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Non contact AC voltage tester

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Discussion in 'General Electronics Discussion' started by Karthik rajagopal, Jan 13, 2017.

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  1. Jan 13, 2017 #1
    Karthik rajagopal

    Karthik rajagopal

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    May 9, 2016
    Hi all,
    I recently made a non contact AC voltage tester with IC 4017. It works well . In the video that I saw the person told that the clock input of this ic would detect the emf produced which causes the LED to blink at a higher rate. My circuit works just as he said but it also detect DC voltages that are produced in AC to DC adaptor. How is that able to detect DC voltages? If so it detects then why only the DC current produced in adaptors and not in normal DC batteries? Please help not able to understand the logic behind.
    The video is :
     
  2. Jan 13, 2017 #2
    Bluejets

    Bluejets

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    Those things have a fairly broad pickup....they can detect AC over the wiring.
     
  3. Jan 13, 2017 #3
    Karthik rajagopal

    Karthik rajagopal

    191
    5
    May 9, 2016
    Do you mean the ac voltage is not completely converted to DC? Please explain.
     
  4. Jan 13, 2017 #4
    Alec_t

    Alec_t

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    AC-DC adapters are not usually well screened, so stray radiated EM fields are present which can be picked up by your body and by nearby wires, such as the one you are holding in the pic.
     
  5. Jan 13, 2017 #5
    Karthik rajagopal

    Karthik rajagopal

    191
    5
    May 9, 2016
    Thank you for the reply.
     
    davenn likes this.
  6. Jan 13, 2017 #6
    davenn

    davenn Moderator

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    yup as a result, any reading you got would be totally unreliable
     
  7. Jan 15, 2017 #7
    Karthik rajagopal

    Karthik rajagopal

    191
    5
    May 9, 2016
    But I have heard many saying all microcontrollers need a very precise DC voltage for them to function. Then how are they able to work with voltages from AC to DC power supply?
     
  8. Jan 16, 2017 #8
    Alec_t

    Alec_t

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    The power supply to a microcontroller needs to be well decoupled (using low-impedance capacitors) so that stray EM fields are 'shorted to ground' and don't adversely affect the micro's performance.
     
  9. Jan 17, 2017 #9
    Karthik rajagopal

    Karthik rajagopal

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    May 9, 2016
    Does the value of capacitors used in decoupling differ for different IC 's?
     
  10. Jan 17, 2017 #10
    Alec_t

    Alec_t

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    In general, yes. ICs drawing heavy current will normally require greater decoupling than those drawing light current.
     
  11. Jan 18, 2017 #11
    BobK

    BobK

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    Typically, you need a 100nF ceramic cap near each pair of power pins.

    Bob
     
  12. Jan 18, 2017 #12
    Karthik rajagopal

    Karthik rajagopal

    191
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    May 9, 2016
    Thank you for the answers.
     
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