This is my first electronics project, I'm really new at this and struggling to understand everything, just learned so solder a few days ago.. :)

I'm using this circuit as my starting point enter image description here

I'm using a 12V power supply, and a 12V LED strip. Instead of TIP31 I have TIP35 (guys at my local electronics shop said that in my case they don't really make a difference..)

I hooked up the schematic but I had problems with audio output splitting, not enough was passed to the TIP35, so I added an amp circuit with LM386 something like this
enter image description here

Having read that powering the LM386 with 12V results in heat and stuff, I wanted to be safe(er) and I'm passing 5V to it with a TO220 regulator that drops 12v to 5v. I'm not using the Gain pins, because I noticed that it doesn't influence brightness, only the "sensitivity", and the LEDs would flash to the smallest sounds (which is not what I want). So I'm passing ~70% volume and amplifying that, so my LEDs react to more lower sounds.

The problem I am facing is that the LEDs do not blink at full brightness. That was the problem I had without having the amp in, I though that might help but it doesn't. The LEDs I believe never reach ~60-70% of brightness compared to just passing 12v and having them on. I thought it might have to do with the length of the strip (5 meters), I cut it down to just a 9 LED length strip - same result with low brightness.

I've stumbled across this pdf that shows different pins for input/ground/output. I've hooked up in this order: input/output/ground while this pdf ground/input/output. Could that have anything to do with it?..

I'm sorry for the newb questions - I am one..

Edit: Here is how I've connected everything (sorry if it's messy..)

Oops I messed up the colors of the output to the LEDs, they should be switched (blue-red, red-blue)

enter image description here

  • \$\begingroup\$ Welcome to electronics.stackexchange. Regrettably, I can't see an actual question (newb or not) in your journal. \$\endgroup\$ – RedGrittyBrick Jan 19 '14 at 11:54
  • \$\begingroup\$ My LEDs don't blink at full brightness only ~60-70% of what the LEDs light up hooked to 12v directly. \$\endgroup\$ – Matt Jan 19 '14 at 11:57
  • \$\begingroup\$ I don't see a single resistor anywhere in any of your drawings. This basically inevitably means you have something fairly basic wrong. \$\endgroup\$ – Connor Wolf Jan 20 '14 at 10:24
  • \$\begingroup\$ Starter for 10: Google for "light organ" or "colour organ" circuits. \$\endgroup\$ – John U Jan 20 '14 at 19:09

If I understand you correctly, you just need to convert the amplitude of the music to the brightness of the LEDs. First, note that human perception of sound is actually very skewed and tends to follow a logarithmic curve. As a result, what you understand to be 70% volume is likely much different than if you were to look at the percentage of the "100% volume" in absolute air pressure.

Moving on from that, your circuit (the original one), seems to be alright. In theory, the voltage across your transistor will be pretty low. You are likely to stay in the saturation region of the transistor (left of the dashed blue line in this diagram.

What this means is that the current through your LEDs will be limited by the transistor in a more-or-less linear way. This could be solved for with a load line (for which I refer you here, since it's not really necessary).

Given your LED strip is spec'd at 12V, I recommend powering it with 12V. The reason is that the forward drop on the LED effectively lowers the voltage available to generate current, and the LEDs likely require <25mA of current. For example, a 5V source with an LED that has a forward drop of 2.2V and a current-limiting resistor of 220R will have

(5 - 2.2) / 220 = 0.0127 -> 12.7mA

which is a reasonable amount of current. Hopefully that demonstrates why 12V might be necessary for the strip. Looking again at your question, you may be providing the strip with 12V (and only limiting the LM386 to 5V, but I wanted to make sure). Going to your point that low frequency sounds tend to make it light up more, the human ear actually perceives loudness differently across the audio spectrum. Sounds ~1kHz tend to be the loudest to us even if the absolute air pressure is the same as for a different frequency. The reaction you see might just be due to our distorted perception.

