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Can I use a relay to wire DC lights to a speaker?

  1. Jun 15, 2017 #1
    I want to wire some LED strips to the bass speakers in my car so that they will light up to the music. I reason that you cant wire the lights in parallel with the speaker because the lights can blow with too much volume. I also read that it will trash the sound quality. ?

    Anyway, my plan is to use a relay on the positive side of the speaker to be a switch for the lights. This way, when the speakers get power, the switch in the relay will close, and the lights can get power as a separate system, that way sound quality is unaffected. Does this plan sound good? Am I overlooking something?


    Sidenote considerations, only if speakers are AC:
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    So when the speaker plays, the current will be alternating very fast correct? Now if we analyze one sinusoidal cycle of the AC current, we can see that there are times when there is instantaneously no current. In theory, assuming an ideal relay with instantaneous response, this will cause the relay switch to be open when there is 'no' current. But for an actual relay, there will still be this rapid on/off, but it should be offset by a minuscule amount of time. However, the rate should still be the same. Now the human eye cannot interpret changes much faster than somewhere around 60 or 100Hz, so, (unless im using like 20Hz speakers) the lights should appear to be constant with the bass speakers.
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  2. jcsd
  3. Jun 15, 2017 #2

    berkeman

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    I would just buy an aftermarket LED unit that has a high-impedance input and gets its power from the 12V supply in the car. I'm sure somebody makes them or the amp/driver part...
     
  4. Jun 15, 2017 #3
    Yeah the LED unit is ready-to-go with appropriate resistors everywhere. It will also run as a separate system on a 12V switch panel that I already have installed, but it will also be just running through the relay that is connected to the speaker, that way, it is only controlled by the speaker, but does not take any power from it at all.

    Im entirely clueless as to what you mean by the amp/driver part lol. I plan on wiring the relay to just wire just before the physical speaker, like where the noise comes from (i think thats called a driver)
     
  5. Jun 15, 2017 #4

    berkeman

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    Can you post a picture of your unit, and maybe a datasheet and a sketch of how you want to hook it up.

    By the amp/driver part, I mean something like this, which has a high-impedance audio input jack and power input, and drives the LEDs in response to the music...(the power input is shown as a battery on the right side of the schematic, but you could have a 12V input from the car's power system instead)

    http://www.eleccircuit.com/wp-content/uploads/2010/11/led-light-based-music-circuit-with-lm3914.jpg
    led-light-based-music-circuit-with-lm3914.jpg
     
  6. Jun 15, 2017 #5
    Yikes, that diagram is confusing! I take it Q1 is a relay-type of switch, D1 is a diode to make current go to the lights only on one direction of the AC current? No clue what the capacitors would be for o_O

    Sure! This is what I mean


    15dudz4.jpg
     
  7. Jun 15, 2017 #6

    Janus

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    Q1 is a transistor. It can act somewhat like a current controlled switch, but also as an amplifier where a small signal can be used to generate a stronger version of the signal. This is what it is be used for here. The "output" of the transistor is applied to pin 5 of the LM3914 IC, which can take a signal input and light the LED's in order depending on the strength of the signal.
    The transistor set up allows for the poteniometer (VR1) to be used to adjust the input level to the IC, while maintaining a high impedance input. C2 is there to isolate the bias voltage for the transistor from the input. C1 most likely acts as a filter. D1 is part of the biasing circuit for the transistor.
     
  8. Jun 15, 2017 #7
    Oh okay. So anyway, I think my diagram is good. Is there anything that you may notice that im overlooking that could raise concern to safety or sound quality
     
  9. Jun 15, 2017 #8

    berkeman

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    No, sorry. It makes no sense so far. It looks like your switches ground out the car battery through the cigarette lighter plug...

    Do you have any friends who work with hobby electronics that could help you locally? That may be the quickest way for you to get something working...
     
  10. Jun 15, 2017 #9

    jim hardy

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  11. Jun 15, 2017 #10
    Oh, i think my diagram is just wrong then. I already wired up two light strips in parallel with eachother to one of the switches on the switch panel and they've been working great for months. So just ignore the switch panel

    So the new diagram would be like this. If the left works (by magic), should the right work? (by analogous magic)
    So actually I guess my question is, can a wire from a speaker successfully open and close the relay switch?
    Sorry for the confusion:oldbiggrin:

    2eq9axl.jpg
     
  12. Jun 16, 2017 #11

    sophiecentaur

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    You can buy 'sound to light' units very cheaply. This may be the cheapest solution if you don't have all the kit needed for DIY electronics. (?)
     
  13. Jun 16, 2017 #12

    jim hardy

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    You seem very unfamiliar with basic electricity. Have you checked any car audio forums ?

    You have not said what voltage you expect across your speakers and at what voltage you want the lights to illuminate.
    This being a physics forum here's the very basics.

    Your speakers are probably two or four ohms 'impedance", let's call that "R" .
    Voltage is usually represented by letter "E", and Power by P .

    The formula for power is P = E^2 / R .

    So 100 watts of power into two ohms of speakers would be
    P = E^2 / R
    100 = E^2 /2
    E^2 = 200, so E = √200 = 14.14 volts . Since they're bass speakers it's probably below 400 hz so an ordinary AC relay would make them flash A 12 volt AC relay would probably work if you have a 100 watt amp and 2 ohm speakers. But it'd only flash on the real loud passages.

    Similarly if your speakers are 4 ohm,
    by same formula
    100 watts into four ohms is 20 volts. A 12 or 24 volt relay would flash them. But you could burn out a 12 volt one if your neighbors don't call the cops first...

    So the answer to your question is "Yes" .
    Not with any degree of elegance,
    but it's a good first step at learning.
    .
     
    Last edited: Jun 16, 2017
  14. Jun 16, 2017 #13

    jim hardy

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    If it's just light you're after,
    a plain old 1156 turn signal bulb wired right across each speaker might work as well.
    You can buy sockets at any auto parts store, even my local Wal-Mart has them in lighting accessories..
     
  15. Jun 16, 2017 #14
    As @jim hardy said, a relay in parallel may kinda-sorta work, but it will mess up sound quality. Amplifier output intended to vibrate the speaker cone is diverted to powering the relay coil instead, and this thins out the sound. Not only that, but the relay itself will be noisy, and act like a variable frequency door buzzer.

    Wiring the relay coil in series with the speaker as you've drawn boils down to the same types of audio problems, it just gets there somewhat differently.

    Did a quick search, and found Velleman makes an inexpensive kit (part # MK103) that may do what you want by wiring it to your LED light bar instead of the supplied LEDs.
     
  16. Jun 16, 2017 #15

    jim hardy

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    Great Find! Only five bucks !!
    It'd probably drive a relay for most anything he wanted to light up.
     
    Last edited: Jun 16, 2017
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