Help with 40 LED Light Bar

[gurpreetsa]

Sorry to bring up an old thread, but I was doing something similar and i need your help. I have built a led light bar that has 40 leds and runs at 3.3v, that part i have sorted out. I also have a tip31c and i wanted to connect the light bar to my home theater system so that I could make the leds react to whatever I'm listening too. I need help on figuring out how to do this without damaging my a/v receiver. Also it would be cool if i could add something like pentiometer to the leds so that i can adjust their brightness say for example if I'm listening to music at low volume the lights will be very dim, but i can adjust their brightness with a pentiometer. Any help would be appreciated

 

[KJ6EAD]

I don't understand why people keep using that terrible destructable, risking their relatively expensive stereo equipment to light up an LED. Why not use one of the many sound to light kits on the market such as the Velleman MK103 or MK186?

 

[gurpreetsa]

My reason is because i like building things myself vs buying something even though it may be harder this way.

 

[audioguru]

A transistor has a diode from base to emitter that draws a huge current if the current is not limited with a resistor in series with the base. The value of the resistor must be calculated with the peak voltage from the amplifier and the base current required. The huge current can destroy the transistor and/or destroy the amplifier.

The maximum allowed emitter-base reverse voltage is shown as 5V on the transistor's datasheet. A diode must be connected from base (cathode) to emitter (anode) to arrest the high reverse voltage from the amplifier.

The transistor, amplifier or LEDs will be destroyed if a brightness potentiometer is set wrong.

LEDs are all different. They should not be connected directly in paallel unless they are all measured and sorted so that they all have exactly the same forward voltage. A current-limiting resistor must be in series with the LEDs.

 

[KJ6EAD]

My reason is because i like building things myself vs buying something even though it may be harder this way.

That's a valid reason to build your own, but not a reason to use a terrible design. You can do much better than destructables with a little searching.

 

[audioguru]

The Instructable (destructable) was designed or copied by a 10 years old little kid who knows nothing about electronics.

 

[kinarfi]

If I were to do something like this, I would us a microphone to pick up the sound and not tie it my amp or possibly have a buffer between the amp and LEDs and I think the only valid way of dimming LEDs that I'm aware of is PWM.

 

[gurpreetsa]

A transistor has a diode from base to emitter that draws a huge current if the current is not limited with a resistor in series with the base. The value of the resistor must be calculated with the peak voltage from the amplifier and the base current required. The huge current can destroy the transistor and/or destroy the amplifier.

The maximum allowed emitter-base reverse voltage is shown as 5V on the transistor's datasheet. A diode must be connected from base (cathode) to emitter (anode) to arrest the high reverse voltage from the amplifier.

The transistor, amplifier or LEDs will be destroyed if a brightness potentiometer is set wrong.

LEDs are all different. They should not be connected directly in paallel unless they are all measured and sorted so that they all have exactly the same forward voltage. A current-limiting resistor must be in series with the LEDs.

So essentialy I need to add a resistor between the amp and the transistor and add a diode between the base and emiiter of the transistor? How would I figure out the base current required for the resistor? I'm sorry for my limited knowledge, I'm only 14 I mess around with electronics for fun, but don't know much yet. I hope to take a class at my local community college on electronics next summer to improve my knowledge but until then its helpful people like you guys that will feed me the knowledge i need.

I watched some videos on youtube about Ohm's law and i think i've figured out the peak voltage from the amp. My amp outputs 130w at 8ohms per speaker channel so by using ohm's law the peak voltage should be around 32v, please correct me if I'm wrong.

If I went the mic route what kind of mic would I need? Something like the one the use in the Velleman kits?

 

[audioguru]

If your amplifier has a real output of 130W RMS at low distortion into 8 ohms then its voltage is 32.3V RMS which is 45.6V peak. If you want the LED to be at its brightest when the amp output is 130W then use Ohm's Law to calculate the base resistor value. You also must calculate how much power (heat) the resistor will make and size it accordingly.

A common mic today is an excellent but inexpensive electret type. Like all microphones its output voltage is only about 0.002V to 0.01V so it needs a preamp with a voltage gain of 80 to 400.

 

[gurpreetsa]

How did you calculate the peak voltage? If you were doing this project, what route would you take? Thanks for all your help.

 

[audioguru]

How did you calculate the peak voltage? If you were doing this project, what route would you take? Thanks for all your help.

The peak voltage is 1.414 (the root of 2) times the RMS voltage.

I designed and made a Sound Level Indicator project 6 or 7 years ago. It has an electret mic that picks up sounds. It has a few ICs and a few transistors. It has 20 LEDs and has a rechargeable battery so it can be portable but is usually powered from a wall-wart. It has a range of 50dB which is from a pin dropped on the floor of the next room to the stereo or TV (or my dogs barking) very loud.

Your problem is that you have an extremely simple circuit. It only has one input voltage that lights your LEDs. If the volume of the stereo is turned down then the LEDs will not light. My LEDs light with ANY sound.