Thanks to all of you who posted great explanations in the comments of yesterday’s post about a dubious way to make a metal detector from an AM radio and a calculator. Mahto submitted his trial video first (shown above), and he gets a Make pocket ref! Some notable comments:

Nick Clark writes:

AM radio operates from a frequency band of 520kHz to 1610kHz. That video suggests that we set our radios to the top of the spectrum, so we’ll say 1500kHz or so. Speaking as an electrical engineer, any radio waves generated by an digital device like a calculator would have to be some multiple of the device’s internal clock. (e.g., a 200kHz clock would broadcast EMI at 200kHz, 400kHz, 800kHz, etc). Most of the noise would be at the clock’s fundamental frequency, with most of the rest of it at twice the fundamental frequency. Thus, in order to be picked up by an AM radio near the upper band a circuit would have to be running a huge IC at 750kHz or so, or a smaller IC at 1500kHz. A metal detector like this _might_ be theoretically possible for a really power-hungry device running at 500kHz or for a not-so-hungry (but still _much_ more hungry than a solar-powered calc) 1MHz device.

Picking up the internal oscillators of the calculator is not at all surprising. The AM radio will pick up fundamentals or AM tones on any carrier if the signal is strong enough (and inverse squares makes that also read “close enough”). AM radios will also pick up lightning in a storm, especially if you are not on a station.

The real question is if there is any oscillator in the calculator that would be sensitive to the proximity of metallic objects. A self-tuned rod inductor oscillator will be rather sensitive to changes in the magnetic flux path, and this is actually half of how commercial metal detectors actually work. Does a calculator have such a circuit? That would depend on the calculator. A lot of LCD bias voltage generating circuits are built that way, so it’s certainly possible.

I’m consistently impressed with the high level of intellect and technical competency of our readers. I learn something new every day. Way to go, guys!

8 thoughts on “Follow up – DIY metal detector”

1. ehrichweiss says:

I like this follow-up and I’m wondering if anyone has thought of a couple of things about this thing.

1) Does the tuning on the AM radio affect the range/tones?
2) Knowing how finicky AM tuning can be, has anyone considered that the metal could be detuning either the RF or IF sections of the radio and that would be making a majority of the noise?

I don’t have any semi-portable AM radios right now or else I’d have a way to test all of this(with accompanying video of course), and I already visited the local Big Lots-type stores to try to find one and they were out.

2. Peter says:

I agree that it is highly improbably that this is the real thing, especially since it has been tested to some degree. However, in Africa there was a large pit with a bunch of Uranium deposits, and whatever the mixture of Uranium and surrounding elements, these deposits created a stable nuclear reaction for thousands of years. That has nothing to do with oscillation, but it goes to show that it is quite possible that a random event has occurred that allows this to work. Also, unless the exact radio and calculator are used, there is no way to be sure of it’s validity.

Well, I had to try this the second I saw it. Definitely got a similar “feedback” noise, but no variation whatsoever from proximity to metal.
Has the original poster stumbled upon a fortuitous synchronicity of products that can’t be re-created by your average radio & calculator?
I look forward to hearing more from the readers!

4. george says:

of course I did this the next day, after buying a cheap radio. After hearing the feedback noise and hearing all the internal loops of the electronic agenda or calculator at first, i had no luck detecting any metal. After 2 days of time-wasting with the radio in diffrent positions and stuff, and after a lot of radio noise i managed to tune up to some freq, where i can hear a slight noise over any metal.