
Detecting a 2.4 GHz transmission for under 1 EUR - nynyny7
https://www.chzsoft.de/site/hardware/detecting-a-2.4-ghz-transmission-for-under-1-eur/
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VLM
"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.

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Zeetah
If you have the time, please do write up what you mentioned. I'd love to read
about it and learn. Thanks!

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baybal2
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.

~~~
antman
Interesting, just that? No power source?

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TrackerFF
I guess the schottky has a much lower voltage drop, compared to using two
LEDs.

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

~~~
kalium-xyz
You can get one for a decent price these days, Ive bene using the digilent
analog discovery 2 lately and its pretty amazing for its proce. Also it allows
for much more freedom because of its nature as USB oscilloscope. i hooked it
up to an RPi and with a battery and screen its now a proper mobile
oscilloscope.

~~~
simias
I've been looking for an oscilloscope for almost a decade now. Every year the
cheapo ones get better but they're still very limited IMO and feel very
expensive for what little they do.

As far as I can tell the Analog Discovery you mention is quoted by Digilent at
$279 and for this price you get two 100MS/s analog inputs, so that means that
you can't sample anything above 50MHz max (and in practice you probably won't
be able to do a lot of useful work above ~40Mhz). With modern circuits
routinely having signals in the hundreds of MHz that's fairly limiting for me.

And if I just want to probe "slow" digital signals like SPI or I2C I can get
90% of the features of this module by using a cheap-o ~$10 Saleae clone off
aliexpress. It's only a digital analyzer but it goes up to 24MHz and with 8
channels you can already do quite a lot of work for a tiny fraction of the
price of an entry-level scope.

Of course if you do a lot of analog work then a digital analyzer won't cut it
and the ~$300 price tag for a basic scope is almost a required purchase at
this point.

~~~
cushychicken
_As far as I can tell the Analog Discovery you mention is quoted by Digilent
at $279 and for this price you get two 100MS /s analog inputs, so that means
that you can't sample anything above 50MHz max (and in practice you probably
won't be able to do a lot of useful work above ~40Mhz). With modern circuits
routinely having signals in the hundreds of MHz that's fairly limiting for
me._

This comment dismisses a lot of measurement use cases out of hand, at least as
far as a hobbyist or rudimentary manufacturing test case is concerned. An
input bandwidth of 50MHz is still more than sufficient to evaluate all kinds
of useful circuits - a few I can think of offhand:

* Buck regulator loop compensation and transient response * Large signal audio debug (not suitable for small signal just due to the limits in the analog frontend and the number of bits in the scope ADC) * Basic digital protocol debug

If you're tuning a PLL, or need to evaluate DDR signal integrity, or trying to
design an audio system with -120dB of THD+N, then yeah, an Analog Discovery is
probably not the right tool. Then again, if you need to do any of those work
items, then you probably already know that.

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rkeene2
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.

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peter_d_sherman
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...

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cushychicken
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?

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baybal2
The lower the voltage drop, the better. I guess, in their part of the world,
germaniums are cheaper than schottkys

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nynyny7
Simple: The germanium diode was the first suitable diode I found in my parts
collection. :-)

A regular silicon diode will not work. An appropriate Schottky silicon diode
will do the job; see the Infineon app-note in the "PPS" of the website. Also,
the point-contact (regardless of the semiconductor material) reduces the
diode's junction capacitance, making it faster, i.e., more suited for RF
applications.

~~~
cushychicken
What's the typical forward voltage of the germanium diode vs a silicon diode's
0.7[V]?

It's a neat hack! Long live the Rigol DS1054Z!

~~~
nynyny7
I didn't measure it, but literature has a value of about 0.3 V for a germanium
diode's forward voltage.

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

~~~
cushychicken
Seems like a safe bet!

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