Now, as I said before, the transistor will limit the current available to the LEDs based on its base current. The LM386 can tolerate 12V. It's spec'd in the datasheet, and I doubt you'll find heat problems. I am assuming that you're using the LM386 to drive the transistor, and if you are, then you should place a current limiting resistor between the output of the LM386 and the base of the transistor. You can see here, that the base current is limited to 5mA, and a good transistor will permit a current multiplication of at least 10x, so it should allow a max current of 50mA minimum. Now, Imagine that your supply is changing, so it isn't just 5V. That's your audio signal. As a result, anything larger than 5V will cause more current to flow. Lower voltages will cause less.

However, to make sure this works (since audio is an AC signal), you have to make sure the voltage is never negative, otherwise you'll only ever get the LEDs to light up half of the time. That's slightly more complicated, and you might be getting a DC offset signal anyway.

As for the document you linked to, it shows the same configuration that I think almost every other BJT/FET uses (at least for a TO220). It is almost always Base/Collector/Emitter (or Gate/Drain/Source). I'm not entirely sure what you are getting at with


but the first diagram you have seems to be correct.

Hopefully all of that info helps. I'd like to add that if I were doing this, I would take the signal, pass it through an op amp (which can provide a 30mA constant current source) which drives an NPN BJT in a low-side drive configuration. I am assuming you want to design/build it yourself, so I won't link a schematic unless you ask for one.

Good luck!

  • \$\begingroup\$ Thank you so much for your response! If I only could understand half of it .. I've tried powering the LM386 amp with 12v instead of 5v, it puts out the same brightness level to the LEDs. I'll try posting the schematic I have, and maybe you could point out what could be done to improve the brightness level?.. \$\endgroup\$ – Matt Jan 19 '14 at 12:04
  • \$\begingroup\$ @Matt that means the 20 gain is not enough. \$\endgroup\$ – Passerby Jan 19 '14 at 22:31

First, typical dumb led strip is setup as multiple 3 led, ~20mA @ 12v sections in parallel. So for every 3 led segment, you need 20mA of current ideally. A 9 led section only needs 60mA, which is fairly small.

The TIP31 is a NPN Silicon Transistor with fairly small HFe (Gain) of 10 to 50. It is a current controlled device (Base Diode). Current at the base is multiplied by the HFe. So for a 60mA load between the collector and the emitter, you need 60mA / 10, or 6mA. Could be smaller depending on the actual gain. The datasheet lists typical, not actual). On the flip side, for a full 5m (assuming 60 leds per meter) is ((60 * 5) / 3 ) * 0.02A = 2 Amps, so you need 2 Amps / 10 or 200mA at the base. Again, could be smaller depending on actual gain, but we want saturation, so stick to 200mA.

So if you take your 9 led strip, hook it up to the transistor's collector, and add a (12v - 0.7v) / 0.006 = 1.8k resistor from 12v to the base, and you would get full brightness.

But the problem is that Audio is not linear, and the way we hear it does not correspond to how it actually looks either. There are multiple frequencies mixed together, and even when averaged out, still results in rapid fluctuation. AND Audio is peak to peak AC, going from positive to negative, centered on ground, while the transistor and led strip are DC Positive, from positive to ground.

Basically, if you want lower (frequency) sounds to turn on the light, instead of quick/quiet sounds (amplitude), you actually need a lowpass/bandpass filter first, then amplify it. The leds will only respond to bass then. If you want full brightness and overall response, you have to amplify it better, and you'll have to deal with it responding to everything. AFAIK there is no better tradeoff.

  • \$\begingroup\$ But as I've said if I max out the LM386 amp, it has no effect on the brightness level even on a 9led long strip, it only gets more sensitive. So should I try adding another amp, another TIP35, something else to the mix?.. \$\endgroup\$ – Matt Jan 20 '14 at 10:10
  • \$\begingroup\$ @matt try replacing the TIP35 with a small 2n2222 or 2n3904 (100 to 600mA) max transistor, higher gain, and see if that affects the brightness of the 9 led strip (60mA). If so, the TIP35 is just not suited for it. \$\endgroup\$ – Passerby Jan 21 '14 at 1:59

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