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Sound Activated Light Circuit

  1. Aug 22, 2010 #1

    I am currently working on a music reactive light system with LEDs, but due to the low amount of light produced by the LEDs I was hoping to incorporate cathode tubes commonly found in desktop computers. I tried amplifying the signal from my mp3 player where the base current was produced by the player and the cathode's inverter was connected to a TIP31B transistor with a 12 V source. This produced a very low amount of light but the light did beat to the music, to a degree. I tried to use multiple transistors but that failed and surprisingly induced no light. Then opened some old computer speakers and tried to use the leads to the speaker as the base current but this produced no effect at all despite the higher current. Strangely when the ground of the inverter came into contact with the positive lead of the speaker and then the leads were placed in the circuit, the cathode would briefly light up and beat with the music but then slowly diminish until this ceased.

    I am a bit baffled at both why multiple transistors and the speakers didn't work. This is my first experience with inverters, are there any special considerations with them?


    These are the cathodes:
  2. jcsd
  3. Aug 22, 2010 #2
    If it helps, the circuit set up is similar to this except in place of LEDs there is an inverter attached to the cathode tube. As stated earlier, that works but poorly. The 12 V is supplied by a modified computer power supply that acts as a bench top power unit, so the dc voltage being used should be very clean with regard to its signal.

    This image is originally from:

    Attached Files:

  4. Aug 23, 2010 #3


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    If you put a lamp or a LED (in series with a protective resistor) across a 12 volt supply, that is as bright as it is going to get. Adding a transistor in series with it will only make it dimmer.

    The transistor acts like a variable resistance in series with the lamp or LED and gives you control over the device, but it can't make it brighter.

    A PNP transistor like the TIP31 should not have its base grounded because this would cause its base current to be very large and it may destroy the transistor. The output of an amplifier is usually through a capacitor, so this probably saved your transistor.
    You would need to have a suitable resistor in circuit from base to ground and then feed the input via a capacitor onto the base.
    A suitable resistor might be 100 K, but you would have to check this by trying it. Make it smaller to send more current through the LED. You can also reduce the size of the 470 ohm resistor, but this is at some risk of letting too much current pass through the LED. Most LEDs will only cope with about 20 mA.
    [PLAIN]http://dl.dropbox.com/u/4222062/NPN%20%20LED%20AMP.PNG [Broken]

    Your cold cathode devices have an internal high voltage supply, so this is probably set up to be self protecting. Again, connect the device across 12 volts (observing polarity carefully) and that is as bright as it is going to get. If it is bright enough to be useful, then you can try to control it with transistors.
    Last edited by a moderator: May 4, 2017
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