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I was watching a video of someone using his smartphone and a custom designed coil to make a metal detector.

The smartphone was connected to the coil via headphone jack.

I couldn't understand how he managed to convert audio signals into radio waves using the app on his phone.

Is this even possible or practical?

PS: Link below to the video

https://youtu.be/xkkI8Qhvf8Y

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  • \$\begingroup\$ there are a lot of these radio transmitters for sale .... here is an example .... walmart.com/c/kp/car-fm-transmitters \$\endgroup\$ – jsotola Apr 30 '19 at 4:33
  • \$\begingroup\$ Most cellphones have a very sensitive magnetometer in them, there are various Apps that allow detection of wiring or metal objects. It's certainly not impossible that the audio output fed to a coil could improve the detection capability. If you feed the audio into a coil you will certainly create an EM field (albeit a weak field). But you didn't supply a link to check this out. \$\endgroup\$ – Jack Creasey Apr 30 '19 at 5:42
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It should be possible to build a metal detector using just a smartphone.

Metal detectors don't use radio frequencies. This overview shows the frequencies used by various commercially available metal detectors.

Excerpt:

enter image description here

All of the frequencies used are within the abilities of the earphone output of a typical smartphone.

You would have to change the operating principle somewhat, but it should be possible.

Typical metal detectors operate by using the search coils in an oscillator. The frequency of the generated signal is compared to the frequency of a second oscillator whose frequency is controlled by the operator.

Twist the knob on the manual oscillator until the unit stops squealing. Both oscillators are now at the same frequency.

If you bring a piece of metal near the search coil, you will change the frequency of that oscillator.

Your smartphone can't operate that way. You don't have an oscillator for the search coil, you have a generated signal driving the search coil.

The phone generates a fixed frequency signal and transmits it through a coil. You have a second coil connected to the microphone input as a detector.

In the phone, you have a program that generates the search tone and "listens" to how loud the received signal is.

In free space, it will have a particular "volume" that it hears from its transmit coil.

If you bring a piece of metal near the coil, the received signal will get louder (or quieter.)

Your software detects the changes in "volume" and changes the display accordingly.

Since you only need one earphone output to drive the coil, you have the second one free to make sounds to indicate the detected metal. If you try, you could probably make it sound like a typical metal detector.


Not to say the youtube video is real. It could still be a fake by someone with more video editing skills than electronics skills.


Now that you've posted the link, I can tell you that it probably works quite well and is no fake.

You video is a test of a detector made by Akyor.

Not a fake, and probably uses a different principle than the ideas I described above.

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  • \$\begingroup\$ Thanks for all the inputs. @JRE thanks a lot for the clarification. \$\endgroup\$ – Sam Kam Apr 30 '19 at 6:33
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The most likely scenario is that the video is fake. There are a lot of fake electronics videos on Youtube.

While it is technically possible to build a metal detector circuit that connected to a smartphone's headphone port, doing so would require a significant amount of external circuitry and a power source for that circuitry, not just a coil ("custom designed" or otherwise).

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