How do metal detectors work


is it like magnets? I feel like it’s magnets

In: 4

Not sure if this is entirely right, but I think in it’s most rudimentary form it uses something like two coils. One has an alternating or pulsed current which then produces a magnetic field. The other coil picks that up, which in turn produces a current again. But the trick is measuring the difference at which the oscillations in current changes between the in and out coils. (Albeit with some offset adjustment, because there’s a little bit of hysteresis or lag and energy loss between the two. The idea is to zero that difference via calibration when there’s no other chunk of metal around.)

Now when you put that near some piece of metal, the eddy currents induced in it also produce a field and that changes the rate of change in magnetic fields between in and out coils. So the difference then goes up or down from the zeroed calibration, which is reflected on an visual indicator and/or those squawking noises on an earpiece or headphones.

Then it’s just a matter of sweeping the thing back and forth and figure out where the signal change is centered. Fancier detectors do even more with more refined ways of measuring at specific frequencies or whatever to try finding specific metals, but I think the gist of it.

I’d think there may be other ways to do it as well. Radar/radio based rather than magnetic field changes, but that’s getting fancier still with things like imaging and all that on top. And then those systems look for stuff like different densities and levels of reflectivity of whatever the radio beam is hitting.

Have you ever tuned a guitar, trying to make two strings sound exactly alike? If so, you’ve heard that when they’re very close but not quite right, you get an undulating sound effect. This is called a beat frequency.

Metal detectors contain two electronic “guitar strings” called oscillators, that you calibrate to play the exact same tone. The tone is inaudible, but if the tuning is out of whack, an audible beat frequency can be heard.

One of the oscillators uses a big coil of wire to determine its tone. The coil sits inside the metal-detecting disc.

If you’ve ever made or seen an electromagnet, you know that current passing through a coil of wire makes a magnetic field. And if you know how an electrical generator or transformer works, you know that if you pass a changing magnetic field thorugh a coil, you get a current. What you may not know is that this happens to any metal, coiled or not. When there is no coil, the generated current just loops around inside the metal. This looping current makes its own magnetic field which affects the current in the original coil. Since the original coil determines the tone of one oscillator, any metallic object sets it out of whack with the other, and the beat frequency is heard, with a higher tone the more and closer metal there is.