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Kitax 28th September 2012 04:33 PM

i want to make music lights, and wanted to clear out a few things... i have got a lm386 chip, wich has a peak to peak output of 10 volts with 12 volt input. firstly i need to decrease the output voltage because the leds needs about 3 volts, but how low should the peak to peak voltage be for the LEDs to blink, but not burn?
also, how much input voltage does a regular mp3 or phone have on max volume?

and... Gain means how many times the signal gets amplified, right?

djoffe 28th September 2012 06:04 PM

LED part of your question
place two LEDs in parallel, but opposite polarity. Place a limiting resistor in series with those LEDs. If the LEDs were rated to take 10 mA peaks, you might choose a 1K resistor to be pretty safe.

Kitax 28th September 2012 06:28 PM

ok... how do i calculate the output amperage? i guess the output watts devided by the output voltage, so max output would be the 0,5w/10v (output peak to peak)?

oh... how much rezistance does a diode have?

djoffe 28th September 2012 06:36 PM

assume the speaker goes from the amp output to ground (to eliminate confusion for bridge tied loads, etc). Let's say that the amp puts out at most +10 V or -10 V wrt ground.

Assume forward voltage on the LED of 1.5 Volts. Then the voltage across the resistor (maximum) would be 10-1.5=8.5 Volts. If you wanted this to correspond to 10 mA, you'd choose a resistor of 8.5/0.01=850 Ohms. The resistance of the LED is rather smaller than this , so could be neglected.

The power dissipation of the resistor, would in the worst case be 8.5 Volts times 10 mA, or 85 mW, so that's really not a problem.

Arty 28th September 2012 07:34 PM

use a trimpot to limit the input of lm386 and slowly raise it till the leds light up at the output.

johnr66 28th September 2012 07:53 PM

It works pretty good with 9 volt supply and two white LEDs in series connected to the output pin (no coupling capacitor needed). Also add a limiting resistor of around 39 ohms. The LEDs are not on continuous, so it is okay to drive them harder. Since they are on for a half cycle, it looks more stobe like to me.

The bass is stronger in most music so the LEDs will flash to the beat. If too much HF signals are keying the LEDs, low pass the input.

Kitax 28th September 2012 09:16 PM

well yea decoupling caps are ment to increase the quality if im right, so there is no point in using them with LEDs :P guess ill get a potentiometer, and set it up...

actually i wanted to know how much harder can i drive them...

Bare 29th September 2012 12:48 AM

Alll right! Tie Dyed TShirts and maybe even Disco Balls.
Frequency/music following colored lights.
What.. will they think of Next?

Kitax 30th September 2012 09:39 AM

well dont forget a disco ball...

sofaspud 30th September 2012 10:18 AM

The output of an LM386 is at Vcc/2, so if you aren't careful your LEDs will be lit continuously (though they will change brightness with the signal). With LEDs in series, Vled1 + Vled2 must = >Vcc/2.

actually i wanted to know how much harder can i drive them...
You've probably seen this before... Read the datasheet. If mA peak specifically.
Output voltage, output amperage, and diode resistance are mostly needless concerns here.

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