Metal detector sensors

Discussion in 'The Projects Forum' started by BigbearWolf 99, Apr 28, 2018.

  1. BigbearWolf 99

    Thread Starter New Member

    Apr 26, 2018
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    I'm attempting to prototype a metal detector device using activity monitoring algorithms, the unit intended is as little as a brecelet but I'm also aware of the fact that for this idea to be feasible I'll need a much bigger power source supply a bigger bettery to create the electromagnetic field metal detectors possess, how do i go about this, the only challenge I have is that the power source is bigger than the suggested unit, imagine a wearable solution that detects metal objects
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    I think you stated the solution -get an appropriately sized battery.

    Why do you think you need much of a magnetic field?
     
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  3. Colin55

    Member

    Aug 27, 2015
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    If the coil is 2cm, the range is 2cm.
     
  4. DickCappels

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    Aug 21, 2008
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    Two cm is not much. If current is a big problem you can probably extend the range for a given amount of current by making the coil larger or if compatible with the design, adding turns to the coil for a given current. Most designs I see use very little current. Do you have a particular design in mind?
     
  5. Colin55

    Member

    Aug 27, 2015
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    adding turns to the coil for a given current does not increase the sensitivity
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    I guess that depends on how the detector works. It increases magnetic flux from the coil but there might be something on the sensing side of the equation that counteracts that effect. Are you thinking of a particular circuit?
     
  7. BigbearWolf 99

    Thread Starter New Member

    Apr 26, 2018
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    The device is intented to read objects from a calculated distance hence the size connotation, the objective here is for the user to obtain the benefit of predetermined experiences by this i mean after the device has detected the hazard object it must some how convey that massage to the user through alarming vibrational signals earning the user a predetermined experience to avoid the predicament at stake
     
  8. -live wire-

    Active Member

    Dec 22, 2017
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    You don't need a powerful battery to get many killowatts. That is, for a tiny fraction of a second. Try making a circuit to pulse supercaps through the coil. They maybe charge 99% of the time and discharge for 1% of the time. BUT, you have a little less than 100x the power, hypothetically.

    So let's say you have a 5V 2A battery. You will probably not be able to keep it small and get too much more power. So here is one approach to get pulses of high power through discharging capacitors. Let;s say you want to acheive a sample rate of at least 10 per second AND pass 100 amps through a coil momentarily.

    First make or buy a cc boost converter. It should acheive a constant current of ~1.5A on the input. It should not draw too much current from the battery, and should limit the output voltage to acheive that. It should have an efficiency of 80+% and have very fast regulation. It should be limited to 32V on the output. Try looking for a commercial product, or modifying one. Then, have that charge an ultra-low ESR capacitor that is about 250uF-10mF and rated for at least 35V. It should be able to handle high power pulses.

    Now here is what happens when you try to charge a capacitor with the only resistance that of the wires.
    http://mustcalculate.com/electronic...scharge.php?vfrom=0&vto=30&vs=32&c=2.2m&r=80m
    That is, if you have a power supply that can supply 30V and 100s of amps. This would almost certainly create a momentary short and damage a lot of stuff. So, you could use a resistor. But that supplies very little power as it approaches the max voltage. So you are still limited to 10W max here.
    http://mustcalculate.com/electronic...ischarge.php?vfrom=0&vto=30&vs=32&c=2.2m&r=90
    Now when it gets to 25V...
    http://mustcalculate.com/electronic...scharge.php?vfrom=25&vto=30&vs=32&c=2.2m&r=90
    Look at how it only draws about 77mA*30V = 2.3W. That is a very small fraction of the input power and less power means more charging time.
    But instead you can use CC to ensure that it is being charged as quickly as possible and you are using the most power you can. Initially, it will try and draw a lot of current on the output. But it supplies a constant current of 1.5A. So it drops the voltage by a lot. Then it needs less current as it is charged, so it can raise the voltage and still get that 1.5A. Then it gets close to the maximum voltage.

    It will never reach that 32V. It will get infinitely close but never get there. You can see how it over 1/2 of the time getting it from 30 to 31.9V.
    http://mustcalculate.com/electronic...charge.php?vfrom=0&vto=31.9&vs=32&c=2.2m&r=10
    http://mustcalculate.com/electronic...harge.php?vfrom=30&vto=31.9&vs=32&c=2.2m&r=10
    Even with CC it will never get there. It will still waste time on the last few volts. So you will want to start discharging it at, say, 30V. This way you get a decent voltage (and power) but can still discharge it many times per second.

