Can an AC signal light an LED?

  1. I am wondering this because my car has very nice looking speakers when viewed in the light, however in the car they don't get much light and you hardly even notice. LEDs are cheap lights...so I was wondering if there is any way I can use these, maybe even run them off the sound signal itself?

    All I know is:
    A sound signal is a complex AC wave
    LEDs run off DC usually, I think.

    If my thoughts have strayed too far, please tell me any ideas you might have on how to get some light to my speakers!
     
  2. jcsd
  3. Astronuc

    Staff: Mentor

    I think most diodes have a more limiting reverse bias voltage, IIRC.

    See - http://en.wikipedia.org/wiki/Light-emitting_diode

    One could use a rectifier to convert the reverse bias to forward bias, with either half-wave or full-wave rectification.
     
  4. Right over my head. Where do I find a 'rectifier'...
     
  5. brewnog

    brewnog 2,791
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    King, I think you'd be much better off just taking power off something else. Depending on where your speakers are and whether or not you want to be able to switch them on and off, you should be able to find something close to your speakers which you can run a couple of wires off.
     
  6. Janus

    Janus 2,453
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    The simplest rectifier is just a diode (non light emitting type). This will get you half-wave rectification. The part of the AC wave that is of the the wrong polarity is simply blocked.

    With four diodes arranged properly you get a "Bridge rectifier" which "flips" the reverse polarity part of the wave to the correct polarity.

    you can get this packaged in a single semiconductor, which you can pick up at any electronics store that sells semiconductors. (You used to be able to get them at Radio Shack, but I'm not to sure about that anymore, since they have been shifting more and more to consumer electronics and away from electronics supply.
     
  7. Janus

    Janus 2,453
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    Back in the day when cars still came with them, and for those of us who didn't smoke, the cigarette lighter always made a good power source. It was even usually fused separately.
     
  8. brewnog

    brewnog 2,791
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    I run a little flashing LED circuit I built off my cigarette lighter socket. It makes it look like my car has an alarm, and deters the scallies from trying to nick it.
     
  9. Okay...so what will this do? Make a flashing light?
     
  10. chroot

    chroot 10,426
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    Most speaker systems have voltages around 60-100 V p-p rms -- measure yours and find out. Use an appropriate resistor divider to put the maximum signal into the 2-3 V (4-6 V p-p) range. Use somewhat large resistors (~1 kohm) to avoid loading your amplifier. Use an op-amp as a voltage follower to produce a low output-impedance "copy" of the signal, and use it to drive the LED (through an appropriate series current-limiting resistor, of course). As long as the LED's reverse breakdown voltage is higher than the 2-3 V negative swing of the signal, you'll have no problem. At audio frequencies, the turn-on time of the LED is not significant -- it'll have enough "bandwidth" to accurately follow the signal.

    - Warren
     
    Last edited: Jun 12, 2005
  11. So how much would this all cost?
     
  12. Adding any circuitry to the audio signal will add distortion though. Use your car's DC power system. Add a 610 to 750 ohm resistor in series with the LED and connect to the car power. Heck, tap into one of the wires used for your driving lights that way the LED's will only operate when you turn the car lights on.

    They make these plastic wire splicers that you just snap onto an existing power wire.

    http://www.radlites.com/images/splice_connector.jpg

    Splice one of the 12V wires and one of the ground wires and you'll be set.

    I'd avoid using the audio source as a power source.
     
  13. chroot

    chroot 10,426
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    Of course, avoid using the audio source if at all possible -- but I thought the idea was to have a light blink with the music? I'm confused.

    As far as the cost of the circuitry I mentioned in my previous post, maybe $1.00 in components. Op-amps are dirt cheap.

    - Warren
     
  14. At audio frequencies the light won't appear to be blinking--well for the most part that is. Anything greater than 30Hz will appear to not be flashing.

    There are a lot of ckts on the net that one can build using a cheap microphone and a bandpass filter to make a blinking ckt. Essentially the bandpass is tuned to a frequency and attacked to a simple non-retriggerable monostable.

    Here's a simple enough looking ckt (the second post to the thread):
    http://forum.allaboutcircuits.com/index.php?showtopic=1640

    Cold cathode Neon light kits that flash to the beat are also available:
    https://www.streetglow.com/cgi-local/shop.pl/adcode=/page=ansa.htm
     
  15. chroot

    chroot 10,426
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    Right -- I thought he wanted something that would follow the amplitude envelope of the sound.

    Perhaps I have no idea what he's trying to do.

    The original post just asks how to make a LED light up -- the answer is: attach it in series with a resistor to 12V and ground.

    - Warren
     
  16. Sorry for being unclear, I understand now how simple it would be to connect an LED to the power wires.

    However, I think it would be awesome to have an LED get brighter as the sound gets louder, and be off when there is no sound.

    An important part I suspect to include would be that my head unit in my car has line-level outputs rated at 2.2 volts (when the volume is at full level, the signal is 2.2V. If there is no sound, it is at 0.) If the 'device' were to be run off this, it would not affect the signal going to the speakers so one need not worry about sound quality. Sound is a complex AC sginal, what does this mean for what I am trying to create? (could my ultimate question simply be how to convert ~2.2VAC to ~2.5VDC?)
     
