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replace LED with relay that activates large LED panel

Discussion in 'Electronic Design' started by Fernando Cassia, Aug 6, 2013.

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  1. Hi there,

    I have a circuit that, under some circumstances, lights a LED.
    The problem is that the LED is too small and can only be seen while in close proximity of the circuit.

    What I'd like to do, is keep the LED in place, but add some additional circuitry after the LED so the original LED keeps functioning, but when it detects the led has voltage applied to it, drive a relay that in turn turns on/off the 12v that powers a large LED panel like this one:

    What would be the preferred/lower cost design to achieve this?

  2. Guest

    You could turn on a scr from the original LED. And have the scr ground oneside of the large LED panel. You would want to have a resistor in the line from the original LED to the gate of the scr, or else the scr will drop the woltage enough that the original LED will no longer be lit. This will not turn the large panel off when the original LED goes off. But you couldput a switch in series with the supply to the large panel and reset the scr manually.

  3. Martin Brown

    Martin Brown Guest

    Do you have something against using transistors?
  4. If the voltage drop above the LEDs series resistor is greater than 1.5V you could
    replace the original resistor with the diode of an optocoupler and a smaller resistor.
    The transistor of the optocoupler then drives a relay or a transistor with sufficient power.
  5. First thing to find the GND of the supply that powers the LED as you will
    need that to be connected to your external 12V supply.
    Then you need to know the voltage of both sides of the LED in on as well in
    off state.
    Depending on the outcome you will likely need a transistor, a resistor, a
    diode and the relay.

    I do not advise an SCR as it can easily be turned on but is difficult to
    turn off.

    Off course, you need to know the current required by the LED panel to choose
    the relay.
    In turn the relay will decide what transistor to be used.
    This and the voltage(s) measured from the LED define the resistor.
    The diode can be about everery general purpose SI diode.

    petrus bitbyter
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