DC Circuit advice

Old 03-10-13, 04:41 AM
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DC Circuit advice

Hello everyone,
This is my first time in your forums which I found trying to find a bit of assistance with a little project that I have on the go at the moment. I am not an electrician at all, and I am only just learning so please bear with me if I ask a stupid question or have done something which is not the correct way to do it.

A bit of background into my project. I have a huge background in PCís, from network security and administrations (which is my qualification) to building and modifying gaming PCís and cases (as a hobby) and have used LEDís on computer modifications, but they have always been easy where I have just measured the voltage going across them and simply just replaced them with another colour of the same voltage. I have never really understood how they work. So, I made the following, which consists of 22 LEDís in parallel which run off a modded USB cable which puts out 5v and 500Ma after a 3 day crash course in google reading up on how it all works.

I found this really easy to do, so decided to make one where the LEDís light up in response to receiving a sound signal from the surround sound and this is where I am now asking for some guidance. I have made a prototype in which I have got connected to a Logitech X-530 5.1 Surround System using the RCA connectors. The diagram below shows how I have planned the LEDís to light in response to the sound/music. (Please excuse the barbaric schematic). Through some research, I found that the transistor opens and closes the circuit when it receives an electrical signal from the positive audio wire which works fine.

There are 28 LEDís all linked together in parallel with their specs as 3.2-3.6 forward voltage, 20Ma, with a 100ohm resistor on the positive terminal of each one and with a single negative wire connecting all of the LED negative terminals together. I was thinking of using a DC Switching adapter puts out 5v, against the 100ohm resistors, and 1.0A (which I understand is 1000Ma?), and this powers everything beautifully.

The issue I am struggling with is, as you can see from here, I have 6 circuits. The subwoofer circuit is not an issue as I can power that straight from the subwoofer which is powerful enough to light them, and I just have the White LEDís with the resistors on them. Therefore I have only 5 circuits which are using the above schematic needing a separate power supply, as the amplifier, or the satellite speakers should I say, are not powerful enough to light up the LEDís.

My questions are therefore this:

  1. Is the manner in which I have used one negative cable the correct way of doing this for effectively 5 different circuits, or at least, will it cause any problems by me using just one negative cable for the entire ? My understanding is that you can use one negative but separate positive cables.
  2. When I connect, for example, a front right and a rear right, it combines them both into one signal if that makes sense, so the lights light up together at the same time and not seperantly for each channel. I am assuming that somewhere in the circutary, it is shorting as such. How would I search for this ideally?
  3. Ideally, I want to take power from the amplifier for each channel, already wired an RCA to the sub speaker so can just plug in a jack into the rear of the amp and into the lights and it works beautifully. However if I wanted to use power from the speakers themselves, is there anyway of boosting the current output to put out enough to light the leds without blowing anything?

Ive been googling for answers, but to be honest, I am not sure of what wording to use to search for these answers which I am sure are out there. Any guidance? Hope my post made sense.
Old 03-10-13, 10:12 AM
Join Date: Oct 2008
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Hi panic and welcome to the forum.
I know you will understand when I say, there's no salvaging your diagram. The design parameters for transistors and LEDs, even at a basic level, requires some understanding as to how they work. I could go into details, but we don't have enough space.

A quick example is that LEDs do not operate on voltage, the are controlled by the current we pass through them, the voltage is then the result of the diodes characteristics. A resistor or other current limiting circuit is ALWAYS necessary to control the current.

I did some searching and found several schematics, but none that I would call simple. Here is one page and it looks like a non-us design. This is their explanation and they have a link to the schematic. audio light modulator/sound controlled light circuit diagram

A good place to start would be a block diagram, what you want for inputs and how they will be combined for the output. Then you can think about how best to fill in the blocks.

Since this is not a unique application, my guess would be there are products out there that would simply plug in and do exactly what you want, so this boils down to either cost or a learning experience, or both. What's your objective? The advantage of buying an already designed box is safety and reliability. Designing electronic circuits has a risk which your electronic level is not up to as yet.

As I am cautioning you on circuit design, you certainly wouldn't want me to design the security needs for one of your clients.

Old 03-11-13, 05:11 AM
Join Date: Apr 2007
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You should isolate the speaker and LED circuits from each other so you don't damage the amplifier. That's one of the reasons the circuit that Bud linked to uses an audio transformer.

An opto-isolator would also work.

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