Making case LEDs react to an audio source

[Zertman]

Hi everyone. I have a cool project I have in mind to do. I have a PC case that has a lot of LEDs and I want to try to make them react to the signal being sent to the subwoofer. The LEDs run on 12v from the PC's power supply, so I know the output to the sub by itself would not be enough to make them light. I have some op-amp chips, could I use them for this project? If I can, how would I hook them up? If I can't use them, what would I need to do this?

-Thanks

Edit:
Here is a link to the datasheet pdf of the Op-Amp Chips I have:
Dual Operational Amplifier | Datasheet Archive

[nigelwright7557]

A low pass filter, then rectified to drive a transistor.
That's how the old sound to light units used to work.

[Mooly]

Check out the LM3914 series of IC's,
Design for audio level meter?

[Zertman]

Thanks for the replies. I bought a TIP31 transistor from radioshack and hooked it up as shown in the circuit in the picture. The LEDs did react to the sound card sub output but only after turning the volume all the way up. Even then, they barely lite up with the bass notes. When I throw the switch so its bypassing the transistor, all the LEDs light up as brightly as they should but always on. How do I make it so that I don't need the volume high to get them to light up when using the transistor?

Here the my LED case circuit:

[Mooly]

First thing... have you got currrent limiting resistors in series with the LED's ? You MUST

(A typical LED drops around 2 volts give or take. So 4 LED's per series chain is around 8volts. 12 volts supply. 4 volts to "loose". Typical current say 5 milliamps. Resistors needed... around 800 ohm. You must fit these or it will end in tears. The preset goes in the bottom halve of the series pair. You then must AC couple the input to the base with a 47uf 16 volt cap. Adjust the preset until the LED's just begin to glow and then back the preset off a fraction.

How to bias the LED's so they respond to a lower voltage. Connect a 10K resistor and a 2K to 4K preset pot in series. Connect this across the 12 volt supply and connect the wiper of the preset to the base of the transistor.

[Zertman]

Thanks for your reply. If I hooked it up like I pictured, what could be damaged? Nothing seemed to go wrong the way I had it hooked up. I want to make sure I have everything correct before I remake the circuit so I remade it in the picture. Please look it over. Thanks.

New LED Circuit:

[Mooly]

That's OK.

It's the LED's that can be damaged. Imagine a perfect adjustable voltage source (like an adjustable PSU). If you connect an LED across this and turn the voltage up, no current is drawn until the voltage reaches a specific level (around 2 volts). Then the LED begins to draw current, say 10 milliamps. If the voltage is increased even a tiny bit above this the LED will draw much more current, perhaps 100 milliamp at 2.1 volts. The current rises rapidly and destructively.

(maybe all your LED's aready have a series resistor somewhere if they were originally fitted to illuminate a case etc... I'm referring to single LED's as you would buy them).

[Zertman]

Thanks for all your help. I have not yet changed the circuit because I have not been able to go to radio shack to get the parts. I couldn't find a 2K-4K pot in any online electronic parts stores. Do you know of one that has the pot I need or could I use a 0-4.7K pot instead?

From the factory, the case fan's LEDs where wired directly to each fan's circuit board. So I'm not sure how they were connected. There was also 5 LEDs on each fan but when I removed them from the fan's boards and connected 5 in each chain, they where really dim and barely lit. So I removed one LED on each fan and they all lit up brightly. I guess they wouldn't have lasted long connected like that because of what you said.

This is a link to the model of case I have:
A380_XClio

[Mooly]

The pots not critical at all. Anything up 10k or even higher would work. Your aiming to apply around 0.6 volts DC bias across the b-e junction of the transistor.

As you found out, removing an LED from the chain causes them to be bright. This is why you have to be careful. Blue and white LED's have an even higher forward voltage so that puts a limit on how many wou can connect in series off a given supply voltage.

[Zertman]

Success! Thanks again Mooly. You are awesome!

I took some video of the LEDs in action.

Here is a link to the video:
Computer case LED mod - YouTube