A Beginner's Two-Component Crystal-Style Wi-Fi Detector

https://siliconjunction.wordpress.com/2025/12/12/a-beginners-two-component-crystal-style-wi-fi-detector/

148•jensgk•1mo ago

Comments

jqpabc123•1mo ago

Can this be used to detect radiation escaping a microwave oven?

A friend of mine has a microwave that noticeably degrades his wifi when it is in use.

zxexz•1mo ago

Sounds like he’s in need of a new microwave. Not tongue-in-cheek; sounds like there’s something up with the shielding, and if it’s not visible, how will you know if it gets worse?

Edit: I see no reason this wouldn’t work, however?

jeffbee•1mo ago

There is not substantial variance from one microwave to the next. They are all made by one outfit, then dressed up in different superficial brands and priced randomly. The emissions of a microwave oven are regulated by the FDA, not the FCC.

macintux•1mo ago

That doesn't mean, however, that an aging appliance can't misbehave.

bgbntty2•1mo ago

I've tested a few microwaves from different manufacturers with my phone a few years ago. I think I looked at some file in my router (OpenWRT), but I can't recall. I got a lot of dropped packets each time. The amount of degradation was similar for the different microwaves.

I had to put the phone close to the microwave to detect this. The degradation was obviously stronger when the phone was closer.

If your friend experiences noticeable degradation regardless of the distance within the room, it might be worrisome.

But I think it's normal to have some interference. That doesn't necessarily mean enough of the 2.4 GHz radiation escapes the microwave to be harmful to an animal, as Wi-Fi, Bluetooth and so on are very weak, comparatively.

Funny thing is, after putting my phone inside a closed turned off microwave, it got Wi-Fi, although very weak. I didn't try that with all the microwaves, but with 2 or 3 of them.

I think the Faraday cage around the microwave was built to be good enough for safety, but it wasn't built with Wi-Fi interference in mind.

Disclaimer: I might be wrong, as I don't have enough background to make any bold claims.

avidiax•1mo ago

> If your friend experiences noticeable degradation regardless of the distance within the room, it might be worrisome.

Probably not. I recall calculating it once, and the legal requirements for microwave oven shielding still allow it to produce a few watts of 2.4Ghz leakage. This is contrasted to 50mW typical WiFi AP power, and 5-50mW BlueTooth powers.

A few watts is totally non-dangerous to humans, especially diffused across the entire door.

shibapuppie•1mo ago

An old Unifi disc AP will do 100mw or 20dbm out of the box, a few cheap Cudy APs I have do about 400mw or about 26dbm.

On 2.4 GHz in the US you're allowed up to 36dbm, 30 of that can be transmitter, the remaining 6 can be a higher gain antenna.

bcook•1mo ago

For devices where connectivity is more important, switch them from 2.4Ghz to 5Ghz to avoid microwave interference.

nja•1mo ago

Other than phones and laptops (i.e. "real computers"), most devices only support 2.4, no? I can't recall the last time I set up a non-computer device that didn't say "make sure you're using a 2.4GHz network"...

(I imagine it's a much lower cost to only handle 2.4GHz?)

tzs•1mo ago

I was curious so checked my network. I've got 21 things on 2.4 and 16 things on 5.

The 16 are 4 computers (2 Macs, a Surface Pro 4, and an RPi 4), 3 mobile devices (iPhone, iPad, Apple Watch), 3 media devices (Fire TV, Nintendo Switch, and Kindle Oasis), one smart plug, a Brother printer, 3 smart speakers (Google Home Mini, two Echos), and an EV charger.

tgv•1mo ago

The article says: "responds to [...] microwave oven leakage".

KaiserPro•1mo ago

You could subsitute the LED for a moving coil meter: https://www.codrey.com/electrical/moving-coil-meter/

They have am much lower current and voltage requirement, so might me more sensitive. You can also do tuning to use different sized antenna. However that strays into analogue eletronics, which I've not really touched for 15 years.

jeffbee•1mo ago

Microwaves degrade everyone's 2.4 GHz ISM band radios. Wi-Fi occupies the ISM band because it's a garbage band that was already wrecked by this type of equipment. There isn't a solution to this; the safe leakage from a microwave oven is millions of times more powerful than wifi field strength. 5 or 6GHz band avoids the problem.

georgefrowny•1mo ago

There is a solution. Spread-spectrum methods for digging out signals from deep in the noise floor. It's just maths, but the amount of noise you can reject that way is really nuts.

pwg•1mo ago

From the fine article, second paragraph:

"you can build a tiny “crystal detector” that responds to 2.4 GHz Wi-Fi, Bluetooth, and even microwave oven leakage"

pseudohadamard•1mo ago

If they're seriously concerned, you can get microwave leak detectors for very little money from most crapvendors. The problem with this design is that while it's a cool toy it'll be prone to false negatives...

ifh-hn•1mo ago

So the question is how to do similar for 5ghz?

avidiax•1mo ago

Faster diode, like HSMS-2850 [1], and different leg lengths (tune for 5Ghz).

