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I was really surprised that this simple hack doesn’t even require taking anything apart! Just use a calculator and AM radio to make a metal detector – [via] Link.

UPDATE: This one might be plausible, folks. Debate in the comments; the first one to prove it’s real/fake gets a MAKE pocket ref!


  • Metal detectors @ NYC toy fair – Link.
  • Build you own metal detector – Link.

Becky Stern

Becky Stern is head of wearable electronics at Adafruit Industries. Her personal site:



  1. vicox says:

    what such a liar

  2. Macadaciouse says:

    Total nonsense, I’m disappointed MAKE.

  3. ad_astra says:

    Unlike every other metacafe “hack” video I’ve seen, using a radio and a calculator to make a metal detector does have some basis in fact. From what I remember, though, you wouldn’t get that kind of range out of it…
    Regardless whether this particular video’s a fake, the fact that metacafe videos make money for their submitters every time they get viewed says to me that maybe MAKE should exercise a little more discretion before posting them.

  4. Blind says:

    Since he gets money for every viewing of the video, why should the burden of proof be placed upon us? If we prove it works or is a scam, either way, the creator is rewarded because the truth of the video cannot be determined without watching the video and giving him his views.

    There are enough people actively scamming metacafe and ruining other sites, like, by posting How-To’s that are nothing more then links back to the video which may or may not be of value. Why allow it here as well?

    Whether it works or not, poor show on your part, Make.

  5. pt says:

    @Blind – you can blame me, after i saw this i suggested to becky we have our smart makers figure it out, i spent about 10 minutes mucking around here and it seems like it’s real, but it might be specific to that person’s radio and calc, i’d need to do more testing and research to know for sure – thing is, it’s a blog post not a printed article from time to time there might be something here we put up as we figure things out too.

    it’s not like we have videos like this all time, maybe 1-2 in the 20k posts here?

  6. rerat15@hotmail says:

    While I have no actual proof, I did just try it out, and aside from a some radio interference-y sounds, this is total bull, obviously. This probably doesn’t entitle me to the pocket ref, but just though I’d let you know.

  7. Becky Stern says:

    Make a video and get it online! =]

  8. mahto says:

    I tried it out and here is the result:

    It seemed like the screen region was responsible for the noise (perhaps the switching display signals?), and I noticed some small sensitivity to a metal spoon, but not like in the above video. Seems to depend on the calculator and radio. Maybe this is possible, but it probably will not be of too much use…

    It did make cool sounds though!

  9. Becky Stern says:

    I tried it too, but with my ipod instead of a calculator (my TI-89′s batteries are dead). No luck, but here’s the video:

  10. engineer says:

    There are two questions here.

    1. Is it possible that there exists a radio and calculator that together are sensitive to metal? Yes. Some calculators may have digital circuits operating intermittently in the AM band, and some radios may contain inductors that can be detuned by proximity to metals. AM radios are prone to interference and sometimes make buzzing noises for no good reason whatsoever.

    2. Does taping a calculator to a radio produce a metal detector? No. Even if this works, it’s particular to a certain kind of radio and calculator and orientation, and even then, it’s a stretch to call it a metal detector. A metal detector is a circuit that detects metal effectively and reliably, which this does not. Any circuit with a non-toroidal inductor can be affected by nearby metals; that doesn’t make all those circuits metal detectors.

    This person seems to have observed this effect and completely lacks the theoretical basis for describing it. He has noticed that one particular calculator and one particular radio can do something and then extrapolated that to *all* radios and *all* calculators, which is cargo cult logic at best.

    The “reflects radio waves” explanation is patently absurd; that is how *radar* works, not how *metal detectors* work. To reflect radio waves at AM frequencies you need very large pieces of metal. That is why radars designed to detect small objects use frequencies measured in gigahertz, not kilohertz.

    engineer says: FAIL

  11. Nick Clark says:

    It would be difficult to definitively prove this, but I would almost certainly bet on ‘fake.’ Here’s why:

    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.

    I can believe that the TI-90 posted above uses some frequency that’s within this range, but it’s very unlikely that the calculator used in that “demonstration” operates at speeds this high. Most of TI’s “simple” calculators from this era used a single IC and had no external timing components, meaning that the clock frequency needed to be generated completely on-board the IC. It would be difficult to generate a frequency anywhere near that range through the sorts of things one could fabricate onto a silicon wafer at the time those chips were initially designed (late 70′s-mid 80′s).