    Last edited: Jun 14, 2005
  17. Cliff_J

    Cliff_J 789
    Science Advisor

    Ok, what is the source of audio for the speakers? If its just the radio and not a seperate power amplifier the voltages will be quite low. The line level voltage means little here, that is a signal to feed a seperate power amplifier. If the radio can deliver 15W of real power to the speaker (a very powerful radio) that would still be less than 11V peak.

    Just pick up a couple LEDs and some 1000 ohm (1 kohm) resistors. Wire it so the current goes from speaker + to the resistor to the LED and then back to speaker - and that should work well enough to give you a brightness that corresponds to the level of music although the overall brightness is not going to be much.

    Yes the signal is a complex AC waveform but it doesn't really matter, a diode is a one way device as long as the voltage is kept below the reverse limit. Also, the amount of current that would flow in a 1 Kohm leg in parallel with a 4 ohm leg is going to be very tiny - like 48db lower (if my quick math is correct) so the radio won't notice and its highly unlikely you would either.

    To fix the brightness problem a solution similar to what was mentioned above would be needed with some op-amps to create a more usable output. In short music has an average level and then many peaks that are much larger and this is typically described as the 'crest factor'. A radio station (and many modern recordings) use compression to reduce the average level to peak level ratio of the signal to something along the lines of 10db, meaning the peaks are only 10 times bigger than average. For a high quality jazz or classical music recording from a source like Chesky Records or Sheffield Labs the crest factor can be 20db or better, meaning the peaks are 100 times bigger than the average level. This requires better equipment to handle a signal that can vary this much, and with an LED directly driven by the signal it would either have its life shortened by the peaks or end up kinda dim. A traditional light bulb would not suffer as much, its thermal mass allows it to handle the peaks without overheating and burning out.

    I had some friends in high school that just used a half dozen christmas tree lights to 'blink' to the music. Those that had subwoofers found it to be the best thing they'd ever seen so others could hear them and see them thumping away. Others bought devices from catalogs like JC Whitney to do other things. And there I was trying to figure out how to save up for better midranges for better clarity...LOL.
     
  18. Thanks cliff, that helps a lot! If I want to power, say, 4 LEDs the same way with the same signal, should I use a different sort of resistor, or wire them a certain way?
     
  19. Cliff_J

    Cliff_J 789
    Science Advisor

    KingNothing - if you search around the internet you'll find all types of sources for working with LEDs and the approach to it. Here's a quick summary.

    Each LED will have a working voltage and current, you want to avoid exceeding either one but most importantly the current.

    Lets say the LED is 3V and 10mA and you have a 12V car battery. Well the battery is more like 12.8V when charged and something like 14.4V when the car is running. It should be obvious that 14.4V is a lot bigger than 3V and would burn this out is no time.

    In a series circuit the current would flow into the resistor and then out and into the LED and back. Since we know ohm's law means that if we know the voltage and the current we want we can calculate the resistor that will allow that amount of current to flow (see link below for a calculator for the easy answer). Also we can use ohm's law to double check to make sure that if the correct amount of current is flowing the resistor will drop enough voltage to bring it down to a safe level for our LED.

    http://home.cogeco.ca/~rpaisley4/LEDcalc.html

    You could wire up many of them in series too, this page has an example of that where the author puts 3 in series and then calculates the resistor.

    http://unclean.org/howto/led_circuit.html

    To complete my example, the easy solution with a 14.4V source and 3V diodes would be to wire 5 in series and that would drop 15V - pretty close and requires no extra parts at the expense of a tiny amount of brightness replaced by a tiny amount of longer lifetime.

    Just make sure when wiring in series that the diodes are all facing the correct direction, the anode (longer one) facing toward the positive.

    And I still think a christmas tree light bulb is much cheaper and simpler since they will be much more forgiving if you overvolt them.
     
  20. Okay...thanks, that does help. What about lighting an LED with an audio signal? I bought some red LEDs from RS a few hours ago. I plugged the prongs directly into an old stereo (the speaker terminals)...it lit, although very dimly. Although with the volume on full blast, it followed the music very well (although it was way too loud)...but that's also one speaker terminal down that I can't use for the actual speaker.

    I won't have any problems running an LED off the 12V battery in the car. Why is the LED so dim when connected directly to speaker terminals? Does it have too much current and not enough voltage?

    I have heard of a 'voltage multiplier' circuit, but only seen them used at much higher voltages. Could that help me at all? How do I make a simple voltage doubler or tripler, even if that won't help here? Im not familiar with a lot of schematic symbols.

    Can someone move this to electrical engineering forum? I think the topic better applies there.
     
    Last edited: Jun 15, 2005
  21. Cliff_J

    Cliff_J 789
    Science Advisor

    An LED is very fast, it can blink many times faster than your eyes can see. Like others, a series of still pictures running at 24 frames a second appears as a moving image in a movie theater. A sound at 24 times a second is outside the range of most musical instruments this side of a pipe organ. A guitar or piano are going to make sounds that vibrate 10x faster than your eye can see - it blurs this fast repetition into a dimmer light.

    You don't need a multiplier, you need something like an audio compressor and a VU (volume unit) meter to do what you want to do. An analog VU meter circuit might have enough output to drive a single LED to varying levels of brightness by itself and could drive a single transistor that could drive any number of LEDs.

    http://www.shure.com/support/technotes/app-meter.html
     
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