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Adafruit MAX4466 mic board connection to audio in on mini DVR

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hoghunter, Jul 23, 2017.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Lithium batteries are dangerous (explosion or fire) if discharged to less than 3V then normally charged. Your idea has nothing to detect a low voltage then disconnect the load. I do not know if your charger will detect a low voltage then attempt to charge safely.
    Their voltage is 4.2V when fully charged so two cells produce 8.4V that might zap your DVR and their low voltage is 6V that is too low for many 5V regulators But a "low dropout" 5V regulator will work.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    ALL ICs need a supply bypass capacitor with short leads mounted very close to the supply pins to avoid oscillation and noise problems.
     
  3. hoghunter

    hoghunter Member

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    I was not aware the mic could take up to the neighborhood of 10v for some reason. Yes, I will definitely reduce the resistance to 15k. I will be sure to only use that calculator for my leds from now on.

    On the last point, I will build another circuit with the regulated 5v feeding everything and see what results I get. I may eliminate the resistor before the mic altogether for that test circuit. So, 5v to the mic, the DVR and the circuit. I guess I can leave the voltage divider as is and expect 2.5v to pin3...
    thanks again for your help and patience.
     
  4. dave

    Dave New Member

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  5. hoghunter

    hoghunter Member

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    I have a BMS pcb connected for charging the 2 li ion batteries.
     
  6. jjw

    jjw Member

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    You can't remove the resistor from supply to the mic.
    There will be no signal from the mic without the resistor.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You need to calculate the current-limiting resistor for an LED because an LED has a range of forward voltage. Look at the datasheet for an LED. A white or blue LED might have a minimum forward voltage if 2.8V and a maximum forward voltage of 3.6V and a "calculator" program does not know the voltage range of your LED. The calculator guesses that you bought thousands of LEDs, measured their voltages and picked one that is 3.2V. You must calculate the resistor value so that if the LED has a low voltage then it does not burn out or if it has a high voltage then it is bright enough.

    Then the mic will not have an output because its output is shorted to the +5V supply. The resistor is the load of the Jfet inside the mic allowing its output voltage to swing up and down with the signal.
     
  8. hoghunter

    hoghunter Member

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    I need the resistor as a pull down so the electret can perform it's function?
     
  9. hoghunter

    hoghunter Member

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    Yes, I use several different types AND colors of leds. From 3mm to high power 'star'' types, red, green and infrared. They differ in voltage up to 2.5v and go from 20ma to 750ma. I do enter that info from the datasheets into the calculator and so far so good. All leds have been performing well.

    I understand. Thank you very much for explaining. It is the same principle as when I have a pull down resistor on the base of my 2N3904's for switching the gate on the transistor.......
     
  10. hoghunter

    hoghunter Member

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    I thank both of you for your input and clarifications. I am learning a lot., :woot:
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes of course different LEDs have different voltages. But I am saying that one LED part number will have some LEDs with a low voltage and some will have a high voltage. A "calculator"simply guesses at one voltage that produces a current too high or too low. You are lucky that your current was not high enough to cause damage.

    No. The load resistor of a transistor to its power supply is at its output, a base resistor to ground is at its input which produces a completely different effect.
     
  12. hoghunter

    hoghunter Member

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    Update... I have a 9v battery going to the DVR (regulated to 5v). I have a separate 12v supply to my preamp & mic. I get perfect audio recorded to the DVR. The preamp circuit is working great! However, I then connected my cctv board camera and monitor to the mix to add video, powered by the same 12v battery and got a noticeable hiss caused by the small 5" led monitor and a loud buzz caused by the camera. I then powered the camera and monitor with another 12v supply and got great audio and video recorded to the DVR. I need to get some wildlife audio/video recorded for a website over the next few days, so next week I will be troubleshooting to see if and how I can power the mic/sound circuit, DVR, monitor and camera all from one 12v, 7AH battery source.
     
  13. jjw

    jjw Member

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    Have you tried to filter the mic supply as AG said in a message #67 ?
    Make the supply resistor R12 of series connected 10k and 4k7 resistors and put 100uF from their connection point to ground.
     
  14. hoghunter

    hoghunter Member

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    No, I haven't yet. I will definitely try that one evening while down at the ranch. Thanks for the reminder. I recall a video omline where a guy had a steady strong buzz in his audio set up on a breadboard and he simply added a capacitor into the circuit and the buzz completely disappeared. Will try it for sure !!
     
  15. hoghunter

    hoghunter Member

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    Like this? I can try it while in south Texas this week....
    upload_2017-8-6_9-34-31.png
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No. The resistor is the load of the junction field effect transistor (Jfet) inside the electret mic and it feeds positive voltage and current to the Jfet. The Jfet in an electret mic uses about 0.5mA and is an impedance converter that converts the extremely high impedance of the "condenser" part of the electret mic to a usable lower output impedance. The electret material permanently stores 48VDC so that the condenser part of the mic works as a capacitive voltage divider that changes its extremely high impedance output voltage as the audio vibrates the diaphragm closer and farther from a fixed part of the capacitive voltage divider.
     
  17. hoghunter

    hoghunter Member

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    Thanks for taking the time to explain. It's a lot to absorb, but I think I get the idea.

    Did you see my schematic above based on jjw's input? Do I have it right?

    Thanks again.
     
  18. jjw

    jjw Member

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    Yes.
     
  19. hoghunter

    hoghunter Member

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    Great!! Thanks for confirming.... I will solder up another board that will include that filter and see what happens. :)
     
  20. hoghunter

    hoghunter Member

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    I have added the camera and monitor to my schematic, along with the mic filter. The DVR gets 12v stepped down to 5v. The mic & sound circuit gets 12v with the mic itself getting approx 4.5v. The cctv camera and monitor get the same 12v.
    So everything is powered by the same 12v - 7AH lead acid or AGM battery. How does this look?
    upload_2017-8-6_18-8-6.png
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I am confused when you say "Aud in -" on the mini-DVR instead of saying "audio in ground".
     

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