Help with LED driver circuit

Discussion in 'The Projects Forum' started by TSOMB, Apr 14, 2015.

  1. TSOMB

    Thread Starter New Member

    Jul 31, 2014
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    Hi, I am having an issue with a circuit I would like to try. Its for a flashlight and enabling modes.

    So I have the buck step down driver which uses the IC ax2002. I would like to cut the enable pin which takes a positive input and somehow use a switched negative output from a 3 mode driver.

    So the EN pin on the ax2002 is connected to the + with a 10k resistor. The board I have for the modes modulates the - side. I need to somehow turn the - into a positive.

    Now I was thinking of a transistor, but im not sure what part is available (if any) to take a neg- input and send out a positive output to control the enable pin on the second board.

    Also, the EN pin input is sensitive enough that even touching it and a + lead will start the IC. in case this matters..

    Any suggestions?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  3. TSOMB

    Thread Starter New Member

    Jul 31, 2014
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    I dont have a circuit yet as I dont know what part would be suitable. I need to take a negative 12v modulation signal and convert it to a positive to drive pin #2 (enable). The boards are china made and I dont know
    what the chip is on the 2nd 3-mode PCB as the info is sanded off. I more or less need a starting point so
    I can experiment with it.
     
  4. TSOMB

    Thread Starter New Member

    Jul 31, 2014
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    I am overwhelmed with all these responses. Please slow down guys o_O
     
  5. #12

    Expert

    Nov 30, 2010
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    The LMC7660 chip can invert a supply voltage, but its high limit is 10 volts. Can you suppress any voltage more negative than about 9 volts so this chip can invert it to positive?

    If we could see more of your basic plan, we might be able to use a negative voltage to cause a transistor to send a positive voltage. Right now, we are lacking so much information that it's all guessing, and we don't like guessing.
     
  6. #12

    Expert

    Nov 30, 2010
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    Don't whine. People all over the planet help here, and some of them have day jobs. Besides, we don't get paid to do this.

    Let's try this approach: I'm rather good at doing level shifting and logic inversions, but I need some clues.

    The power supplies are x voltages and are capable of y amount of current.
    The 3 mode device has 3 output states,
    The first one has this many volts and that many amps.
    The second mode has this many volts and that many amps.
    The third mode has this many volts and another amount of impedance.
    When this output happens, I want this other thing to happen.

    Do you have any clues for me?
     
    Last edited: Apr 14, 2015
  7. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    21
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    I have a 3mode PCB shown on the right here in this pic:

    [​IMG]

    The negative is the modulated side, the positive goes straight through, but is required or the 3mode board does not work, I tried removing it and the board no longer works.

    Then I have this type driver board with the AX2002 IC. Pin #2 is the EN pin that requires a positive input from the rail and the chip turns on. Remove the positive and the chip shuts off...
    [​IMG]

    So I need to take the negative signal from the the 3mode PCB and convert it to positive like you could with a relay, but not by a mechanical switch / relay. Im looking for something faster. One of the modes is approx 20khz. I tried placing the 3mode output neg- to feed the r-set, and it worked, but not for the 3rd mode. The driver came on, and worked with the slower 'strobe' mode, but the 20khz mode was lost. This is when I figured that the enable pin would be the way to go, but the the output signal from the 3mode PCB is negative.

    My first thought was to use a transistor, but im not sure where to start, or at least what type to start with. I would like to keep it simple and am, not worried about efficiency etc. Also, the small 3mode board is china made, the numbers sanded off the IC. All im sure of is its capable of handleing 1-1.5A to the load (which is not needed here) and may not be rated for any higher than 9v. The EN pin on the ax2002 triggers at voltage down to 2.5v if im not mistaken.

    Thanks for the help, I was just being a smart ass in the other post. No offense intended.
     
  8. #12

    Expert

    Nov 30, 2010
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    I'm going to call this adapter circuit, "the inverting driver".
    Fact: Enable needs a voltage from +2.5V to Vcc at 20 ua minimum.
    What voltage are you really using for the maximum positive supply?
    What voltage are you really using for the negative supply?
    Do you want to remove the 10K resistor from Vcc to the enable pin and use the inverting driver that will obey the negative command from the 3 mode chip (and change it to send a positive voltage to the enable pin)? or, are you wanting to leave the 10K resistor in place and merely defeat the negative signal so it doesn't disable the AX2002?
    Is the 3 mode driver always connected to the enable pin?
    How about just put a diode in the 3 mode command line and let the 10K pull the enable pin high, as usual, because the 3 mode driver can't send negative to the enable pin with a diode in the way?

    Edit: no. A diode would wreck the 20KHz signal. Right? Or is the 20KHz signal in a different place from the enable pin?
     
    Last edited: Apr 15, 2015
  9. #12

    Expert

    Nov 30, 2010
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    Please declare the purposes of the wires in the photos by color and photo.
    What's going on where the red wire and the white wire sneak under something silver? Just a connection?

    Edit: I am obviously having difficulty integrating all this information. That's what all the questions are for.
     
    Last edited: Apr 15, 2015
  10. #12

    Expert

    Nov 30, 2010
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    Here is an idea. Very little confidence.
     
