Turning An Ordinary Pen Into A Covert Radio Receiver

[Ben Krasnow’s] latest project will be good for anyone who wants a complicated way to cheat on a test. He’s managed to squeeze a tiny FM radio receiver into a ballpoint pen. He also built his own bone conduction microphone to make covert listening possible. The FM radio receiver is nothing too special. It’s just an off the shelf receiver that is small enough to fit into a fatter pen. The real trick is to figure out a way to listen to the radio in a way that others won’t notice. That’s where the bone conduction microphone comes in.

A normal speaker will vibrate, changing the air pressure around us. When those changes reach our ear drums, we hear sound. A bone conduction mic takes another approach. This type of microphone must be pressed up against a bone in your skull, in this case the teeth. The speaker then vibrates against the jaw and radiates up to the cochlea in the ear. The result is a speaker that is extremely quiet unless it is pressed against your face.

Building the bone conduction mic was pretty simple. [Ben] started with a typical disk-shaped piezoelectric transducer. These devices expand and contract when an alternating current is passed through them at a high enough voltage. He cut the disk into a rectangular shape so that it would fit inside of the clicker on the ballpoint pen. He then encased it in a cylinder of epoxy.

The transducer requires a much higher voltage audio signal than the litter radio normally puts out. To remedy this problem, [Ben] wired up a small impedance matching transformer to increase the voltage. With everything in place, all [Ben] has to do to listen to the radio is chew on the end of his pen. While this technology might help a cheater pass an exam, [Ben] also notes that a less nefarious use of this technology might be to place the speaker inside of the mouthpiece of a CamelBak. This would allow a hiker to listen to music without blocking out the surrounding noise.

26 thoughts on “Turning An Ordinary Pen Into A Covert Radio Receiver

      1. I recall discovering this when I was playing with my dads 6 ft amplifier and a pair of amp headphones as a kid. Plug headphones into input jack and voila, microphone.

        I find it amusing that as humans we can understand something with 100% clarity and still bitch as though it does not compute because of semantics. “Whats that you said speaker? Now there is no way I can understand or appreciate this, too bad it didn’t say mic..”

        1. It’s not that nobody can understand it, it’s just wrong. It’s the sort of precise mindset that finds this annoying, that makes people good with technical stuff. Literal rule following.

          In the case of language you can cut someone some slack, but calling a speaker a mic is stuff someone writing on electronics should know better than, it’s hardly like getting some complicated circuit’s name wrong. Wrongness really annoys some people. I’d say it was OCD but it’s nothing like OCD, that’s another popular misconception.

    1. Well I feel silly. Somehow I had the word microphone in my head consistently through this article. I definitely intended to write “speaker”, but my fingers kept typing microphone. Weird.

      1. Apparently feeling silly about a weird error is not enough to get the editors to fix the text in the article. This is another example of quantity over quality, we are not talking spelling, grammarnazi, or the fine points of semantics, a speaker is the only applicable transducer to use in a receiver and HaD readers deserve better editorial oversight or they will start seeking content elsewhere.

        1. I dunno where else they’d go. HAD does pretty well, you should see some of the “techy” websites out there.

          That said, most journalistic outlets employ sub-editors especially to correct things like this. It’s surely justifiable in a budget, when things like this happen all the time. Get a sub, HAD!

  1. One app that would be also not so above the board, but maybe be able to listen to music while at work. Stick your pen in your mouth while you are at your desk and catch some tunes or sports cast. Or more above the board, anywhere you need your ears to be available to hear what is going on in your surroundings but you need to listen to something important. Hunters could use it out in the field where they need to listen but it gets boring when nothing is around so you can listen to weather broadcasts or news while out in the field and not have your ears totally incapable of hearing game walking through the forest.

    1. +10000

      Especially as you get to higher levels of education, the only people that you know that can help you on your “advanced microelectronic fabrication” exam (or whatever your flavor is), are probably sitting right next to you worrying about their own grade, not yours.

      1. You can make teeny, godawful, FM radio transmitters that would fit into a pen. So you’d have two-way comms with a friend with Internet access and some textbooks. Maybe a throat mic would be good. Hide it behind a tie.

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