requirement: 1v x .02 A = 0.02 Watts. Doubled the requirement to 0.04W. You could use any resistor rated for 1/25 Watt or higher.

    To keep your parts count low, you could use SIP resistors. Digikey has 6-pin SIPs in stock:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=770-61-R51-ND
     
  10. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    i have had a slight change of mind i'm still looking at the 4015 circuit but what i need to do is to drive a set of 2 led's x8 light chaser actualy all i want now is to make some lights like taxiway lights on a runway
    as me and the wife to be love aeroplanes and i want a chaser light set for my garden path to welcome her home after the wedding
    thanks all
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you want to emulate taxiway lights, you could use the 4015 circuit, but a 4017 Johnson counter would make things easier - and the lights would always strobe in the same sequence.

    Cheap super-bright LEDs like you might find on E-bay won't work very well. They have a very narrow focus, and will appear very dim unless you are looking at them within about 25° of their centerline. You would either need to use some kind of conical reflector for each LED, or wide-angle LEDs like a Phillips LumiLED.
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    I covered that in Chapter 9, Light Chasers in my article, where I mentioned using it for runway lights.

    How long do you want the runway to be in terms of LED count?
     
    Last edited: Oct 10, 2009
  13. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    i want led's 1/4 watt 100ma each x 8 and obviously the same for the other side.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the Vf at 100mA?
     
  15. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    3.2 to 3.6 fv
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Here is how I would do it...

    [​IMG]

    The LM317 will need heat sinked. The resistor R2 can use a ¼W resistor, and you can tweak it by using parallel resistors.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill,
    He's using a 9v supply. There won't be enough headroom for the LM317.
    With an average Vf of 3.4v @ 100mA, that leaves 2.2v. The LM317 needs 3v+ headroom when used in current limiting mode.

    The 4017 will be hard-pressed to source enough current to reliably saturate a single BJT with Ic=100mA, as by the book 10mA base current will be required. However, according to ONsemi's datasheet for the PN2222, if base current is >=3mA, Vce should be about 0.1v with Ic=150mA. If the base resistors were reduced to about 2.7k, that could be enough. Of course, this means that PN2222/2N2222 transistors will be required. 2N3904 or 2N4401 would require more base current.

    So that leaves about 2.1v to drop. 22 Ohms is a standard value. 1/2 Watt should be a sufficient power rating for the resistor, replacing the LM317.

    Marko, you should still use the LM317 circuit to test the Vf of your LEDs. Most of them should fall about midrange (3.4v), but you'll get some that will have higher and lower Vf's. You can use a low Vf LED in series with a higher Vf LED to wind up with the same average voltage drop.
     
    Last edited: Oct 11, 2009
  18. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    i will use a 12 volt supply for this circuit bill has chosen for me.
    what do you recomend for Q1 to Q8
     
  19. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, so use Bill's circuit as-is.
    PN2222/2N2222.

    You could use 2N7000 MOSFETs, but they will be sensitive to static.

    You could also use the ULN2804, which would replace the eight resistors and Q1 through Q8.
     
    Last edited: Oct 11, 2009
  20. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
    0
    so i'm going to make bill's circuit just one more thing where the hell can you buy a 12.5K OHM resistors from i have looked on ebay farnell and most of the usual places
     
Loading...