Amplifying Audio signal for use with TIP31 power transistor and LED array

I have an analog circuit that takes audio input and amplifies it to 0v-5v to power an LED array and has a potentiometer to reduce the gain. I am using 5v because my power source is a portable USB battery. Anyway, how do I amplify the audio so that weaker audio signals can light up the LED array better? I have on hand some 2N3904 transistors and a couple of TL082 opamps, but I'm not sure how to use them to amplify the audio signal.

I would be very thankful if someone could provide me with a circuit using 2N3904 transistors of a TL082 opamp to amplify an audio signal for use with a TIP31 power transistor.

Several things puzzle me about this project,

How do you expect to drive a matrix from an audio signal, a matrix needs precicse signals driving coloum and rows? Where does the Arduino fit in? Why a TIP31? These things have a very low gain. What do you hope it looks like at the end?

It sounds to me like a crappy instructables project.

I am using the TIP31 because it initially made it extremely simple to amplify my audio signal to 0v-5v. I am not driving a matrix, it's just a bunch of LED's in parralel. It's also completely analog, so no Arduino (yet). Simply put, I just need to know how to make a (pre)amplifier circuit using either a TL802 opamp or some 2N3904 transistors.

You need to know what the input voltage is, then you can set the gain of the circuit correctly. You also need to tell us what value of supply voltage you have available.

There's about a bazillian pages on the web that tell you how to use an op-amp to amplify a signal.

Why a transistor? Why not use a chip that's designed to take an audio input and light up some LEDs?

eg. AN6884, LM3914, etc.

The AN6884 even has a built-in amplifier....tada!

You can turn on & off* LEDs with a transistor, but making a linear amplifier requires a few resistors & capacitors, and some careful design. Depending on how much gain and you need, you may need multiple stages (with at least one transistor in each stage). [u]Here[/u] is an example.

Sure, in the "old days" amplifiers were designed with discrete transistors but it's a LOT easier to use op-amps. With an [u]op-amp circuit[/u] you just need two resistors to set the gain. (But most op-amps only about enough current capability to drive one LED.)

  • With PWM (built-into the Arduino) you are turning the voltage on & off rapidly and that allows you to dim an LED (or appear dim, or off, to the human eye). With PWM, you can use a single transistor or MOSFET in a simple circuit to "boost" the voltage/current and dim/control multiple LEDs.

Grumpy_Mike: You need to know what the input voltage is, then you can set the gain of the circuit correctly. You also need to tell us what value of supply voltage you have available.

Other than the 5v USB battery that I specified above, I don't know what you mean.

fungus: There's about a bazillian pages on the web that tell you how to use an op-amp to amplify a signal.

Why a transistor? Why not use a chip that's designed to take an audio input and light up some LEDs?

eg. AN6884, LM3914, etc.

The AN6884 even has a built-in amplifier....tada!

Like I said, they're what I have on hand. I don't have time to order/get new components.

DVDdoug: Sure, in the "old days" amplifiers were designed with discrete transistors but it's a LOT easier to use op-amps. With an [u]op-amp circuit[/u] you just need two resistors to set the gain. (But most op-amps only about enough current capability to drive one LED.)

I tried following the non-inverting amplifier circuit exactly, using a potentiometer for R1 and R2 and my audio signal going directly into Input +. I assumed that 0V means ground, and wired it as such. I measured the voltage of Output with my multimeter and the voltage stayed constant regardless of any adjustments to my potentiometer.

Here is my exact circuit:

I assumed that 0V means ground,

No.

First off what op amp is it? What sort of microphone? 0V is the bias level, it is only ground if you are supplying the op amp with a negative supply as well as a positive one. It is normal to create a 0V by having two 1K resistors between + and -, the junction creates a 0V. There should be a resistor, say 10K from the +ve input to the 0V. The pot should be connected to 0V not ground. Depending on the type of microphone you might need a capacitor to couple it an and some DC bias fed into it. The other end of the microphone goes to 0V and not ground.

Grumpy_Mike:

I assumed that 0V means ground,

No.

First off what op amp is it? What sort of microphone? 0V is the bias level, it is only ground if you are supplying the op amp with a negative supply as well as a positive one. It is normal to create a 0V by having two 1K resistors between + and -, the junction creates a 0V. There should be a resistor, say 10K from the +ve input to the 0V. The pot should be connected to 0V not ground. Depending on the type of microphone you might need a capacitor to couple it an and some DC bias fed into it. The other end of the microphone goes to 0V and not ground.

Like I said above, I'm using a Tl082 op amp.

Grumpy_Mike: 0V is the bias level, it is only ground if you are supplying the op amp with a negative supply as well as a positive one. It is normal to create a 0V by having two 1K resistors between + and -, the junction creates a 0V. There should be a resistor, say 10K from the +ve input to the 0V. The pot should be connected to 0V not ground. Depending on the type of microphone you might need a capacitor to couple it an and some DC bias fed into it. The other end of the microphone goes to 0V and not ground.

^^^This was EXTREMELY helpful, and the first time I've seen this SPECIFIC information in all the weeks of research I've done into making LED's flash to music.

I'm using a Tl082 op amp.

Not a good choice. The data sheet shows +/- 5V as the lowest you can use. With just a singe 5V supply in effect you are running this from +/- 2.5V. Also you can only get to within 2.5V of either supply rail. Use an op amp that is designed for a lower voltage, like the MCP602. There are lots of others to choose from as well.