Follow up – DIY metal detector

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.

CaladanJan writes:

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!