LED powered by phantom power supply

Discussion in 'The Projects Forum' started by computerlen, Nov 26, 2011.

  1. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    I was asked to modify a simple microphone xlr male to xlr female muting box for a customer at the store I work for. The customer asked for an LED to be installed, showing the state of the selector switch-mic on or muted. There is a DPDT footswitch on it where half the switch is made use of. Pins 2 and 3 of the XLR connectors are shorted together to mute the microphone. A phantom-powered microphone will not be used but the customer asked for the LED to be powered by the phantom power supply on his mixer. Is this feasible? Using the unused section of the footswitch's spst contacts and resistors in series with the LED, plus a resistor to cut down the 28 to 40 volt phantom supply, causes popping sounds when the switch is pressed. [of course!] Do you think that there is a way to do this without causing any noise in the microphone's mixer? Thank you for any help you can give. Len.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You might try adding a capacitor, maybe 0.1µF ceramic or such, across the pins to act as a high-frequency cuttoff. The cap will "short" any AC component to ground while having no effect on DC. This is a total guess on my part, but easy to try.
     
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  3. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    I was thinking of next trying a circuit that uses a transistor to drive the led instead of just using a series resistor before the led. But probably there is no way around the noise problem. It's really not a good idea from the beginning. The customer who wants this mod performed knows that he could simply use a battery to power the led but I suspect he doesn't want the bother of replacing the battery in due time. Len.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I don't see how adding a transistor would change anything. And it's reasonable to use the power already in the cable, if you can get away with it, instead of a battery. But any current surges in the cable may introduce noise in to the signal. An inductor in series might prevent rapid current changes, which lead to noise on the signal.
     
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  5. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    I was thinking that maybe a transistor being used as the driver of the led would cause less noise because its base, for example, would be the controlling device or switch to the led. And let's say that the transistor's circuit would somehow always be 'on' and acting as a load to the phantom supply's voltage source, therefore a very small voltage to its base would turn on the led and not cause such a loud noise when this happens. What do you think?
     
  6. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    What about a circuit that uses a transistor that continuously places a load on phantom supply with either a resistor or the led? I would then switch on the led or switch it off thereby switching to a resistor that takes the place of the led.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Well, again I think your problem is a transient and the challenge is to find the simplest way to eliminate that transient. Using a transistor might offer an advantage, because removing the transient from the base of the transistor involves less current than the same transient at "full" current. You might get away with a smaller capacitor. But I doubt the expense of the filtering capacitor (or whatever arrangement you need) is your first priority. Why not first try some simple, standard filters? Like a small inductor in series or a cap in parallel.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Use one of the new "High intensity" LEDs. I have some here rated for 6000 mCd that give a usable indicator brightness at 0.5mA, or even less.

    At such a low current and with a cap across the LED that should fix the popping sound.
     
  9. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    I was able to use the transistor idea and it worked out beautifully. I sent the idea in to the manufacturere of the muting pedal and they are considering it for production[I hope]. If you're interested in the wiring diagram, please write me at computerlen@hotmail.com. Thanks for all of the advice.Len.
     
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