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Electret microphone noise

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hoghunter, Jul 13, 2017.

  1. hoghunter

    hoghunter Member

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    Last edited: Jul 14, 2017
  2. hoghunter

    hoghunter Member

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    I tried a quick test on the breadboard. I connected the DVR to 5v. I then connected the lead from the 'RECORD' pin to ground using a 22 ga solid wire like a switch and it did not trigger recording.
     
  3. hoghunter

    hoghunter Member

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    Here is a pic from the datasheet of the DigiKey mic with wires. It has 3 pins :eek:
    upload_2017-7-14_12-46-21.png
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A microphone has a very low output level. A microphone preamp increases its output level to make it more sensitive. We do not know if the input of the DVR is for a microphone level or for a preamp's output level.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The detailed datasheet from Knowles of the MR-28406-146 mic shows that it has an awful +8dB to +20dB peak in its output at 5kHz. It is usually powered from only 1.3V but it is functional up to 20V. It's circuit has a 10k resistor added from its output to its black ground wire. Its black wire should connect to the shield of its connecting cable.
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    You might need a DC-blocking, audio-coupling capacitor between the mic-output and the VCR-input.
     
  8. hoghunter

    hoghunter Member

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    I'd probably try that before anything else, thank you.
     
  9. hoghunter

    hoghunter Member

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    I have a very small cctv 'spy' camera that included an electret mic soldered to the board. I hooked it all up and got video and audio recorded on the DVR's micro SD card. Obviously any required circuitry for the mic is already on the tiny board. The audio was acceptable. I need to get some capacitors now.
    What specs should I look for in a capacitor? How many microfarad? I watched a youtube video of a guy that was testing an audio circuit with an electret and he was getting a steady buzz on the audio. He inserted a capacitor into the circuit and the buzz/humm instantly went away and he had clear audio. This is the camera/mic I tested. You can see the electret at top right.

    [​IMG]
     
    Last edited: Jul 14, 2017
  10. hoghunter

    hoghunter Member

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    What specs would you suggest I look for so I can go ahead and get a few ordered? I will plan on soldering the mic to a board along with anything else that is needed to get the audio I need. This is all going inside a weather proof enclosure. When I finally get all of this stuff configured properly, I can then think about getting a PCB designed and manufactured.
     
  11. ci139

    ci139 Active Member

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  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The Knowles mic you selected from Digikey is designed with "bellows" on it to be used under water. That is probably causing the awful peak in its frequency response. An ordinary omni-directional electret mic (most have only 2 wires, signal and ground) is sealed and rain drops make noises but do not enter it. The case and wires might get corroded of it is exposed to weather.

    The circuit with the spy camera mic has a preamp on it. Is that what you will use? It might already have an output capacitor.
     
  13. hoghunter

    hoghunter Member

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  14. hoghunter

    hoghunter Member

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    I will probably use the 'spy' cam and mic for something, but mic is not sensitive enough, so I want to choose a high sensitivity omni-directional electret to use with this new product I am putting together. When I look at the Hz & impedence, etc.. on the DigiKey list, how do I know which mic is more or less sensitive and what voltage rating should I be targeting? My product will have a main 12v circuit and a stepped down 5v circuit for the DVR.... thanks again for all of your help!
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is not the mic that makes sensitivity, instead it is the gain of the preamp that is determined by the value of a single resistor in the preamp circuit. This resistor can be a potentiometer then you can adjust it for as much sensitivity as you want.
    Since your recorder works with the mic that has a preamp but needs more sensitivity then a super-sensitive mic still needs a preamp circuit or it will not produce anything. So simply select a mic with "normal" sensitivity and a preamp circuit for it.

    All the electret mic circuits I made have an ordinary 2-wires Omni mic and a preamp that has a gain of about 150 times. I power the electret mic through a 10k ohms resistor that is fed a filtered 5V to 9V. The output capacitor value is calculated from the lowest frequency you want and the input impedance of whatever it feeds with a simple RC formula.
     
  16. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    The size of capacitor depnds on the frequencies wanted and the relevant impedances, however at a guess I'd say a 10uf 63v electrolytic with the + connected to the mic would be a good start.
     
  17. Mosaic

    Mosaic Well-Known Member

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    Mics have different 'pickup' patterns, if you need sensitivity perhaps you should browse the pick up pattern, e.g. cardioid. Omnis have the lowest directional sensitivity and collect more ambient noise.
    You do a DIY 'shotgun' type arrangement by placing the mic in a small can fed by straws of tubes of different lengths to optmize frequency range selectivity.
    Then there are shock mounts to consider if your application is exposed to vibration

    There is a yahoo groups on mics and preamps thats fairly active with all kinds of mics, custom made. Google 'Alice' microphone designs.

    You mention 'product' so I suggest you do a bit of research into the subject matter as there are a lot of design options and preamp configs based on what you're trying to accomplish. We can't advise unless we know the full specs and purpose of the product. Piecemeal info requests will get you piecemeal answers.

    There even maybe a way to use an LM324 opamp......(audioguru takes a deep breath now :D) .........ok maybe not.
     
  18. hoghunter

    hoghunter Member

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    Ask and ye shall receive.....:happy:
    Description: This mic will be installed in a small plastic case that will be placed adjacent to a deer feeder. It will be used for hunting predators at night. The unit is powered by (3) 18650 batteries. The purpose of this unit is to power up after sunset via LDR , detect motion via a motion sensor which will trigger a transistor and turn on (2) 3W leds as well as trigger recording of video and audio on a mini DVR with a micro SD card installed. The camera runs on 12v. The LEDS will have 12v and an LED driver board. The DVR is powered with 5v via a buck converter. I am still trying to figure out how to trigger recording on the DVR as well as the mic specs and circuit. Let me know what other info may be needed. Thanks.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    In the bush he wants to pickup animal or bird sounds all around so an Omni pattern was selected. The sensitivity is determined by the gain of the preamp.
    An LM324 is much too noisy (lots of hisss) to be a mic preamp. An audio opamp should be used instead.
     
  20. hoghunter

    hoghunter Member

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    My apologies.... the description above is for a 'feeder light' unit. The schematic I posted early on is for a unit to be used in the blind that will use an led monitor instead of lighting a feeder area. However, very similar circuitry requirements. Both will have the need to record clean audio, but sensitivity will need to be much higher at the feeder. The feeder light DVR will be triggered by motion where the other unit will start recording when the REC button is pressed manually.
    Sorry for any confusion
     
  21. hoghunter

    hoghunter Member

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    I ordered some of these a few weeks ago and still waiting on delivery:

    Will this do the job?


    They are only $3/pc
    http://www.ebay.com/itm/MAX4466-5V-Electret-Microphone-Audio-Amplifier-AMP-w-Adjustable-Gain/182338075949?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
    [​IMG]
     

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