Sound capture (Part1)

Part 12345
On one side we have sound, on the other side we want volts. In between, we need a device which will convert air pressure changes into electrical signal changes: a microphone! The one we will use is of the electret type, nammed after the type of membrane in use. It is small, cheap and easy to set up, thanks to its permamently charged (lightweight) membrane, and to its integrated preamplifier. Follow this thread for more information.

Feeding the electret capsule is as easy as feeding its positive side (Terminal 1) to the power supply through a resistor, and collecting the signal from this same pin, through a capacitor. The negative side of the capsule (Terminal 2, the one which is wired to the capsule body) is directly wired to ground. Follow this thread for more information. Check this other thread for an insight of electret microphones and a fancy mod.

Unfortunately, this is not enough. The output from this type of configuration is no more than a few hundred µV, which is far beyond the specifications from Arduino’s a/d converter (5 volts in 1024 steps makes somehow 5000 µV of resolution! So that we need an amplifier in order to adjust the output level from the microphone to the input level of Arduino’s ADC. A good’ol LM358 will do the job for us. The non inverting input from one OpAmp is biased to half the DC voltage so that the signal poutput shall be offset by 2.5V. The inverting input receives a fraction from the output of the OpAmp as feedback signal. The gain is determined by the ratio of the two resistors forming the divider bridge at the output of the OpAmp.

There are a few extra passive components which role is to limit the noise generated by this amplifying stage. As usual, the goal is to a minimal number of extra components to achieve the most acceptable result.

The next task to accomplish is to sample the signal in a fast and accurate way. Here is the starting point to fast signal sampling

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  1. Allen says:

    Thank you for this series of posts.

    What is the value of C2 above?

    Regards, Allen.

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