LED Strips That Pulse With Music

Hey Everyone, I am working on a project for a custom subwoofer box for a friends car and I want to make sure I am covering all of my bases because I have never had a request like this before so I want to make sure it works. I have done a bit of research but I feel like I am still missing something. The project is to have the subwoofer box lined with LED strips and power those lights from the single going to the subwoofer from the amp. I plan on using standard 12volt LED strips, TIP31 Transistor, Switch (Two or Three terminal?), and 18 gauge automotive wire. I will connect the transistor to the positive and negative going into the subwoofer and then wire the leads from the transistor to a switch so that it can be controlled when the lights are to sync with the music or when they are to be off. My questions are: Do I need a resistor somewhere in the circuity to prevent the LED strips from blowing? How should I wire it to the switch to get the ideal effect? (2 terminal switch that is just an interruption for the power wire leading to the LEDs or a 3 terminal that is wired with the negative coming from the transistor?) Am I missing anything else? Thanks, CBux Here is a picture I found that kind of illustrates the circuitry I had in mind but does not include the switch. Also note that I will be using 18G wire instead of a 3.5mm jack.

What if you use an LP instead of a single?    I assume you mean using the existing sub wiring only. Try the LEDs across the wires and see how that works. Most simply do that. Series resistors are only needed if sub voltage exceeds LED specs in which case a series diode (1N4004 etc) should be included. To do more - eg, high illumination at low volumes - requires a suitable amp... I don't see any need for the transistor you describe unless in current limiting mode to (maybe) protect the LEDs. An SPST switch can be inserted anywhere in series with the LEDs. Ensure the added LEDs don't overstress the amp. That's unlikely with LEDs being mA compared to a sub's Amps, but in case lots of LEDs parallel are used...

oldspark wrote: What if you use an LP instead of a single?    I assume you mean using the existing sub wiring only. Try the LEDs across the wires and see how that works. Most simply do that. Series resistors are only needed if sub voltage exceeds LED specs in which case a series diode (1N4004 etc) should be included. To do more - eg, high illumination at low volumes - requires a suitable amp... I don't see any need for the transistor you describe unless in current limiting mode to (maybe) protect the LEDs. An SPST switch can be inserted anywhere in series with the LEDs. Ensure the added LEDs don't overstress the amp. That's unlikely with LEDs being mA compared to a sub's Amps, but in case lots of LEDs parallel are used...

What do you mean LP instead of a single? I wasnt going to connect the LED's to the amp directly. Instead I was going to run the + and - to the transistor from the speaker wire input to the sub coming from the amp so that way the lights power is generated by the signal going to the subwoofer itself which in turn I believe will cause them to be brighter the harder the bass hits. Am I wrong? In regards to the switch, I can just through the SPST coming from the 12v power source and it wont cause any issues?

As I understand it, you are only using the two wires to the sub from the amp. Where you connect across them is irrelevant. (I assumed in the sub box.) The strongest signal (brighter & harder) is with LEDs straight across the + & - of the sub. (Adding a transistor can only decrease the signal.)

oldspark wrote: As I understand it, you are only using the two wires to the sub from the amp. Where you connect across them is irrelevant. (I assumed in the sub box.) The strongest signal (brighter & harder) is with LEDs straight across the + & - of the sub. (Adding a transistor can only decrease the signal.)

But off I dont add the transistor wont I blow the LEDs incredibly quick?

oldspark wrote: As I understand it, you are only using the two wires to the sub from the amp. Where you connect across them is irrelevant. (I assumed in the sub box.) The strongest signal (brighter & harder) is with LEDs straight across the + & - of the sub. (Adding a transistor can only decrease the signal.)

Will adding the wires directly to the terminals for the subwoofer cause any sound distortion?

You'll only blow the LEDs if their specs are exceeded - ie, at greater than 12V RMS or peaks greater than they handle (both forward and reverse). A series diode protects reverse voltage (PIV) being exceeded (and probably adds an extra 0.7V drop forwards - ie, they can then handle 12.7V RMS). A resistor is all that is required to reduce excessive voltage but that also reduces the signal at low volumes (tho not linearly - there will be some inherent compression). To do this properly requires more than a transistor. At best 2 transistors as a current limiter but using an LM317 is probably easier (that adds a 1.2V drop). Otherwise a musicolor type circuit - ie, a PWM circuit that PWMs LED current (brightness) in proportion to the sub voltage (with whatever filters & compression or gain circuitry is desired) but that requires external power - eg, a 12V supply, hence 2 more wires. Distortion using your 2 wire approach should be negligible unless LED power is significant compared to sub power.

oldspark wrote: You'll only blow the LEDs if their specs are exceeded - ie, at greater than 12V RMS or peaks greater than they handle (both forward and reverse). A series diode protects reverse voltage (PIV) being exceeded (and probably adds an extra 0.7V drop forwards - ie, they can then handle 12.7V RMS). A resistor is all that is required to reduce excessive voltage but that also reduces the signal at low volumes (tho not linearly - there will be some inherent compression). To do this properly requires more than a transistor. At best 2 transistors as a current limiter but using an LM317 is probably easier (that adds a 1.2V drop). Otherwise a musicolor type circuit - ie, a PWM circuit that PWMs LED current (brightness) in proportion to the sub voltage (with whatever filters & compression or gain circuitry is desired) but that requires external power - eg, a 12V supply, hence 2 more wires. Distortion using your 2 wire approach should be negligible unless LED power is significant compared to sub power.

I picked up the LM317 yesterday. How would you suggest wiring it into the circuit?

Do you need it? Properly done it is one current limiter per LED string. (Circuit as per here and installed as above - ie, in series and probably with a series blocking diode). That's only to limit the total current that can go thru the circuit (as opposed to protecting each individual string of LEDs). I suggest that before doing any of the above you simply connect a sample LED string across the sub and see how they react and if they blow. The string need only be one string of series LEDs. They are typically arranged in groups of 3 LEDs for commercial 12V LED strips but could be more (eg, 5 or 6 for red) Try to get them to blow. If they blow, repeat with a series diode (whatever you have handy - eg,1N400x or 1N914 etc for under 1A & 100mA strings) & repeat.    If they blow, repeat with a series resistor else LM317 etc. When they no longer don't blow confirm that you're happy with their overall performance at typical volumes. Or do all the above in one go... That should give you the basis for the design - ie, the minimum resistor value per strip (use the next size up for longer LED life) or current limiting setting. You might find you want to use voltage limiting (clipping) instead... Or calculate the above requirements based on the LED specs and output to the sub (ie, voltage across it).