Led, help

[howitzer]

Hi guys nd girls
Im wanting to connect some leds up to my speakers for the awsome effect.
ive tried today nd managed to get them to work but after a while a orange flash nd they were dead

I connected up 3v blue leds to my 95w stereo, and ive done some research and im guesing i need to put in a resistor, im not to good with figuring out what components i will need. sooo
if any one can help me with this i will be very gratefull
Cheers

 

[mark_3094]

How have you connected them to the stereo?
each LED needs to be protected by a resistor. For 3v going into the LED's, you'd be looking at 330 Ohms

 

[howitzer]

i connected them by the wires at the back of the sterio were the speaker wires clip into, i had it working before looked awsome
soo 330ohm resistor should help?

 

[mark_3094]

Yeah, you definitely want to have a resistor on every LED.

I know you said that they were 3v LEDs, but are you sure that you are putting 3v into them?

The higher the voltage, the higher the resistor. For example, if you have 9v, you'd probably want to use a 560 Ohm resistor or thereabouts.

If you're not sure, start with larger resistors first. At worst, a resistor that's too large will just make the LED a little more dull (because not as much current will flow).

 

[howitzer]

Does it matter if they origionally came from a 3.2v dc circuit?
because i did some tests and i get around 20.5v maxing out from the sterio on a ac circuit...

 

[mark_3094]

I'm afraid I don't really know much about AC, so I couldn't say for sure...
Are the LED's in series with the speakers or parallel?

 

[howitzer]

They are in paralell to the speakers

 

[mark_3094]

I think it sould be fine. Maybe put in a 560 Ohm resistor and LED in series with each other, but make the whole thing parallel with the speakers.

At worst, if it goes wrong, it will just pop the LEDs. If that happens, you know you need a bigger resistor

This might be of some interest:
Condom Sound Activated LED

 

[colin55]

Put a LED around one way with a 330R resistor, across the speaker.
Put another LED around the other way with a 330R resistor.
See which LED works the best or if both are similar.

This is how to work out how to connect the LEDs.

Add more LEDs in series with the best LED until the effect diminishes. This way you will work out the voltage being delivered to the speaker.

You will possibly end up with a number of LEDs in series because the voltage (AC voltage) being delivered to the speaker will be greater than 3v, as suggested above and 330R will be insufficient for a single LED.

 

[howitzer]

They are in paralell to the speakers

 

[kchriste]

Try something like this:

 

[audioguru]

Like I said on the other website, 95W into 8 ohms is 39V positive peaks and 39V negative peaks. If the LEDs are 3.3V blue ones then with a 1k resistor the peak current is (39V - 3.3V)/1k= 35.7mA. The absolute max allowed continuous current for most LEDs is only 30mA so they might burn out if the amplifier is playing at full blast continuously.

The volume control will cause the LEDs to be extremely dim when the volume is turned down.

 

[howitzer]

Thanx every one

 

[kchriste]

One LED will handle the positive peak and the other the negative. So the average current in each LED would be less than 18ma even with full clipping.
I agree that the LED will be quite dim at low volume levels. howitzer should really build a color organ once he gets more skilled.

 

[howitzer]

Hi me again
I had it wired up with a 330ohm resistor and replaced it with a 1kohm resistor and is brighter but.... gets up to around 4v :O

 

[audioguru]

The speaker signal is AC. But an LED works with DC. The reverse voltage half the time with AC will destroy a single LED unless it has a second LED connected in reverse to it.

You need an ocilloscope to see the peak voltage of the AC. A meter just shows average low frequency voltages.

A 330 ohm resistor allows more current to flow in the LEDs than a 1k resistor so the 330 ohm resistor will make the LEDs brighter.

 

[mark_3094]

Would there be any way to calculate the peak voltage without an oscilloscope?
Any formula or something?

 

[colin55]

I have already explained how to add LEDs until the best eefect is achieved. From this you can add the voltages of the LEDs and arrive at the values you require.

 

[rmrunner]

Like I said on the other website, 95W into 8 ohms is 39V positive peaks and 39V negative peaks. If the LEDs are 3.3V blue ones then with a 1k resistor the peak current is (39V - 3.3V)/1k= 35.7mA. The absolute max allowed continuous current for most LEDs is only 30mA so they might burn out if the amplifier is playing at full blast continuously.

The volume control will cause the LEDs to be extremely dim when the volume is turned down.

audioguru. i saw you have a formula to calculate voltage using wattage and resistance. can you please tell me a recommended resistor to use with a 200W RMS (house amp) @ 4 Ohm.

THank you

 

[audioguru]

Only an amplifier with vacuum tubes and an output transformer must always have its rated load. A solid state amplifier works perfectly without a load or with a load impedance that is too high.

A 4 ohm resistor rated for 200W is huge, expensive and dangerous.

The RMS voltage is the root of (4 x 200)= 28.3V. The peak voltage is 28.3V x 1.414=
40V.