LED Current Indicator for Speaker Wire

Discussion in 'General Electronics Chat' started by ljmccon, Nov 28, 2015.

  1. ljmccon

    Thread Starter New Member

    Nov 27, 2015
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    My background: My knowledge of circuits is extremely basic. I aced physics in college several years ago so that I could get into med school – then promptly dropped out of med school and forgot everything I learned. :) So, you’re gonna have to talk to me like I’m five.

    The situation: I have an A/V receiver that supports an ‘A’ set of speakers (which are in my living room) and a ‘B’ set of speakers (which are outside on the patio). My receiver is hidden away in a closet, and it allows audio to output to A only, B only, or A and B simultaneously. So, just because the speakers are on inside doesn’t necessarily mean they’re not on outside, too (which, if I’m cranking the volume, is an unwitting nuisance to my neighbors). I need to be able, at a glance, to determine whether the speakers are on out on the patio without having to get up, open the door, and stick my head outside – or make a trip into my closet where the receiver is located. (I know, I’m lazy.)

    What I want to do: I want to install an LED (into a wallplate in the living room) that will illuminate when the outside B speakers are on.

    What I’ve tried: I found some LED current indicators that I thought might do the trick. I bought this one and this one (0.75A turn-on), strung my speaker wire through them, and tested them out. Long story short, neither worked very well. The NK Technologies inductor has a lower turn-on point, but it’s aperture is smaller, so I could only get my 12-gauge speaker wire looped through 3 times. The CR Magnetics inductor has a higher turn-on point, but it has a larger aperture; so I was able to loop the speaker wire through 5 times. The results were about the same. At high volumes, the LEDs worked pretty well (but still weren’t terribly bright) – but turn the volume down, and the LEDs stopped working. I need the LED to be solidly illuminated regardless of the volume.

    My questions:

    1. Am I on the right track using an inductor/LED to accomplish what I want to do, here? Any easier way of solving my problem (that of needing to be aware when my outside speakers are on)? I’m open to suggestions.

    2. Is there any way to make the indicators I have already bought work in this application? (e.g. Could I use a higher-gauge speaker wire, wrap it several times around the inductor, and somehow wire it in parallel to the thicker speaker wire feeding my outdoor speakers?) Any other ideas for modifying what I’ve already bought? I’m not opposed to some modification and light soldering if someone would be so kind as to provide instructions.

    3. How difficult would it be to build my own inductor/LED current indicator with a much lower turn-on point (perhaps 0.05A)? (Remember, my knowledge is way basic – but I’m handy, I’m a quick learner, and I can follow instructions; so don’t be afraid to suggest a project if you think I can handle it.)

    4. General question about inductors: does the inductor attenuate the current running through the wire around which it sits? If so, I can’t imagine that would be good for sound quality -- so is it a bad idea to be using an inductor on speaker wire? Or is the effect so small as to not be noticed?​

    Some (perhaps) relevant info about my receiver and speakers:

    Receiver:
    Pioneer VSX-1130 (100 W in 2ch; likely much less in all channels driven)
    Speakers: Polk RC85i and Polk RC80i (8 ohm)​
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd feed one of the B outputs to a comparator and have it drive an LED.
     
  3. ljmccon

    Thread Starter New Member

    Nov 27, 2015
    27
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    Thanks for the suggestion, Dennis! I'm not familiar with comparators, but it looks like something I'd have to build from the ground up. Is this something a complete circuit amateur like myself could handle?
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Shouldn't be very difficult. LM393, a resistor, LED, decoupling cap, socket, perfboard, wire, and a power supply.

    Read page 10 of the attached datasheet.
     
  5. ljmccon

    Thread Starter New Member

    Nov 27, 2015
    27
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    Thanks for the info. I see a couple problems with going this route:
    1. It's over my head (did I mention I forgot everything I learned in Physics? :D) -- I might be able to figure it out if I sat down and studied some basic circuit principles for a while, but that may not be worth it because...
    2. This would require its own power supply. Considering it will be buried in my wall, I probably won't know the battery's dead until my unhappy neighbor comes knocking on my door, again.
    For those reasons, I'm thinking the self-powered inductor/LED would be the better option. Is it possible (for a noob like me) to build one that would detect a low enough current for my application? Or is it possible to use the ones I already bought in some way I haven't tried? Or is an inductor just a bad idea on speaker wire (and if so, why)?
     
  6. ljmccon

    Thread Starter New Member

    Nov 27, 2015
    27
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    Okay, given the lack of any other responses, I figured everyone must agree with the comparator idea. So, I sat down and read up on what it's all about. And I created a circuit powered by an old cell-phone charger -- and it works! Kinda...

    I have my reference voltage at half the supply voltage (so reference voltage = 2.5V), and I am drawing from the supply with a trimpot to provide a bias voltage to the inverting input...set juuuust below 2.5V. The voltage coming from my speaker is set with another pot to be in series with the bias voltage (w/ capacitor inline) to bump the total voltage into the inverting input up over the reference voltage if my speakers come on.

    But the problem I’m having is that I can’t get the LED to stay solidly lit at my desired low volumes. It flickers, but I want it to be ON. As I said, I’ve adjusted my trimpot (for bias voltage) to be as close as possible to the reference voltage without tripping the comparator. And I have my speaker pot turned all the way up, so the circuit is taking all the voltage my speakers are offering — still no dice.

    I used my voltometer to measure the voltage across my speaker terminals, and at the volumes I’m interested in, it fluctuates between 15-25mV. That seems shockingly low. Turning it up to the top of the range at which I’d normally listen, I get 200-300mV; and at the absolute max volume my receiver is capable of, I get 1-3V. My A/V receiver is a Yamaha HTR-5860 (110W per channel) — is it odd that the voltage is that low?

