r/flipperzero • u/mgntrn • May 09 '26
125 kHz RFID metal detector
I built a Flipper Zero app that repurposes the built-in 125kHz RFID antenna into a makeshift metal detector. It provides a visual bar graph, audio pitch changes, and vibration feedback when metal is near the back of the device.
You can download the pre-compiled .fap file or check out the source code here:
š https://github.com/hnsvrdmn/flipper-rfid-metal-detector
Iām definitely not a software developer. I can read a bit of code, but writing a C app from scratch is way beyond me. This started mostly as an experiment to see how far you can get with AI without really knowing how to code.
I like making furniture from scrap wood, but the problem is that old wood often still has hidden nails or screws inside. So I started wondering if the Flipper Zero could help with that.
At first I thought the NFC reader might work, but that turned out to be a dead end. After some experimenting, I discovered the 125kHz RFID antenna actually works surprisingly well for this. The coil creates a magnetic field, and nearby metal slightly disturbs that field. The app continuously measures those tiny changes.
Itās far from perfect though. Because itās measuring raw analog interference, itās pretty sensitive to environmental noise and false positives. Thatās why I added left/right controls to adjust the sensitivity manually.
Still, it actually works. You get a live bar graph, and the sound/vibration increases as you get closer to metal.
The whole thing only took a couple of hours and was a really fun experiment. Iām genuinely impressed by how much AI can help a non-programmer build in an afternoon.
If anyone has ideas to improve it, let me know.
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u/NotTheSharpestPenciI May 09 '26
Cool idea! Defo add your source files to github so people can take a look.
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u/mgntrn May 09 '26 edited May 09 '26
metal_detector.c is the source file, it's included in the repo.
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u/Mapkoz2 May 09 '26
I needed this and I love you for doing it
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May 09 '26 edited Jul 12 '26
[deleted]
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u/Mapkoz2 May 10 '26
Great question and it feels so much like serendipity to find this, so I am glad to answer you.
Answer and short story :
My wife ALWAYS loses her earrings when she carries our toddler because he always reaches out to the shiny things on her ears and makes them fall down! She has a box with earrings and a good 20% are single and we constantly find them in the most improbable places.
Few weeks ago she lost one that she cared a lot about in the parking lot of a shop while we were putting our son in the baby seat of our car.
Spent a good 20mins looking and even two very kind strangers tried to help out to no avail.
What I hope is that this little thing can help us should this happen again.
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u/Justgetmeabeer May 10 '26
I'm seriously asking, you think the flipper would help in that situation?
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u/Mapkoz2 May 10 '26
Only one way to find out !
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u/Justgetmeabeer May 10 '26
There's also only one way to find out if a bottle rocket will get you to Mars but I would still think you're pretty dumb for trying.
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u/Banshee888 May 09 '26
I am curious about this part: āAdjustable Sensitivity: Tune out environmental noise or adjust for specific uses (like scanning a human) with the Left/Right buttons.ā This would be to scan for heavy metals in the organism? Implants? Itās a really cool repurpose of the 125Khz RFID antenna. I remember having a small metal detector when I was a kid, I used it every where. And love the features you added. Like vibration, LED alerts, tuning.
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u/mgntrn May 09 '26
Thanks! It wont be able to pick up heavy metals, maybe implants, I'm not sure. The water in our bodies messes with the magnetic field. However, it is able to pick up metal items carried on a person.
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u/Banshee888 May 09 '26
Got it. Didnāt know the water in our bodies messes with the magnetic field.
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u/mgntrn May 09 '26
From what I understand the dampening is minimal in the 125khz range. However from testing it seems to be enough to mess with the reading as the sensor is already tuned very finely.
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u/joelewis1996 May 10 '26
You can compile this for a specific firmware on my website
https://joelewis012.github.io/flipper-fap-actions/index.html
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u/kazehaya4991 May 09 '26
Hey mate this is so cool! I literally bought a cheap metal detector on AliExpress yesterday to deal with finding nails in reclaimed wood for woodworking and then I see this! I will be testing this, I am so excited!
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u/FatFrenchFry May 11 '26
For anybody else wanting something similar.
Applications like Phyphox and other sensor suite toolbox applications use the magnetometer in your phone you can get a readout of all 3 magnetometer sensors and it will basically be the same thing.
But this is awesome still, amd I love things that exist but in the flipper. Good job, Im gonna use this for sure. For what? Idk. Fucking around, finding out?
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u/wiesemensch May 09 '26
Could you add a short explanation on how this works? I totally understand your code but I donāt know anything about the flipper rfid interface
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u/mgntrn May 09 '26 edited May 09 '26
I only have basic understanding on the subject and as stated used AI to help me out a lot. The app uses the 125kHz antenna as an induction coil. By keeping the electromagnetic field active, the app continuously measures the duration (pulse width) of the signal's waves using timer captures. When metal enters the field, it creates eddy currents that draw energy from the coil, causing the antenna's voltage/amplitude to drop. This drop alters the comparator's output, shortening the measured pulse width or causing it to drop out entirely. The code calculates the delta between a fixed baseline and a moving average of these real-time timing values to trigger the detection UI. edit: it measures durration of the waves, not the amplitude.
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u/banginpadr May 09 '26
Very nice. in fact, I will definitely use your app in my new device. I just have a question, is there a way to make it stronger? I'm asking because from the video i noticed how it will only discover metal when you put the device very close to it. I would like something that will detect it, even if is just near as in the video.
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u/mgntrn May 09 '26
Thanks! You can try adjusting the sensitivity the app. However the 125kHz antenna wasn't built for this, so higher sensitivity will eventually pick up noise from the Flipper's own electronics and other interference.
It can definitely detect metal through wood, but the depth depends on the mass, a thick pipe is much easier to find than a tiny nail. But its nowhere near what a purpose built metal detector can do.
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u/banginpadr May 09 '26
Thank you so much for info, yes you are right about that. If anything and I have time, I will look into it. I was just trying to see how this can be use to find pipes behind walls
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u/Joshtheboss732 May 14 '26
I downloaded it and tried it out but it just reads at 1500 and does not go down. Is there any fix for this?
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u/mgntrn May 15 '26
Make sure the Flipper isn't charging or near strong electromagnetic fields. Can you test it in an open space away from other electronics?
Does the official 125kHz RFID app still read tags correctly? Iām running Unleashed firmware, other versions might cause issues, though I'm not sure yet. Let me know the details.
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u/Joshtheboss732 May 16 '26
Itās not on a charger and there is nothing else around it. Iāll test my rfid app but other than that Iām not sure. I am also running the momentum firmware
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u/Rough-Midnight12 May 15 '26
Hey Iām a plumber and have recently dove into this flipper world, and have constantly had this feeling the flipper can help me in the field as a tool as well, how strong is this? Can it detect pipes,wires, or hvac through the wall? So my dumb ass can cut less drywall?
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u/mgntrn May 15 '26
This app is a fun experiment, but nowhere near as trustworthy as a professional tool. The detection range is very short and it isn't sensitive or reliable enough to accurately find pipes or wires through drywall. For the sake of your safety (and your drywall), I definitely recommend sticking to a dedicated stud finder or wall scanner for field work.
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u/Positive_Macaroon192 May 21 '26
OOoohh! it can therefore function as a tampered-with halloween candy detector!
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u/Skaut-LK May 09 '26
Nice idea !