Well I am building a circuit for the set of a show that is submitting a pilot episode to Adult Swim. Basically it is 7 LEDs that need to light up in bar-graph fashion. I was going to use this exact circuit with a potentiometer to Vin.

However since they will be modulating this knob to the sound of voices I thought why not go the extra step and just make it respond to sound decibel levels.

Is there any easy/cheap way to do this?

Thanks guys.

 

You could use an IC for that - http://www.national.com/mpf/LM/LM3915.html#Overview

If you want a VU meter display with a log response, try the LM3916 - http://www.national.com/mpf/LM/LM3916.html#Overview

 

ok that's cool.
can I hook a small electret mic right up to the chips signal input?

And I am kinda confused on the reference input. Is that to change how much voltage is required to get the next LED to light?

 

You will need a preamp to raise the signal level out of the microphone - http://www.newcircuits.com/circuit.php?id=aum002

You might find that a more modern op amp will be a good substitute, like a TL092.

Is that to change how much voltage is required to get the next LED to light?

Yes, the reference voltage determines the voltage spacing between comparator stages. More voltage means it takes more input signal to light the next one in the series. Try using a trimmer pot as an adjustment to make the response look right to you.

 

Any other opamp suggestions? digikey only has that one in SM.

Or does the opamp not matter so much as I'm not interested in sound quality just voltage amplifying? Cause I have a number of different opamps. including the 741's and some audio ones. How about a TL071?

 

For a volume meter and audio amp the TLO71 would probably be OK, frequency response is the big issue. Figure an amp with a gain of ?what? 100 at 15Khz? This is pure guess on my part, but the old crop of op amps (such as the LM741 or LM324) have surprisingly little gain at those frequencies.

 

It might be best to look for a single supply op amp. The LM358 is one, but the TL071 is not. I may have made this more difficult than necessary by suggesting a more modern device.

 

You can usually use a dual supply op amp if you use a simple resistor divider to create a pseudo ground.

BTW, I am in the middle of a rewrite on my LEDs, 555s, Flashers, and Light Chasers (forum edition), it gets transfered to the blog when it is polished.

I haven't put the schematics up yet, but the flickering LEDs will apply, as it would be the front end of your circuit.

http://forum.allaboutcircuits.com/showpost.php?p=117649&postcount=13

The LM3915 already has a peak capture and hold as I understand it, so you may just want a simple amp. A simple transistor amp would also work instead of a op amp amplifier for this application.

 

ok so basically this will be people talking into the microphone and the LEDs need to light up based on volume level. So whatever frequency normal spoken language is in. lol

I will not be using a dual supply. so no negative voltages...
So you think the LM358 will work? I'm ordering parts now.

Also I need to design this so the output (the led's) are NOT twitchy. Say if you hold a note the LEDs should sit steady on whatever output it provides.

 

Depends on what you mean as twitchy, since volume varies so will the LEDs. The capture and hold will slow it down (go to max then slowly go down).

We've been cross posting here.

 

Ugh this gets too complicated too easily.

The mic will output an AC signal will it not? when this goes through a single power supply opamp it will only amplify the positive half wont it? so already the wave is looking choppy...Will this choppyness not affect the bar-graph by causing it to flicker?

I will attempt a schematic once I get home...

 

Yes, although some modern mics require biasing circuitry. A simple speaker also acts like a mic, and outputs pure low level AC (audio).

For your application you can loose half of the waveform and it won't matter.

I'm still not sure what you mean by flicker. Audio constantly varies it's volume, by my definition this is flicker if translated to light. It is going to bounce up and down. The capture and hold takes the max volume and holds it there (no flicker), but you don't want it to sit too long or it is useless.

Still using that fader circuit?

 

Ok so when the positive pulse hits the bargraph chip it will obviously take that voltage and light up the correct number of LEDs based on my bias. But in between positive pulses will it not cut the LEDs off? Or will the frequency of spoken word keep you from noticing? Could a capacitor in parallel with the input smooth things out?

I guess I just need to go ahead and build a prototype to see if this is feasible. The requirements of the people i'm making it for just wanted a knob that they would turn while they speak. But I want to go above and beyond. I have a 2 week deadline.

what fader? A project from a long time ago? probably not using it much. all my previous projects where to basically learn things. Now that I have a full time job I am somewhat rusty on my hobby electronics.

 

The voiced speech of a typical adult male will have a fundamental frequency from 85 to 180 Hz

says Wiki.

So I need an amp that will amplify that frequency. will the 741 or TL071 or any other common opamp do this?

Also on the Mic pre-amp can I just connect the non inverting input to ground? Wouldn't that just amplify the positive pulse since i'm not using a negative supply and don't care about the negative pulse? Or would there be no buffer for electrical/background noise? ugh...I need to brush up on op-amps. And it doesn't help that I have never used them for audio...

And I just noticed the preamp schematic is showing an inverting amplifier...Is this a problem since I don't have a negative supply?

 

http://www.youtube.com/watch?v=0-81vTGPemk