Little electret mic preamp

Konusu 'Elektronik Devreler' forumundadır ve guclusat tarafından 2 Ocak 2016 başlatılmıştır.

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    Russ said he had trouble using a bare electret mic as an input device in his Arduino 201 sensors class. I hope to help by making up a couple of simple mic preamp boards. There was an initial prototype, and a short ‘production’ run with a better layout, allowing a smaller board and putting the header pins closer to the edge so they don’t obscure as much of the breadboard. I stuck a couple of the mics pulled out of the Sansa recorders on to try them out. I also played with code ’til I got a decent one-LED PWM “applause meter”.

    PreampSchematic.png

    It’s a very simple one transistor amplifier, and the output still rides on a DC offset, just like with a standalone electret mic. That was mostly so the students would have to deal with the DC offset. Overall gain is about 100. With one sample board, the analogRead() DC offset is about 320 (of 1024) and a loud noise gives a peak around 650 higher than that, so it’s a pretty good match for the Arduino’s A/D range.

    I tried a couple of approaches in the code to automatically remove the DC offset and “rectify” the signal. Final result was running batches of 600 reads (about 70 ms) and keeping both the max reading and the average over that time. With that many samples effectively at random times within the audio waveform, statistically we’re pretty likely to hit close to at least one near-peak reading each run – and it seems to work pretty well in practice. Students can start with a simple version of the code ( loop() is only 9 lines). There’s also a fast attack/slow release feature in an independent, additional block of code. The code is in the W88 shared dropbox under Arduino201ScratchSpace.

    While the target use case is on a breadboard, since I’m lazy and wanted to try it out on a bare Arduino, I put the 3 header pins in A0,A1, A2. I used one as the obvious analog input, and the others as digital outputs, one constantly putting out HIGH, the other LOW to provide approximately +5 and ground. Worked fine. It responds to normal conversation maybe 2 feet away from the mic with a visible brightening of the LED. A handclap shows a bright attack/release. I hope Russ finds the boards useful.
     
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    2 transistors NPN BC548 (experiment with others if you like for example: 2n2222, c1056, etc)

    1 Jack plug Male Stereo

    1 Switch

    2 Electrec mic

    1 Little piece of hose transparent from my old Acuarius Tank

    1 Battery holder 2 AA
    FFAEGZGMBXEV2Z4NZA.LARGE.jpg
     
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    Small Electret Microphone Pre-amplifier

    electret-microphone-pre-amp.jpg

    The mic pre amp described here is designed to connect a small electret microphone and uses an integrated circuit type NE5532.

    As you can see, this scheme is very simple. The use of a dual operational amplifier type NE5532 is a large part in simplicity, although it was quite possible to simplify even more by using a single transistor.

    Polarization of the microphone
    The electret microphone is fed Due to the resistor R1, 4.7 K. The value of this resistor should be adjusted (increased) if the power is chosen higher than the one proposed here (9V). The capacitor C1 prevents this voltage reaches the microphone in the first op amp, the reference voltage can be shifted and a problem of distortion of the high levels (asymmetrical clipping).

    Small electret microphone gain
    The op amp provides a first amplification of 10 (20 dB), this value is equal to the ratio of resistors R3 / R2. The second op amp is mounted in exactly the same way as the first, and also provides a gain of 10, and the ratio R5 / R4. The overall gain is equal to 100 (40 dB). If you want to be able to vary the gain, simply replace the resistor R5 of 100K with a 220K potentiometer in series with a resistance of 10K. The overall gain in this way can vary from 10 to 200.
     
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    mic pre.png

    Farklı bir şema daha
     
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    FFMLVLDGMON5YBF.LARGE.jpg

    Q1 = 2N3904
    R1 = 10k
    R2 = 100k
    R3 = 10k
    C1 = 0.1μf or if you prefer 104 or 100n
    C2 = 0.1μf or if you prefer 104 or 100n
    MIC1 = a common electret microphone
     
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