String of LEDs in parallel w/correct resistors and a TIP31 Transistor.

• Defkon
In summary, the problem is most likely a DC offset in the circuit, caused by improper wiring. The solution is to bias the transistor and then connect it to the computer via a series capacitor.
Defkon
I have a string of leds hooked up together with the correct resistors in parallel, going out to a TIP31 transistor which then goes to the power cord and to a 3mm jack. This 3mm jack goes into my computer and my leds "dance" to the music, I assume is the change in voltage. Anyways, I have hooked this up to one of my computers and it works flawlessley, but when hooked up to my other computer, the LEDs light up very bright and do nothing else. I am not sure how to begin debugging or how to fix it.

If it makes any difference, I am using a sound card in the second PC. Any nudges in the right direction or solutions are appreciated.

It sounds like you are using the transistor as a switch. Think about how much current it would need to "turn on", and what the current requirements likely are of the sound cards. They may be intended to drive different things.

What do you think the input impedance is of your LED circuit? Have you thought of measuring it across different frequencies?

Good job tinkering, this was one of my first projects!

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Sounds like a problem of DC offset to me. Can you show us the schematic diagram?

If you are connecting the base of the transistor directly to the computer sound card speaker output, then this could be risky.

It would be better to bias the transistor properly and then connect to the computer via a series capacitor. Maybe 10 uF would be a good starting value.

Measure the current your LEDs use. Divide this by the current gain of the transistor to get the base current.

If you don't know either of these, assume 2 mA for the base current. These transistors have a gain of up to 50, so this would allow up to 100 mA for the LEDs.

Then calculate the size of the base bias resistor.
It will be (supply voltage minus 0.6 volts) divided by the base current (in Amps).

So, for example, if the supply voltage was 12 volts and the base current was 2 mA, the base resistor would be (12 - 0.6) / 0.002 or 5700 ohms, so you would use a 5.6 K resistor from the +ve supply to the base.

You may have to increase this resistor value until the LEDs just light up with no sound and then light up to full brightness with loud sound.

Thank you for the responses, I have to travel for work the rest of the week, but I will start testing next weekend. I believe I need different resistors b/c this is not an onboard sound card. By the way I'm using 100 ohm resistors. and a 5v psu with 700ma.

Got it working, I just pulled the other card out and used the onboard sound. Thanks for your help!

What is a TIP31 Transistor?

A TIP31 Transistor is a type of electronic component that acts as a switch or amplifier in an electrical circuit. It is commonly used in LED lighting circuits to control the flow of current.

Why is a TIP31 Transistor used in a string of LEDs?

A TIP31 Transistor is used in a string of LEDs to regulate the amount of current flowing through the circuit. This helps to prevent the LEDs from receiving too much current, which can cause them to burn out.

What is the purpose of using resistors in a string of LEDs?

Resistors are used in a string of LEDs to limit the current and voltage in the circuit. This prevents the LEDs from receiving too much power, which can cause them to burn out or become damaged.

How many LEDs can be connected in parallel with a TIP31 Transistor?

The number of LEDs that can be connected in parallel with a TIP31 Transistor depends on the specific characteristics of the LEDs and the transistor. It is important to consult the datasheets for both components to determine the appropriate number of LEDs to use in the circuit.

What happens if the correct resistors are not used in a string of LEDs with a TIP31 Transistor?

If incorrect resistors are used in a string of LEDs with a TIP31 Transistor, the LEDs may receive too much current and become damaged. This can also cause the TIP31 Transistor to overheat and potentially fail. It is important to use the correct resistors to ensure the proper functioning and longevity of the circuit.

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