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  11. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    21
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    My Bad... I should have described the pics. Lets start with the first pic:
    The board on the left is a boost driver and has nothing to do with this. The leads going to the
    smaller board is white neg- side to the modulation input. The red pos+ is needed but just passes through
    the board. The white and red leaving the small PCB is the output and the silver thing is an LED. This smaller board just modulates and doesnt control current or voltage.
    [​IMG]

    So the idea is to tie the output of the small PCB into the EN pin of the 2nd larger board below. The input for this PCB is on the underside of the PCB and rated 3.6V ~ 18vdc. The small black and red lead on the top of that board is the regulated outputs for a high current LED.
    [​IMG]

    So what im trying to do is change nothing aside from cutting the enable pin from the pos+ and interrupt it with the modulated signal.

    I had a view of your diagram, now bare with me, im still green at electronics so im learning as I go. From what I see for your circuit, I will need a NPN and PNP, is this correct? Would a 2n3904 and 2n3906 be suitable for this?

    Regarding the 10k resistor, that would still be needed between the new modulated + source and the EN pin. So same config, just interrupted with the modulated signal...

    Thanks again! Its starting to look possible lol.
     
  12. #12

    Expert

    Nov 30, 2010
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    The npn is what I am assuming is inside the 3 mode controller.
    The pnp converts a low output from the 3 mode controller to a high positive voltage limited by a 10k resistor.

    Any P.O.S transistor will work, including a 2N3906
    I think I better move a resistor.
     
    Last edited: Apr 15, 2015
  13. #12

    Expert

    Nov 30, 2010
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    From left to right;
    The first resistor limits the base drive of the inverter transistor to your maximum voltage range divided by 10,000. Maybe that's about 1 ma to 2 ma.
    The second resistor turns the transistor off when not in use and wastes about 60 microamps when the inverter is "on".
    The third resistor grounds the enable pin when the inverter is not on, but I have a lot of doubts there. Do you have a ground voltage? If you don't, that resistor will have its lower end at the -12V you mentioned. Is that voltage safe for the enable pin? I don't think so. Is the resistor needed? I think so.
     
    Last edited: Apr 15, 2015
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  14. #12

    Expert

    Nov 30, 2010
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    I really need your voltages!
     
  15. TSOMB

    Thread Starter New Member

    Jul 31, 2014
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    The power supply is 3 li-ion cells in series so 12.75vdc at highest. Im thinking a few diodes to lower the voltage into the 3mode board on the neg- side. The 3mode board runs on the output of a boost driver meant to run between 5-9v so I have to test and see what the upper limit is.

    So the the transistor on the left in your drawing is an assumed part already built in? I just need a PNP? Just want to be sure im not missing anything and will go out in the morning to get transistors, only have npn on hand. Which transistor would you recommend? Im not aware of all the differences and dont want to use the 2n3906 if its a crappy choice lol
     
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  16. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    Hello,
    soon i will be dealing in Digital RTC LED clock cam we use driver for it?
    to make it compact and less weight.!
     
  17. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    21
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    Not sure if its suitable outside being a decent switching regulator for a single LED:D
     
  18. #12

    Expert

    Nov 30, 2010
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    I wouldn't run right out and spend money. Only when I know the voltages can I fine tune this. You will spend more money on gasoline than on parts. I see what? 12.75 volts positive and the batteries slowly fade. What negative voltage? Any at all? Is it just one voltage and what we call, "common"? How about a drawing? Photograph it if you don't have the slick tools I have.

    Repeating, any Piece of S#!7 transistor will do. This is not a sophisticated, high gain, wide bandwidth, low distortion amplifier. It's a switch; a low voltage, low frequency, switch. Any small pnp you have, a 2N3906, or the re-marked rejects that Radio Shack sells. I can't remember a small transistor that won't work.

    and now is 4 AM here. Time for bed!
    ps, Please state what country you are in.
     
  19. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    21
    2
    On hand I only have a small assortment of parts. Just so happens pnp are not amongst them. Will have to grab some anyway. I made a quick sketch of what im thinking. I hope this contains everything I wasnt clear on or left out. The voltage will drop to no less than 7.5v before the batteries need to be charged, so the lowest will be that and fully charged @12.75v. All just a basic LED driver and an attempt to make a slight change. I didnt include your circuit in this just to keep clear on the idea and then go from there. Thought it would save any confusion.

    I was also thinking to drop the voltage before the mode PCB, that using a voltage reg like a 7805 or lm317 would be the better route, but illustrated diodes in series pending advice on additional elements.
    DSC02141.JPG

    6:30am here, time for my 8hr clarification zzzZZZ

    Oh and im in Canada eah ;)
     
  20. #12

    Expert

    Nov 30, 2010
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    The AX2002 is safe up to 25 volts. Please do not cripple the power voltage to other chips with a bunch of diodes so the AX2002 can send them a voltage high enough to damage them.

    There is NO negative 12 volts! Why am I designing for negative 12 volts?
    Because that's what you said in post #3, "I need to take a negative 12V modulation signal and convert it to a positive".
    That's why schematics were invented. Words are a miserable way to describe circuits. (That's your lesson for today.)

    OK. This being a flashlight circuit which uses WAY more power than any switching logic, I am going to design more for noise immunity than microscopic power savings. Oh. I just did. Rearrange a couple of grounds and we're good to go.
     
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