All-band Receiver Hides In Plain Sight

This handheld radio has a little secret. You’re not going to be able to listen to Limbaugh since the original FM tuning circuit has been removed and replaced by a diode detector. Now [Miguel A. Vallejo] a discreet way to look for interesting radio signals in public.

The first step that he took was to remove the circuit board from the case and depopulate the tuning circuit while leaving the audio amplifier hardware. Next he referenced a proven design and built the diode detector circuit on a piece of protoboard. Finally he patched the new circuit into the original audio amplifier (seen in the image above) and put everything back in the case. Now he can listen in on data burst from a keypress on a computer keyboard, RF data communications, and slew of other noise sources.

This would be really handy for tracking down the electrical noise that’s screwing up your project.

[Thanks Superlopez]

17 thoughts on “All-band Receiver Hides In Plain Sight

  1. Wait, there are people besides me who would want to listen to Limbaugh on this site?

    Now I’m confused, because most of the tech sites I visit want him dead in unpleasant ways.

    On-topic: reminds me of one or two conversions I’ve done myself.(…that would be all of them…)

  2. For troubleshooting EMI investigations: He could replace the stock antenna with a small loop antenna appropriate to the frequencies he’s interested in. The loop wouldn’t work well for ‘discrete’ applications in public, but would work nicely in a lab, especially if it was attached to the radio by a length of coax. The loop would add a bit of directionality and isolation from other signals in the vicinity.

  3. This looks like a fantastic project.

    I think this could be used as a “bug” detector. If there is a device in a room using a microphone and transmitting over radio, there should be a feedback loop when the device gets near it. I think.

    In any case, this would be a really interesting thing to have.

  4. “paranormal investigation” ??

    Sure ! don’t forget to bring the photo light level meter too ! (like the aluminum siding conman on an old “All In The Family” episode – who used it to measure ‘energy loss’).

    Also don’t forget to turn out the lights and talk in whispers !

    (a bag of Scooby snacks could come in handy as well!)

  5. With some signal processing it’d be interesting to turn this into a TEMPEST-style keylogger, requiring only proximity to a USB HID.

    @Volfram: Have no fear, you’re not the only one. Now that we’re both exposed, pardon me while I don my nomex suit.

  6. That all-band receiver is pretty interesting, especially that FM demodulation was unintended but works. I guess you actually could still get Limbaugh if you were close enough to the transmitter! I certainly wouldn’t though. That guy is whack!

  7. I made a shifting peak oscillator that transmitted a moving peak from 48-50mhz for a final project. Basically FM transmitting of a triangle wave generated from a 555. It was battery powered, I gave it to my professor at the end of the class. The main purpose was to screw up labs as that class does all its labs in the 48-50mhz range and if you are trying to get specific gain and someone puts this baby across the room you won’t know why your peaks keep moving :-) Added bonus, you could hear the harmonics on the local country station causing an annoying pulsing noise over the sound of the station.

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