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Kitax

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Joined 2012
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?
 
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djoffe

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Joined 2008
akitika.com
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.
 
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johnr66

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Joined 2009
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.
 
sofaspud

sofaspud

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Joined 2005
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...
Click to expand...
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.
 
sofaspud

sofaspud

Member
Joined 2005
The LEDs will get their voltage as long as Vf is met or exceeded. No more and no less (in practical terms). If you are speaking of LED dynamic resistance still, it isn't a concern in anything but the most unusual of circuits, which this isn't. And the series resistor isn't overly critical; the 39 ohms suggested earlier seems like a good starting point. Or start with a larger value, say 100 ohms, and solder another resistor(s) in parallel if you need brighter output.
I'm not sure what units "0" refers to in your post.
 
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Kitax

Member
Joined 2012
how should i make a low frequency filter, so the lights react more to the bass... i used RC filter, but it wasnt very effective, and i almost couldent feel the difference (just put a rezistor right after the diodes and the capacitor paralel, between the ground and output)
matbe i should modify the schematic to change the amplified frequencies, but i dont know how to...
(i used the datasheet schematic called "Amplifier with Gain = 200") i had it laying around
i think that the cutoff frequency should be around 50hz, but it depends on how good the filter is...
 
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johnr66

Member
Joined 2009
Sounds like your using the LM386. You need to filter on the input side. Use the RC impedance formula to find the pole frequency of 50Hz. If your using the output coupling cap, the one on the data sheet (220uf I think they used) is too small. Increase value to 1000uf if your driving power LEDs.
 
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