The title says it all, really. Is there any way to create a sound dependent variable resistor?

For a halloween costume, I'm building up a mask featuring red LEDs (around 6-12) under a visor and a microphone hidden in a respirator.

Ideally want to set it up so the lights are half lit when I'm silent, but increase in brightness as I speak.

Now, I have no idea if this is possible, never done anything involving sound in circuits before and my knowledge is pretty limited to begin with.

Assume size isn't an issue for circuitry, the mask itself is hooked via a tube to a seperate unit, which I can increase in size to fit the circuit board, components, power supply, etc.

Thanks in advance!

 

Well, it is possible. A microphone is actually a sound to voltage/current converter if you think about it. Its like a very miniature power plant. As you speak louder the output of your microphone is larger. Now you need an amplifier to be able to use this correctly, but if you put the mic into an amplifier, if you speak louder your output is larger. Proportionately larger.

So you can easily come up with ways to make certain LED's be on while other LED's are off using this fact. I have recently discovered how useful comparator op amps are. Also you could use digital logic circuits.

The rest is up to you, but if you gave me about an hour I could probably come up with something LOL. But I wan't you to do it.

Can you please describe in a little more detail what exactly you want the LED's to do when you speak louder or more quietly? What kind of Halloween costume is this LOL?

 

I'm working on something similar. Not there yet there, have some other projects to finish first.

LEDs, 555s, Flashers, and Light Chasers (forum edition)

 

Bill, that looks like it'd be a great wealth of knowledge but my extent of electronics knowledge is a few months of college, before I changed courses. So naturally, that's all leaked out by now, I'm only really brushed up on the usage of basic components and how to knock it all together at the end.

Count, it's an odd little costume, we originally intended to 'acquire' some scrap metal and make MadMax-esque 'raider' costumes, but we ended up going a little overboard, and now we're adding mock respirators, lights and the like.

If a microphone is essentially changing sound to current, could I use that in conjunction with an op amp and a (primarily insufficient) power supply?

IE: the leds are dim because the voltage of the circuit is low, but as sound comes in, the voltage increases and the brightness changes?

 

The brightness would change disproportionately, you wouldn't see much change (it would be dim or very bright in a short range). Without a PWM circuit you will not get the desired effect as the light output from an LED does not have a linear relationship to the input voltage or current.

A PWM circuit can be built with a quad op-amp or a 555 timer and an op-amp, pick your poison.

If you wanted more of a VU meter feel, National Semiconductor make the LM3914 which has most of the components you would need for such a meter.

 

Hmm, if LEDs are an issue, is there an alternative?

Got enough room, for maybe 2 standard size LEDs side-to-side in the mask, but I could always fiddle a little extra room.

 

Unfortunately a light bulb and other types of lights all suffer from nonlinearity.

A simple PWM circuit will work. Here is one example of a triangle wave generator:

http://falstad.com/circuit/e-triangle.html

It is then only the addition of a comparator and a preamplifier for the microphone to do. Let us know if you need any help with those. This circuit will vary the brightness with a linear response. Using a quad op-amp like the LM324 would work.

 

Had a little mess around with that java applet (fantastic little thing!), and I'm having trouble working out how those pieces would fit together.

Very much appreciate all the help sofar guys

 

What do you need?

The PWM generator?

The circuit I linked to you was a triangle wave generator. If you fed in a signal and compared it to this triangle waveform, you would produce a series of on and off pulses. The on and off pulses would approximately correlate to the signal fed in. This means the LED would be on for say 10% of the time and off 90%, creating the illusion, through persistence of vision, that it is 10% bright. The LED has to flash fast enough to create this illusion, usually 1000Hz (1kHz) is enough.

The comparator?

The comparator is very simple, just look up "op amp as a comparator."

The microphone amp?

This would be a non-inverting amplifier for the microphone.