Detecting a 2.4 GHz transmission for under 1 EUR

by nynyny7 on 5/31/2020, 10:19 AM with 29 comments
  • by VLM on 5/31/2020, 4:37 PM

    "If one wants to make it more sensitive to certain frequencies than to others, one needs to add a resonant circuit, usually a combination of an inductor and a capacitor"

    Coincidentally for OP, the ideal antenna element length for a 2.4 ghz dipole antenna is around an inch and a quarter, which handwavy looks like about how long his diode leads are.

    Even a pitiful attempt at a tiny air wound coil choke would decouple his scope probe wires from interfering with the natural dipole antenna pattern.

    I bet, that if OP decoupled his scope probe wires, he could plot the natural radiation pattern of a dipole by rotating the diode and its antenna around and noting the voltage on the scope at various angles relative to the transmitter. If I have time I'll try that in the lab today just for fun.

    All it'll take is a couple inches of hookup wire... OR more sneakily could coil the diode leads themselves at the correct distance from the diode... Or I could tack solder some coaxial cable and BNC connector with a sloppy homemade choke balun ... hmm...

    I bet for TWO EUR I could plot antenna radiation patterns pretty reliably. I wonder if OPs mouse has a horizontal or vertical polarized antenna? I bet I can test that for two EUR.

  • by baybal2 on 5/31/2020, 11:47 AM

    An even simpler recipe from me. Get an epoxy cased LED and twist its leads around a schottky, polarity reversed.

    It will light up near wifi/bt sources.

  • by FreeFull on 5/31/2020, 11:16 AM

    Of course, the price in the title is assuming you already have an oscilloscope :)

  • by rkeene2 on 5/31/2020, 4:52 PM

    I've done something kind of similar (but much less low-level) using the nRF24L01+ attached via SPI on the GPIO pins of a Raspberry Pi Zero. I'm in the middle of writing it up, but it will probably be a couple of weeks.

  • by peter_d_sherman on 5/31/2020, 9:10 PM

    Opinion: HN should have the equivalent of folders/tags for articles; this article and others like it would be perfect for a folder/tag marked "Cool Hack For Under $10" -- or some designation similar to that...

  • by cushychicken on 5/31/2020, 1:28 PM

    I've never understood why germanium diodes are special here.

    Am I missing something? Does the diode have to be germanium for this trick to work?

  • by anfractuosity on 5/31/2020, 12:03 PM

    I presume those little key chains that flash when your phone is ringing do something similar?

  • by Magnets on 5/31/2020, 7:54 PM

    NRF24l01 + ESP8266 (or arduino) + OLED screen can do the same for less all-included