    Now what does this kind of discharge look like? You have charged it to 30V and now are discharging it through a .3 ohm load. Let's say it's mostly the coil with it's many turns but also a little bit through the other wires.
    http://mustcalculate.com/electronic...scharge.php?vfrom=30&vto=.5&vs=0&c=2.2m&r=.25
    You can get much more power than on the input. Over 30V at 120 amps! But again, you spend a lot of time towards the end and do not get a lot out of it. For the second half you get less than 3% of the power initially. You want a long duration, but at a certain point it is not worthwhile, given the increased charge time.
    http://mustcalculate.com/electronic...ischarge.php?vfrom=5&vto=.5&vs=0&c=2.2m&r=.25
    So let's only discharge it to 10V, then start charging it again.
    http://mustcalculate.com/electronic...scharge.php?vfrom=30&vto=10&vs=0&c=2.2m&r=.25

    Now one more really important thing. YOU WILL GET HUGE INDUCTIVE SPIKING IF YOU DO NOT INCLUDE PROTECTION! You will need a really powerful diode or other protection system. It will try to continue to push 100 amps through your components. This will lead to very bad overvoltages and destruction. So make sure to add that diode or other system. You can alternatively use a fet made for brief pulses of higher power, or add resistors and other things. And just to be safe add fast zener diodes to prevent overvoltages across all of the components there.

    Now how do you switch from charging to discharging? You need a scmitt trigger or something similar to control some MOSFETs. Its input should be connected to the capacitor to measure its voltage. When it is below 10V it would turn on a low power fet that only needs to handle the 1.5 amps. You would have an inverted output for that. It simultaneously turns off the high power mosfet (designed for such pulsing) controlling the discharging, through the normal output. Then once it reaches 30V, the trigger pulls the charge fet low and the power fet high. But you will need a good driver for that that is designed for pulsing a fet like that. The driver should be connected to the battery. It would use the 5V 2A to drive the fet, but would be controlled by the trigger. It may require an amp or two to control the fet with that much power.

    Now for the microcontroller part you have to have an output of the trigger go to it. That way it knows if it is being charged or discharged and can know to take measurements. You will want it to have a fast clock so it can react quickly and take accurate measurements. You could probably find a low power wifi board to send data to your phone. Some boards even have built-in wifi.

    In terms of making it wearable, you could probably make it watch-like and have a long, kind of tall housing for the electronics. Some components may be a little big, but you will probably be able to fit it all in in the end.

    Obviously you can use different values than I stated in the example. A more powerful battery would certainly not hurt, but a do not use one that is too much weaker. You can try increasing the voltage for the capacitors more. Just don't use anything too high, for safety reasons and practicality. I do not recommend above 100VDC. And also you really should not go lower with the voltage for efficiency reasons. If your coil has a different resistance, you will want to adjust some component values.

    You will want to maximize the inductance but minimize resistance. More turns means more inductance but greater resistance. So you will want to use many turns of low resistance (thicker) wire. But not so thick that you get a lot less turns. Google the resistance per foot for the AWG you use and try to get as many turns as possible while not exceeding 250 milliohms.
     
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  9. BigbearWolf 99

    Thread Starter New Member

    Apr 26, 2018
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    Thanks this has been very much helpful and insightful, from henceforth I'll incorporate the use of eletronic principles received from this detailing with more cautiousness and care should I get stuck in the middle of this project I'll always come back here for applicable suggestion, recommendation and annotations.
     
    Last edited by a moderator: Apr 30, 2018
  10. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I can't follow Post #7. Do you only have to detect the presence of an object?

    What function do you want upon detection? Vibration?

    Are you trying to make a miniature metal detector for your wrist?

    A safety device........or Maxwell Smart/007 device?
     
  11. BigbearWolf 99

    Thread Starter New Member

    Apr 26, 2018
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  12. BigbearWolf 99

    Thread Starter New Member

    Apr 26, 2018
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    What is it that you don't understand I'll explain,
     
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