Voltage Trigger.

Discussion in 'The Projects Forum' started by mystamo, Aug 7, 2013.

  1. mystamo

    Thread Starter Member

    Jun 6, 2009
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    Hey guys.

    I'm using a Current Sensor IC to detect the current going to an IR LED. I'm making a board that can light up 4 blue led's while the IR LED is functioning correctly and the blue LED's should turn off when the IR fails.

    The Current Sensor IC has an output that is at 3.3v when the IR LED is not functioning. The Current Sensor IC output gets down to 1.3v once I connect the IR LED and it's functioning.

    What I am trying to do:

    Make a circuit after the Current sense IC that turns the Blue LED's on or off depending on the voltage seen on the output of the Current Sense IC.

    So at 1.3v I should see 4 LED's on
    at 3.3v I should see the lights turn off.

    Anyone know of a nifty circuit that can make that happen? I figure I can do it on a micro controller. But I need to build 200 of these and programming them would not be an option. I don't have very much room on the board to make an ICSP header.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    A comparator would work fine. Give it a reference on one input of, say, 2V. If the voltage goes past the reference, the output status changes.

    You could even rig a single transistor to switch on when the base voltage rises more than 3 diode drops (at ~2.1V total).
     
  3. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    Sorry would you be so kind to elaborate with a circuit?

    So I've got one pin coming off the current sensor.. I have GND and 3.3v available.

    I like the idea of a single transistor. But how exactly do I achieve that?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I can't figure out how to turn the LEDs off with just one transistor. Turning them on, yes, but not off.

    Anyway, please excuse the crudeness of the drawing - I'm just getting started with Eagle.

    The idea is that as long as the voltage on the 2 leftmost diodes is less than ~2V, the left transistor will be off and the right transistor will be held on by the resistor to Vcc, lighting the LED(s), the rightmost diode. If the voltage rises on the left, the left transistor pulls the base of the righthand transistor to ground and shuts it off, shutting off the light.

    From left to right, resistor R1 might be a 10kΩ, R2 a 1kΩ or less and R3 is the current-limiting resistor for the LED.

    [​IMG]
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Here's an idea. What is the LED forward voltage and required current ?

    3.3 v PNP LED driver.png

    EDIT: You really don't need R2 if the high voltage is close to 3.3v
     
    Last edited: Aug 7, 2013
  6. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    wayneh,

    Your solution is elegant and fantastic. SOLD!

    Thanks for your explanation. Really useful circuit for inverting logic as well I can see.

    Thanks to everyone.

    Edit: If you have any questions with regards to eagle. Shoot me a PM. It's all I use exclusively here at my office.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Tubeguy's solution is comparable and eliminates a transistor by using a PNP instead of 2 NPNs. I tend to always think in terms of NPN unless there is no other way. Silly, but that's me. If you have a PNP, it's the more elegant choice.

    As for Eagle, I just have to get up the learning curve. I don't even know what to ask. Everything I try to do is so frustrating because I know how to do it so quickly in the programs I'm used to, which are drawing programs, not dedicated circuit design apps. But I suppose I'll eventually learn to get around.

    Oh, here's one question, where do I find a ground symbol? There was nothing in a search of the library for "ground".
     
  8. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    Lol. It's funny that you should ask that. I jumped into eagle with a lot of previous schematics done before hand for me.

    I use the cut and paste tool to take entire parts of circuits that I need from one place to the other.. My ground connections were all taken from previous schematics.. Aka I don't have your answer.. Lol

    Sorry.

    As for the PNP vs NPN.. I too am an NPN man. N-Fets also... PNP's just seem illogical to me. :)
     
  9. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    So I'm back... Lol

    I tried both those circuits and none of them work. I think maybe it's because the changing 3.3v signal I have coming off the current sense IC carries little to no current. Can I still be switching a transistor with such low currents? Perhaps I could make use of a logic level FET?

    Mo
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    What is the part number of the transistor you used for testing the circuit?
     
  11. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    BD680 as my PNP..No go

    2N3704 as my NPN .. No love

    Mo
     
  12. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    EDIT: Double Post.. My bad.
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    Just realized this. You said blue LED's correct?
    Some of those need 3.1 to 3.2 volts to turn on.
    Transistors drop some voltage even when saturated.
    Do you have any 2 volt LED's (red, green) ?

    Edit: the PNP is a darlington = too much drop...

    Try Wayneh's idea with some lower volt LED's. :)
     
    Last edited: Aug 13, 2013
  14. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    Been testing it out with green LED's anyways.

    I just tried some darlington transistors as well. Still not getting results.
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Can you post a drawing of your exact circuit, part values, and the part number of the current sensing IC?
    I wasn't very clear, darlington's likely won't work with only 3.3 volt supply.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    +1
    And a regular transistor may not work either if you don't have enough current to turn it on.
     
  17. MrChips

    Moderator

    Oct 2, 2009
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    You cannot just go swapping NPN and PNP transistors willy nilly. Transistors don't work like that.
    You must post your circuit diagram and specify the transistor number you plan to use.
     
  18. mystamo

    Thread Starter Member

    Jun 6, 2009
    12
    0
    Gents,

    I used the two circuits provided to me by the members of this form.

    I used NPN transistors for the circuit wayneh suggested

    and used PNP for the circuit tubeguy suggested..

    I supplied the Transistors I used in each circuit in my previous post.

    I'm not sure at what point in came across that I was just switching transistors "Willy nilly" I simply swapped out the ones I was attempting to use for a darlington alternatives. PNP and NPN respectively per circuit.

    I was also aware that current and voltage levels could be the reason I'm not able to turn the transistors on/off. I found some fets on digikey that have 3v logic gates. Going to see what I can muster up with those.

    Mo
     
  19. MrChips

    Moderator

    Oct 2, 2009
    12,414
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    Sorry Mo. I did not see in your previous posts you said that you were testing two different circuits, one with NPN and a second with PNP transistor.
     
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