Magnetic field detection using tank circuit

Discussion in 'Test & Measurement Forum' started by shibin_varghese, Aug 16, 2019 at 12:57 AM.

  1. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
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    0
    Hi Friends,
    I have a doubt.
    I would like to make a tack circuit (LC circuit) which can detect a magnetic field.
    Its purpose is to make a water meter. When the dial in the meter attached with a magnet rotates my LC circuit has to charge to 3.3V or discharge to 0V.
    Then only my microcontroller can count the number of rotations and thus the water consumption.
    Can anyone help with this?.
     
  2. Picbuster

    Well-Known Member

    Dec 2, 2013
    933
    121
    Use a partial sealed hall sensor a lc circuit is more sensitive to external signals like lf,hf,vhf and pulses.
    Filter and use a Smith trigger to feed the pulse into mpu.

    However water volume meters(pulse) are available on the market.
    For household volume (20 ltr/minute approx) not expensive. Even cheaper when a non calibrated is needed.
    Pulse modules for common water meters are also available. consult water meter supplier in your country.

    Picbuster
     
  3. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
    49
    0
    Thank you for your reply.
    I know a hall effect sensor is enough for the same but I would like to make it with an LC circuit.
    Do anyone have any idea about the same
     
  4. bertus

    Administrator

    Apr 5, 2008
    19,686
    4,051
    Hello,

    Could you make a drawing of the flowmeter you want to make?

    A LC circuit would work, when you make an oscillator with it.
    The magnet will change the frequency when it is near the coil.
    The frequency change can be measured/detected.

    Bertus
     
  5. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
    49
    0
    This meter has a rotating dial with a magnet placed on it and will complete one complete rotation let's say in every 100 liters.
    In order to sense the rotation of the dial, I would like to use an LC circuit.
    which will make an interrupt when it gets a magnetic field.
    When the magnet reaches the field of LC circuit, it should charge or discharge so I can easily determine one rotation is completed by rising or falling edge trigger as an interrupt.
     
  6. LesJones

    Well-Known Member

    Jan 8, 2017
    2,196
    653
    I don't understand why you want an LC circuit. Just an inductor will generate a small voltage as the magnet passes by it. The problem I see is that the voltage it proportional to the RATE of change of the magnetic field. (There is no voltage no matter how strong the field is when the field is not changing.) This would mean that with very low flow rates you may not detect a voltage. You would not have this problem with a hall sensor. The method suggested by bertus would work but it will not be the magnetic field that changes the frequency it would be the presence of ferromagnetic material (The magnet) close to the inductor that would change it's inductance thus changing the resonant frequency. (It would probably lower the Q of the inductor when it was close to it as well as increasing the inductance.) Why do you want to NOT use a hall effect device ?

    Les.
     
  7. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
    49
    0
    I
    I have already made the same using a hall effect sensor and is working fine.
    Now I am looking for some new and interesting methods to reach the same goal.
    But I don't have a clear idea about this.
    That is why I am looking for some ideas over here.
     
  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
    6,230
    1,352
    An example where a coil is used to detect a magnetic field was an old mailing machine that had to detect rotations of a shaft so it had a small coil with a small iron core and the magnet would pass by the coil and generate a spike like voltage. However, in this application the shaft rotated fast and there were a lot of turns on the coil so the voltage was significant even though it was present for only s short time for each rotation.

    The thing is, slowly changing magnetic fields do not generate much voltage in s coil so as Bertus suggested you might try an oscillator that depends on the coil to oscillate and when the magnet comes near it will change the frequency and that can be detected and acknowledged by the microcontroller as one rotation. This circuit and coil would probably be similar to a metal detector, although you could make the coil smaller.

    Oh yeah BTW an iron core works better than an air core in this kind of application.
     
  9. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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  10. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
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    Can you explain it in detail? Or do you have a schematic diagram?.
     
  11. shibin_varghese

    Thread Starter Member

    Jan 14, 2019
    49
    0
    Can you explain it in detail? Or do you have a schematic diagram?.
     
  12. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Well there is no schematic really it's just a coil of wire wrapped around an iron core of the type used for transformers and inductors.
    There was a steel sprocket that turned and each tooth that would pass the iron core tip would generate a spike that would be detected as a partial rotation equal in degree to the number of teeth on the sprocket divided into 360 degrees.
    The 'circuit' would be maybe an amplifier and a microcontroller.
    But again, you might not be able to do this anyway because you have a very slowly changing magnetic field which is not the same thing, so you need to go to the oscillator circuit. I'll see if i can find one if no one beats me to it.

    I just did a search for "oscillator using a coil" and it came up with a huge number of hits that contain circuits and videos of tutorials.
     
  13. LesJones

    Well-Known Member

    Jan 8, 2017
    2,196
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    This is what MrAl is describing in his first paragraph. I used it to drive a tachometer on my lathe before I changed to a hall effect sensor.
    IMG_0346.JPG
    The coil was remove from an old CFL lamp. There is a magnet fixed to the top of the iron core so that the gear wheel does not need to be magnetised. This is called a variable reluctance sensor. I changed to a hall effect sensor as this did not work well at very low speeds.

    Les.
     
    MrAl and shortbus like this.
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