    So, big question: Any thoughts on how to get that LED to stay solidly lit at these low voltages? My goal is to have that LED on SOLID if ANY voltage is in the line of my outdoor speakers.

    (P.S. Dennis, thanks for setting me on the right path toward the comparator. :))
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Unless you have a sophisticated/expensive AC voltmeter it won't register music signals accurately. It will be sluggish and tend to give a 'sort of average' reading rather than a true RMS measurement.
    Post a schematic of how you've wired the comparator and bias arrangement.
     
    Exo-electrico likes this.
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Please post a schematic.
    Does seem low, but who knows what your voltmeter is measuring. The AC range may not have been intended to measure audio frequencies. Can't say for certain because you didn't give enough information.
    I'm thinking a retriggerable oneshot, but might think of something simpler after seeing your schematic.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Now that my personal day-long living hell of an inet outage is over, I mustn't agree...

    When you say you "strung your speaker wire" through the sensor, did you run both wires? An inductive current transformer like the ones you have will not work if you run the speaker *pair* through it because the current in the two wires cancel out each other. Pick one speaker to monitor, separate the two wires, and run only one of them through the sensor. If it doesn't blink on loud audio, loop the single wire through the sensor more times.

    And while technically this kind of sensor does extract some energy from the power going to the speaker (that LED power has to come from somewhere), and technically an LED is a non-linear load meaning it is introducing a tiny amount of distortion into the speaker signal, you can't hear the difference.

    As for your comparator circuit, adding the right capacitor in the right place will eliminate 95+% of the flickering. Schematic!!!

    ak
     
    Last edited: Dec 8, 2015
  10. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Maybe adding a capacitor and diode to the comparator input for a long peak hold will be sufficient but then it may need to be buffered.
     
  11. ljmccon

    Thread Starter New Member

    Nov 27, 2015
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    Thanks for all of the replies and great info. (Very interesting regarding music signals/less expensive voltmeters.) Schematic is below.

    (AnalogKid: With regard to the inductors, I only strung one speaker wire through the sensor as you suggested. I looped the wire around as many times as I could -- I could only get my fat 12-gauge wire through that little hole five times. And that still wasn't enough to make the LED light at lower volumes, and certainly not consistently. C'est la vie.)

    Speaker Comparator Circuit.jpg
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd try this:
    upload_2015-12-8_14-9-9.png
    Any signal should turn the LED on constantly.

    If you're going to crank up the volume, consider some input protection for the inverting input.
     
    cmartinez likes this.
  13. wayneh

    Expert

    Sep 9, 2010
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    Why not wire the LED indicator to the speaker B selector switch?
     
  14. ljmccon

    Thread Starter New Member

    Nov 27, 2015
    27
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    Dennis -- definitely liking the simplicity of your suggestion, but it's behaving randomly in two ways, neither of which is what I want:

    Behavior A: When I switch the B speakers on, the LED will light, but not on full -- it still pulsates. When I turn the B speakers off, the LED goes out as it should.
    Behavior B: Switching the B speakers off will actually cause the LED to turn on to full, solid brightness with no flicker -- turning the B speakers back on will cause the LED to dim and start fluttering, again.​

    The frustrating thing is, I cannot figure out what is determining this different behavior. If I sit there and switch the B speakers on and off repeatedly, sometimes I see Behavior A, sometimes I see Behavior B. It seems to be random. Any other ideas?

    Wayneh: the idea of opening and messing with my new (and under warranty) A/V receiver doesn't thrill me -- and I wouldn't know how to identify the switch even if I did open the case. Can I do what you suggest without permanently modifying anything inside?
     
  15. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Can you post a pic, or link to a spec sheet of your A/V receiver? There might be a workaround.
     
  16. ljmccon

    Thread Starter New Member

    Nov 27, 2015
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  17. dl324

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    Do you have a decoupling cap on the supply pins for the comparator? Comparators can oscillate when the two inputs are at about the same voltage or the input is changing slowly around the reference voltage. The usual way to deal with this is to add some hysteresis. That be as simple as putting a small cap between the inverting and non-inverting inputs. Do you have a few caps in the 22pF-100pF range you can try?

    Do you have this problem if you do the switching with the speaker volume higher.
     
  18. AnalogKid

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    Aug 1, 2013
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    Add an electrolytic capacitor from LED+ to GND. It will hold the LED+ pin low (on) when the 393 output is fluttering, but allow the LED to go off when the 393 stays off for a while. Size of cap determines the cutoff time delay period. Start with 470 uF and see if that is long enough.

    ak
     
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Pardon me, but I'm under the impression that that approach will only work when sound (current, voltage) is being pumped into the speakers. Isn't there some other feasible scenario to accomplish what the OP wants?
     
  20. ljmccon

    Thread Starter New Member

    Nov 27, 2015
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    Yes, I just tried it at max volume, and the LED still fluttered. That also fried my LM393 -- oops, good thing I got spares!

    I do not have any capacitors other than the 100v 0.1uF capacitors I bought for my first prototype. Happy to make a run to Fry's, though -- what should my shopping list be, you reckon?

    Also, I considered hysteresis, but knew it wouldn't work with my old prototype due to the bias voltage. My research indicated that you accomplish hysteresis by connecting the LM393 output to the non-inverting input and the 5V supply with resistors (Schmitt Trigger). You're saying I can accomplish the same thing with just a cap between the inverting and non-inverting pins?

    I'd prefer the LED go out nearly immediately when the B speakers are turned off (so that I know I can stop hitting the toggle button on my remote), but this is a good plan B. I can add it to my Fry's shopping list...what range of capacitance should I buy? (And what voltage rating?)
     
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