Condenser Microphone

Reported by Soyoung Park,May 2007

This report is for getting proper values from this small microphone for any projects. People easily get this, but the value is not stable. So we can't use it as a real microphone. It works like wind a detector, because it just give us high value when we blow into it. I saw many students have trouble, not only me but also other people did. Actually, we can use a big condenser microphone with complicated amplifier device for detecting sounds. However generally it's too big to use for projects and expensive. If this small microphone can catch sounds well, we can save money and make clean projects. On this account, I decided to challenge it for people.


Unidirectional Condenser Microphone Element from RadioShack $3.69 / Model: 27-101, Catalog #: 270-101

Product Summary This unidirectional condenser microphone's small size makes it an ideal replacement in hands-free microphones, digital recorders,and custom projects needing a high-quality condenser microphone element.

  • Allows your to fix that old microphone
  • Makes replacement of the element easy

Features

  • Sensitivity: 47dB
  • 100-20,000Hz frequency response
  • 6mm diameter-2.7mm high

This is the only data sheet on the back of it's package.


Before we go to connect with micro controller, here's about basic SOUND INPUT.

-The microphone is the most basic of all audio transducers. Microphones aren't as easy to use for analog input as the variable resisters. Working with microphones presents a few challenges. Microphones convert air pressure changes to changing electrical voltages. Audio electrical signals are AC signals. This means that they're typically centered at volts and vary in both the positive and negative direction. They are divided into two categories: microphone-level signals and line-level signals. Microphone-level signals range across a few microvolts or millivolts. Line-level signals vary from -1 volt to 1 volt. Line-level signals are produced by CD players, MP3 players, turntables, and other amplified devices. In order to bring a microphone signal to line level, we need an amplifier. Any voltage from an audio input goes up and down in an AC wave. When the sound energy is converted to electrical energy, the volume is represented by the height of the wave above or below the horizontal axis. This height is referred to as the amplitude of the sound. For our purposes, amplitude is the same as volume or loudness. In order to detect sound levels on a micro controller, you will have to amplify the signal coming from your microphone.

- source from Physical Computing Chapter 13 " Controlling Sound"


First step : Amplifier

We need amplifier to make signal bigger.

$1.39 from RadioShack

-Operational amplifier

-LM386 data sheet

circuit diagram for setting up LM386 - source form Physical Computing

amplifier on breadbord.

value after amplifying

The value is line-level signal from the amplifier. It varies from -1 volt to 1 volt. When I blew into it, value jumped up.


Second step : Rectifier

Rectifier serves to create a unipolar signal from a bipolar signal.

We can set up the rectifier by using diodes. -Diode bridge

bipolar signal changed to unipolar signal.


Last step : Low Pass Filter

It removes the high part of the signal, allowing the low part to pass.

I tried lots of capacitors and resistors, finally 10kohm resistor and 0.01uF worked.

Now value is stable!!!


I checked every step's signals to make sure differences.

You can see differences at a look.

This is the Final circuit diagram.


Limitation

Now we can get stable value from this small & cheap microphone, but it won't work with complex sounds like speech or multi tonal musical sounds like chords. However for simple pitches, it will work well.

tip: If you put a horn, it gathers sounds better.