    Also, calculators designed to run off of solar power would almost certainly be in an effective ‘standby’ whenever no buttons were being pressed, meaning that the chip would not be switching a significant portion of its transistors on and off. It would certainly not be doing enough switching to create a significant amount of EMI.

    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.

    A calculator like this probably doesn’t operate at anything faster than 100-200kHz (if that, even!), and almost certainly could NOT absorb enough power through its solar cells to transmit an amount significant enough to be reflected back to an AM radio.

    A final point: Calculators from this time period were not shielded for EMI, either on the front or the back. They were pretty much just a chip soldered onto a thin PCB with traces that went to the LCD and keypad. They would have radiated any EMI fairly uniformly around the calculator, which means that the radio would have been able to pick it up regardless of a “reflector” like a spoon or other piece of metal.

    My verdict:

  12. Alon says:

    This type of “trick” is based on what the professionals call RFI – Radio Frequency Interference.
    RFI is usually an unwanted side effect, caused when two or more electronic devices are placed in proximity, usually at least one is a radio transmitter or receiver.

    The important thing about RFI is that they are normally unpredictable. A minor change in the equipment or the surrounding can make them appear or disappear.
    This is what makes the RFI debugging process so difficult, and the worst nightmare for electrical engineers.

    For that reason, I think it’s not possible to “prove” this video is a fake.
    Theoretically, this set-up can work.
    Practically, it is very much dependent on random variables, just like the stories about toasters that started to receive radio broadcasts.

    I guess you can improve your chances for success, by replacing the calculator with a simple oscillator ( 555 ?) tuned for a frequency somewhere in the AM range, or the local oscillator of the radio – 450kHz probably.
    Of course, this will not be as casual as using a ready made calculator.

  13. Nick Clark says:

    Slight correction to my precious post: I meant the TI-92 posted by Mahto can believably run with some clock on the order of 500kHz-1MHz, not the “TI-90″ that I wrote.

    I am still certain TI Education Pack calculator used in the original video does not operate anywhere close to high enough frequency (or high enough power for that matter) to generate any significant amount of EMI (electromagnetic interference also called RFI and other things).

  14. CaladanJen says:

    I suspect that he is pushing a calculator button to get the distinctive “beep” noises as the metal object approaches. Pushing a button changes the operating mode of the calculator, and the signals it generates. With that said, there is no reason to rule out metal detection in general.

    AM radios are notorious for poor tuning and excessive sensitivity in their front end. Any strong signal will cut right through the filter and show up, and the cheaper the AM radio, the better. I used to use an AM radio to pick up the noise of my telephone tone generator for running cables. There was no RF carrier, just a nasty two-tone square wave that you could generate with a 556.

    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.

  15. Bayne says:

    He’s pushing a button on the calculator with his thumb.

  16. Becky Stern says:

    Mahto, you’re first, so you win the Pocket Ref! Email pt to claim your prize. Look for a follow up post soon guys, thanks for all the clarification!

  17. Joe says:

    Wasnt this exact project in “sneaky uses for everyday things”?

  18. Blind says:


    You might have misunderstood me. I don’t mind that you posted a video. I also don’t mind a “can this work?” type post (I rather enjoy Hack-A-Day’s HackIT posts). My concern is that the uploader of that video is getting money per view. If it were a confirmed “Hey, this works, check out this video” type post, I wouldn’t care, but in this case it’s a “Does this work or not, why not try it out?” and we risk rewarding the uploader for something that doesn’t work.

    Like I said, there are enough entries at Instructables that consist of “watch this video at metacafe” without any real explanation of what the video actually shows and seem to be there for no reason other then to raise revenue for the uploader. I would hate to see Make continue to do the same because I expect a bit more out of you.

    So to sum it up, my complaint is merely that the post is posting a video of questionable value that generates revenue for the uploader and a question of “does this work?”. Since my last post it has been shown that it can work and explanations as to why, but frankly, this is info that I think should have been in the original metacafe video anyhow.

  19. SDMike says:

    Tried with a couple AM radios (one very old transistor version) and two calculators. No combination worked. No change in sound with small metal objects (spoon) or large metal objects (rifle barrel). Beep sound is certainly faked.

  20. Jon M. says:

    Looks good but of no real practical use.
    I made a website that has information on making a more advanced pulse induction type unit