How can I build a sound controlled LED circuit without a programmed chip?

[ulverbeast]

Hey.

I am currently working on building a circuit that should receive sound input from an electret mic, and then based on the output voltage, light up a certain number of LED's. For example, if the sound is low it should light one LED, and if it's very high 3 LED's. I imagine that 3 LED's total is a good number.

I have tried different setups with no luck so far. Does anybody here have any experience with a build like this, and is it possible to make without a programmed chip?

 

[dlgoff]

Welcome to PF ulverbeast.

Is this a school project? What you are trying to put together is called a http://en.wikipedia.org/wiki/VU_meter" .

 

[ulverbeast]

Hello.

Yes, that is what I am trying to make, it is however, not a school project.

Thanks for your suggestion, I found a circuit though that'll do the trick.

http://www.discovercircuits.com/PDF-FILES/voltst1.pdf"

The only problem I have now is that I don't know how to connect the electret mic to the circuit, at the voltage input. Anybody that knows?

Edit: What I need is just to connect the electret microphone to the circuit, so that when it receives sound input, it'll send a DC current with varying voltage into the circuit.

 

[vk6kro]

[PLAIN]http://dl.dropbox.com/u/4222062/microphone%20amplifier.PNG

This amplifier gives a gain of about 70, depending on the transistor used.
It can give about 2 volts from an electret microphone.
I have used it many times and it works well.

The data sheets for the LM3914 and LM3915 LED driver chips say it is OK to apply an AC signal to pin 5 of these chips (the normal input) as long as the signal does not exceed 35 volts positive or negative.
This means you don't need to rectify the signal before using it. Rectifying it would raise problems of losing some of the signal across diodes.

So, you could just take the signal from the microphone amplifier above, directly to pin 5 of a LM3914 or LM3915, and then drive LEDs with it.

 

[ulverbeast]

Do you think I could connect that amplifier circuit to the circuit I posted previously, with the LM339 operational amplifiers, instead of an LM3914/5?

 

[dlgoff]

The circuit you posted uses two voltage dividers (made with 1% resistors) to set a high limit of 5.2 volts and a low limit of 4.8 volts. So with a 2 volt max. input, the LOW LED would always be on.

 

[vk6kro]

Yes, that is right. You would really want all the lights to go off if there was no sound.
Also, you would probably need to rectify the input.

Radio Shack, Fry's and Sparkfun don't carry the LM3914/5 but Futurlec have them for US$1.75.

Maybe they are becoming rare, but they are great chips if you can find them.