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Problem switching a 5V mini relay from a low power LED (1,2v 2 mA) input

Discussion in 'Electronic Design' started by [email protected], May 27, 2007.

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  1. Guest

    I am trying to make a 5V mini relay switch when the HDMI indicator
    in my DVD player lights up.

    I have unsuccesfully tried to trigger the mini relay (5V, 40mA) via
    optocoupler (4N33 - infrared/darlington) "piggybagged" on the
    indicator LED (1,2V 2mA), but the relay doesn't switch, only "hums"
    slightly. Supplying an input of 1,5V from before the indicator LED
    resistor (220ohm) generates the same result. I even tried using the
    (normally unused) basis of the optocoupler as input, hoping it was
    more "sensitive" - but still only "humming"

    The setup works when "piggybacking" a standard 3V LED (>10mA), where
    the relay swithes fine. What have I overlooked ?

    Any suggestions for (simple) curcuitry I could use to trigger the 5V
    relay from the 1,5V 2 mA input (I only have 5V supply power
    available) ?
  2. LEDs in parallel do not share current equally.
    1.2V is enough to open a si transistor.
    Do you need to be opto-isolated?
  3. Numbers, numbers!
    2mA<< 40mA
    1.5V<< 5V
    Do something about those inequalities, may start working.

  4. Guest

    Hi Jan,

    The solution does not need to be opto isolated. I just thought it had
    the most sensitive input, and I didn't have to worry about any
    "feedback" currents.

    I think I need another alternative; anything simple that can make a
    5V, 40mA relay switch from 1,2V 2mA.

    Stanislaw: I don't understand your point ?

  5. mpm

    mpm Guest

    A comparator that has enough drive on the output?
    May not be the simplest solution, however.
    Plus, this suggestion comes well into a Memorial Day 6-pack...

    I can't remember the part#, but it was from TI.
    LMC7215? (Or whatever your favorite is.)
  6. Maybe it's not getting DC, but rather some short duty cycle pulses.
    You could try using a CMOS 555 as a monostable and drive the relay
    with that. Pull pin 2 low with the optotransistor.

    Best regards,
    Spehro Pefhany
  7. ehsjr

    ehsjr Guest

    Jan's suggestion would be something like this:

    +5 ------------------+------+
    | |
    [Relay] [D1]
    | |a
    | |
    | /c
    +---[220R]---| NPN
    | \e
    [220R] |
    | |
    [HDMI LED] |
    | |
    Gnd ---+-------------+

    Most any NPN you have on hand will work.

  8. Hi, I understand you _do_ have a separate 5V supply available?

    There are several ways, this oen for example using your current setup:

    | |----------
    | k |
    R diode [ \ ] relay coil
    10 | a |
    | |----------
    --- c ----------- |
    opto coupler --- b c
    --- e --------------- b NPN

    The resistor is for current limit.

    An other cool way would be to siply use a LDR (CdS cell - light dependent
    resistor) glued to the LED.
    It perhaps will be low enough R to switch the relay when the LED is on.

    If you have 5V DC on the set itself, try to fin what drives the LED, likely
    a 5 or 3 V output from a micro controller or some chip, and use that voltage
    to drive a transistor via resistor, or via a FET.

    c ---- as above.

    --- 1k ------- b

  9. Right :)
    One of them.
    Problem why I did not publish that is that often the LED is on the +
    and pulled down via a resistor...
    Then we should use a PNP :)

  10. You are so smart, yes I missed a clue 'buzzing'...

    Maybe just capacitor parallel to the relay would work too?
  11. Guest

    Any easy way to determine if the LED is being "pulsed", before trying
    something else ?

  12. Sure, make room dark, move head while looking in direction of LED.
    If you see many dots it is pulsed.
    Capacitor will smooth it.
  13. Guest

    How many uF ? Don't want it to hang open forever ...
  14. Jamie

    Jamie Guest

    stick a cap across it and see if it gets brighter.
    100 uf sounds good.
  15. Guest

    Indicator LED was indeed pulsed (which also explains the low power
    100 uF capacitor in parallel with relay solved problem

    Thanks all,
  16. Jasen

    Jasen Guest

    hook it to an oscilloscope, or through a capacitor to headphones, or
    through a capacitor to an AC voltmeter ...
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