emoquin
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Occupation: HRIS Analyst (mom helping 2nd grader)

We are trying to build a metal detector from and AM/FM radio and a calculator (both with batteries). We have the radio on AM at a high static station. As soon as I touch the calculator to the back of the radio it makes a loud humming, but does not change when put near metal. What are we doing wrong?
Thank you

Former Expert
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Ok, let me see if I understand what you are trying.

You are using the radio as your "metal sensor", with the idea being that if the radio is near metal, the metal will affect the radio transmissions received by the radio and the static noise will change in some way.

What you observe is that a calculator brought near the radio will change the sound, but metal does not affect the radio.

What you have learned is that your idea probably has some useful application (as you did find a way to affect the radio sound), but not the one you hoped for and you are not sure why. There are really two questions to try to answer, and the way to do it is yet more experimentation.

Science is often like this in real life. You get a strange result and it prompts more questions, and you run more experiments to see if you can understand what is happening.

question #1
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Why does the calculator affect the radio?

The reason is probably something to do with the fact that it is an electronic device. Electric current is passing through it and interacting with the radio waves or the radio receiver. You can test this idea by removing the batteries from the calculator and seeing if the radio static changes. (I recommend taping the battery to the outside of the calculator so the material composition of the calculator is the same in both tests, but the batteries aren't providing power to the calculator).

If the sound changes even though your calculator lacks power, try it again with just the batteries and just the calculator (the composition of each is quite different, you may have invented a battery detector or a printed circuit board detector instead of a metal detector)

question #2
......................
Why does my metal NOT affect the radio?

This is a bit harder because it is always harder to test for an effect that did not happen, vs one where you know you can make a change happen and just are tinkering until you discover which part of your experiment made it happen.

If you have some reason to believe metal should affect the radio reception, but the metal you used did not affect the reception, then the thing to try is to change the metal you are using in your test, to see if you can figure out when metal does or does not affect reception. Here are some ideas:

A. use more metal (sometimes there is an effect, but your sensor isn't good enough to notice unless the effect is stronger)
B. move the metal all around the radio (radio reception sometimes varies based on where the radio is compared to the broadcast station - try all the angles and see if the metal affects any of them)
C. use different metals (metals have very different electronic properties. an obvious one is that a magnet will stick to steel but not aluminum. Magnetism is a variation of electric current (you get a magnetic field when a current goes in a loop). A piece of steel might behave differently than aluminum or copper or tin.
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When you have more information you can make theories about why your observations occur as they do. Just to pick a possibility, if your calculator causes the radio to hum with power on, but does not cause it to hum with power off, then what you've invented is an "electronic device detector" not a metal detector. If instead it causes the radio to hum only if its batteries are near the radio, it's possible that, say, the copper in a Duracell battery is having an effect and trying a piece of pure copper metal might also get the similar result (so you might have a copper detector, rather than an electronic device detector)

HowardE
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Joined: Thu Nov 20, 2014 1:35 pm
Occupation: Science Buddies content developer

emoquin-

That's a fun way to recycle AM radios. I've never done it myself, but this Youtube video (https://www.youtube.com/watch?v=LsF3hsCIyNQ) has some pretty clear instructions. Maybe it will help you figure out what you may have done wrong.

Howard

wolfxxi
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Joined: Sun Nov 15, 2015 2:35 pm
Occupation: Parent

I know this is an old thread, but my son and I have built a functioning metal detector in this fashion. But now I'm trying to explain to him how it works, but I don't know myself and can't find the answer after some vigorous searching. Can someone please explain why an am radio at the top of the band and a pocket calculator produce this result?

Thank you.

HowardE
Posts: 496
Joined: Thu Nov 20, 2014 1:35 pm
Occupation: Science Buddies content developer

That is indeed a good question. I hadn't heard of it until the original asker asked back in March. I found a few of the same videos you probably did and at first blush they seemed promising. My only theory why this *might* work is that the metal object interferes with the reception of the weak signal coming from the calculator's microprocessor. I thought if you tuned the radio to any very weak station, you could cause the station to disappear back into static by getting a large metal object nearby.

I also looked tonight and found several other videos that suggest you'll hear tones. Those would have to be faked by pressing a calculator button. There's no legitimate explanation for why a metal object nearby would cause beeping. You can't always believe what you see on the internet.

It seemed like something worth a few minutes to confirm so I got out a couple of AM radios and some calculators.

Getting the AM radios to pick up residual EMI interference from the cheap 4-function calculators was easy. The radios I have are all decent quality and didn't seem to care whether a metal object was nearby or not. I couldn't get any credible reaction from the radio by moving even a large steel frying pan near the setup. However you can some small effect without the calculator if you live in an area with very weak AM radio stations. At night I can pick up a station from Toronto out here from New Hampshire. If I tune it in and pass the steel frying pan near the radio's antenna, I lose the signal a tiny bit.

So why does this 'metal detector' ever work at all? The best answer I can offer you is that the calculator's microprocessor is running at frequencies that will interfere with AM radio reception. When you get the right part of the calculator near the radio's antenna you'll hear buzzing. If you then get a metal object near the setup and you have the right kind (poor quality generally) of radio, the metal will interfere with the radio's reception and change the buzzing. The tuner in an AM radio uses a long coil of wire wound around a ferrite core. When you get a metal object near one that's poorly designed, the inductance of the coil changes which changes the received frequency of the radio. That's going to have some audible effect on the interference you're picking up.

My question for you is: when you say it's functioning, what does it do? How small an object can you detect and at what range? What does a detection sound like? You may have lucked out and had just the right kind of radio to make this work. While most of the people reporting they've tried this had no luck, a few have. You might be one of the lucky ones. Congratulations to you and your son on your success!

Howard

Monday317
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Joined: Sun Feb 28, 2016 9:57 am
Occupation: Parent

Not easy to answer in the Digital Age, we must look at how analog radios work for the answer.

An AM radio is also known as a "superheterodyne reciever". The audio we hear as a program has been mixed (technically the term is "heterodyned") with an Intermediate Frequency (IF) of 455kz to improve the signal quality and simplify the circuitry (Wikipedia explains this fully). This signal is again heterodyned (more heterodyning is why they term it "superheterodyne) with the broadcast carrier wave and transmitted. Your AM radio receiver reverses the process by first subtracting out the carrier wave as you tune your radio, then the IF, leaving only the program to play through the speaker.

Now a digital calculator uses a clock chip to coordinate the math circuits and Input/Output (I/O) devices onboard the calculator. I/Os would be buttons and the display unit. The display uses the most power on the calculator, and has the least shielding, so it is the most affected by metals.

How this all world is fairly simple:
1. Tuning the radio to the high end of the AM range eliminates most AM radios signals which would overpower anything the calculator sends out. It also happens to be a spot where the IF in the radio will likely find the clock chip signal from the calculator coming through the display.
2. When the calculator is near the radio, its clock heterodynes with the IF, which gives an audio signal that sounds like a whine you can hear. So the calculator is our detector's probe and the radio is our amplifier and output device.
3. As the calculator/radio nears a chunk of metal, the display will "feel" the change in its electrical environment and reflect it back to the radio. The audio detector circuit in the radio will subtract the change and process the difference as a sound (called a Beat Frequency) you can hear.

It may NOT work because:
1. Your radio is all digital, deriving its IF from a divider circuit rather than an oscillator circuit.
2. Your radio or calculator use metal casings, shielding them from outside influences, and limiting their emissions needed for this all to work.
3. Your calculator is solar powered: they use far less power inside than battery-powere calculators, and are ineffective as a metal detector probe.

The cheapest, oldest, radio & calculator will together you the best results. Good luck!