[1] https://datasheet4u.com/pdf-down/H/S/M/HSMS-2862_AgilentTech...

ifh-hn•1mo ago

Thanks. Appreciate the reply. From a brief look that might detect 2.4ghz too?

Really wish I was clever enough to do electronics!

avidiax•1mo ago

Depends on the length of the antenna. I'd bet you can get a length close enough to be a quarter-wave 5Ghz and 1/2-wave 2.4ghz antenna. Just buy a 2.4/5Ghz combo antenna and slap the diodes across it.

ifh-hn•1mo ago

I love simple solution like this. It'd be cool just wandering around being able to detect mobile and stuff with a tiny led.

This sort of thing is definitely why I come the HN.

pixelpoet•1mo ago

Just curious, what does your username stand for? :D

ifh-hn•1mo ago

Nothing. obviously the last bit stands for hacker news. Just randomly pressed letters.

pixelpoet•1mo ago

Okay, I was just curious because you were saying nice things about HN and there's a popular abbreviation IFL for I Fucking Love, so I had to wonder!

ifh-hn•1mo ago

Oh, I didn't know that and searching for ifh abbreviation on urban dictionary brings back what I think your curious about. Unfortunate coincidence. All my usernames are random strings. Good to know though.

ninalanyon•1mo ago

For 2.4 GHz each leg of the antenna should be 30 mm. For 5 GHz just under 15 mm.

See Dipole Calculator: https://www.omnicalculator.com/physics/dipole

And you need a faster diode.

cluckindan•1mo ago

Shorter antenna and faster diode. A Schmitt trigger would allow for much more sensitivity, but it would not be a passive device (requires a power source)

didntknowyou•1mo ago

curious, so if you build a "wall" of these, it would use up the wiress energy in the air so will absorb the excess waves around you?

thenthenthen•1mo ago

Depends if you are standing infront or behind the wall ;)

orbital-decay•1mo ago

Of course. Ambient radiation is a viable power source for distributed sensor networks and similar low-power stuff.

https://en.wikipedia.org/wiki/Energy_harvesting

cluckindan•1mo ago

An array of these would also allow you to better discover individual sources in a room with lots of devices.

thenthenthen•1mo ago

I made hundreds of these. Never worked

ides_dev•1mo ago

Hundreds? Really? I can see making two or three before giving up, but to build hundreds..

BugsJustFindMe•1mo ago

Especially if they never worked.

thenthenthen•1mo ago

I never give up. Tried soo many different diodes, but the ‘good’ ones seem very susceptible to clones/fakes since they arent manufactured anymore. There is a company that makes dedicated IC’s that can harvest the emf very efficiently, bit expensive though… and a black box..

wowczarek•1mo ago

Remember those little LED keychains for your "mobile phone" that would magically light up when taking/making calls and sending/receiving texts?

thenthenthen•1mo ago

These stopped working since 2G, since it uses radically less power. Altho I did recall seeing some fancy phone cases with blinkies on 3G… never could replicate it sadly. I guess with a lot of joule thief/stepup circuits it might work…

zoklet-enjoyer•1mo ago

I don't remember that, but I do remember my speakers buzzing just before a call or text came through.

b00ty4breakfast•1mo ago

they still do that, but I think that this is RFI from the processor rather than a deliberate rf transmission

zoklet-enjoyer•1mo ago

I haven't heard it in years, since around the time of the song your username references

pavlus•1mo ago

This is how FPV jammers are often field-checked in Russo-Ukrainian war. It doesn't test frequencies or spectrum quality, but is a useful indicator, that jammer is actually emitting (has power and is turned on), so you can be sure, that if it's up to spec and covers frequently used frequencies at your location - it actually tries to do it's job, instead of being a costly paperweight.

peter_d_sherman•1mo ago

>"Using just two components — a

high-speed [1N5711] Schottky diode

and an LED — you can build a tiny “crystal detector” that responds to

2.4 GHz

Wi-Fi, Bluetooth, and even microwave..."

That is interesting!

I never knew that a Schottky diode could rectify at 2.4 GHz -- that's pretty darn impressive!

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A Beginner's Two-Component Crystal-Style Wi-Fi Detector

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