LED's to use with speakers

[R1futly]

I built a cooler stereo to use at my pool area or on float/camping trips. I was wanting to add LED's to each speaker that will flash to the beat of the music. I am using an amplifier that produces 50 watts RMS @4 ohms going to each speaker. I bought some rectangular LED's at RadioShack that have the following specs:

Intensity: 350mcd
Wavelength: 465nm (min) 470nm (max)
FW current: 20mA
FW supply: 3.2V (typical) 3.8V (max)

I was playing with one just connected directly to the speaker cable, turned it up a little too loud and let the smoke out of it. My question is what kind of resistor do I need to get the best light output but not to blow any more of them?

Thank you very much for any advice.

 

[BobK]

P = I * I * R
50 = I * I * 4
12.5 = I * I
3.54 = I
Vrms = I * R
V = 3.54 * 4 = 14.2
Vpeak = Vrms * sqrt(2) = 14.2 * 1.414 = 20V

So the max voltage peak will be 20V.

Now, what is the drop across the LED? Assume 2.5 so:

V = I R
20 - 2.5 = 0.02 R
875 = R

This will be the minimum absolutely safe resistor. But that would probably not produce much output at normal listening level.

Bob

 

[Resqueline]

Don't forget that LED's are rated for only 5V reverse. The best way to ensure that is to add another LED in antiparallell to the first one (in parallell but the opposite way).
Also, each LED only receives any current for half of the time, so you can safely up the current to almost the double value. The 20mA is also an average current rating.
So my suggestion is to calculate for a 50mA peak value, which gives about 35mA rms to both LEDs, and thus only 17.5mA average to each LED.
R = (20V - 3.2V) / 0.05A = 336 ohms
P = approx. (20V - 3.2V) / 1.414 * 0.035A = 0.42W
If you use a 330 ohm 0.5W resistor I believe the LEDs should survive even at party levels.

 

[FuZZ1L0G1C]

Assuming V (Vcc in equation) is 20V
Rs=(Vcc - Vf)/If
Vf = LED forward voltage - is this the 'FW Supply' you mention?
'If' is the LED forward current.
Cooler effect?
Colored ultra brights anti-parallel.
U/B rating 3V 30mA (0.03A) so
Rs = (20 - 3) / 0.03 = 17/0.03 = 566R (560R standard value)
Increased If rating: allowing 50% duty cycle cool-off:
Rs = (20 - 3) / 0.05 = 17/0.05 = 340R (390R std val)
Rs = (20 - 3) / 0.075 = 17/0.075 = 266.66R (270R std val)
Each LED should have its own series dropper Rs. (0.5W each).
If you series LED's (of same rating) this also increases Vftot (Vf*Qty) so R wattage requirement.

/ resistor -------- a LED |> k --------\
~-----------\ __ k <| LED a ----- resistor ______/--------~

 

[Bluejets]

Try looking for a circuit " colour organ".

 

[Audioguru]

I bet your cooler stereo is powered from a 12V motorcycle battery. Then its output is 50 Whats which is 14 Watts RMS per channel bridged into 4 ohms. Recalculate the resistor for 14 Watts in each speaker.

The brightness depends on the loudness. It is bright at full power but dimmed when turned down. Do you play it at full blast all the time? Maybe you need a circuit to detect the beat of the music at any level then light the LEDs at full brightness. Then the amplifier will not be damaged by overload.

 

[Externet]

Made this, has been working for years...