Sweep for the readers you can't see.

Specter turns your Flipper Zero into a pocket counter-surveillance bug-sweep for active 13.56 MHz NFC readers. It passively listens for the RF field that a powered-on reader is constantly emitting — a hidden card skimmer slipped into a payment terminal, a covert reader behind a door panel, a rogue logger taped under a desk — then tells you where it is, what kind of thing it is, whether the room is clean, and — left on watch — the moment one appears while you're away. It never transmits.

_{The readers are invisible. Specter makes them visible.}

📟 On the Flipper

_{Sweep (quiet)  ·  Sweep (reader locked)  ·  Fingerprint  ·  Survey (running)  ·  Survey (verdict)
Watch (clear)  ·  Watch (reader!)  ·  Logbook  ·  Calibrate  ·  Settings}

✨ What it does

Specter answers four different questions — where is it, what is it, is the room clean, and did one show up while I was away — and each has its own screen.

🔍 Sweep — where is it?

The showpiece: an analog-style EMF gauge. The needle rides the FIELD % in real time, a peak-hold marker remembers the strongest hit, and the top of the dial is a hot zone. A live waveform traces the noise floor while it's quiet.

The instant a reader's carrier is sensed the screen frames itself in an alarm border, a throb ring pulses around the dial, and the strip flips to ● ACTIVE READER with a proximity readout (FAINT → NEAR → CLOSE → STRONG). Optional geiger clicks speed up as the field gets stronger, so you can sweep a surface with the Flipper in your pocket and hear yourself getting warmer.

OK resets peak/contacts · hold OK logs the reading · LEFT calibrates to the room you're in.

🔬 Fingerprint — what is it?  _{NEW in 2.0}

Found something? Hold still on it. Specter stops chasing proximity and starts timing the carrier's on/off edges, then tells you what kind of emitter you're looking at:

You get the polling period, burst width, jitter, duty-cycle and a confidence bar — plus a logic-analyser pulse train of the raw carrier along the bottom, so the verdict is never asked to be believed on its own. A reader's polling loop is crystal-timed, so a genuine poll shows up as an unmistakable square wave.

OK saves the finding to the logbook · hold OK restarts the measurement.

🗺️ Site Survey — is this room clean?  _{NEW in 2.0}

A timed sweep of a whole space. Start it, walk the room, and get one verdict at the end instead of having to watch a needle the entire time:

• CLEAN — nothing crossed the noise floor
• TRACE — brief or faint hits, worth a second pass
• ACTIVE READER — something was up and emitting for real

…with max/average field, contact count, and how much of the survey a carrier was actually up. Runs for 30 s, 60 s or 2 min, and auto-logs the result.

🛰️ Watch Mode — tell me if one shows up  _{NEW in 2.1}

Where Sweep is you hunting and Survey is a bounded verdict, Watch stands guard indefinitely. Arm it, set the Flipper down, and walk away. It keeps a running clock and a detection count, remembers when the last contact was, and — the whole point — wakes the screen and sounds off the instant a reader appears. Each new contact is auto-logged with its timestamp.

It deliberately ignores stealth: a silent, dark guard that never tells you it saw something would be worse than useless. OK re-arms it (clears the count and clock).

📖 Logbook + live CSV  _{CSV NEW in 2.1}

Every finding is written twice, from one action — so it's readable on the device and already a spreadsheet, with no export step to remember:

2026-07-18 14:35:11
  SURVEY 60s ACTIVE max74…
2026-07-18 14:32:07
  READER POLLING 204ms…

timestamp,type,detail
2026-07-18 14:35:11,SURVEY,60s ACTIVE …
2026-07-18 14:32:07,READER,POLLING 204ms …

Both live in apps_data/specter/ on the SD card. Nothing leaves the device.

🎯 Auto-calibration  _{NEW in 2.0}

Press LEFT on the Sweep screen and Specter listens to the ambient noise floor for 3 seconds, then sets the detection threshold just above whatever it measured — saved as your Custom sensitivity. Every room has a different RF floor; this tunes to the one you're standing in rather than a number baked in at build time.

🕶️ Stealth mode  _{NEW in 2.0}

Keeps the screen and LED dark for the whole sweep, so the Flipper doesn't glow while you're the one doing the looking. Sound and vibration keep working — the point isn't to disable the feedback you're sweeping by. Exiting a stealth screen re-lights the display, so BACK always lands you on a lit menu _{(fixed in 2.2 — it used to leave the screen dark, which made BACK look dead)}.

📏 Reading the meter  _{FIXED in 2.3}

Why a reader you're touching doesn't emit 100% of the time. The detector measures one physical thing: what fraction of the time a 13.56 MHz carrier is actually up. But readers poll — a short burst, a sleep, another burst. A typical terminal or access reader is only radiating 20–35% of the time, so raw duty-cycle saturates around 30% no matter how close you get.

Specter used to print that raw number on the gauge, which made a perfect detection look like a third of one — and left CLOSE/STRONG and the survey's peak test permanently out of reach. The meter is now scaled against that real polling band:

MAX means pegged — you're as close as this measurement can resolve, and moving nearer won't change it. That's stated rather than hidden, so a stuck needle never looks like a broken one.

The raw duty is never lost: the noise floor, auto-calibration and the Fingerprint screen's DUTY all still work in true duty-cycle, because those describe the signal, not your distance from it. Prefer the literal number? Settings → Meter → Raw.

💾 Everything persists  _{NEW in 2.0}

Sensitivity, survey length, sound, vibe, LED, stealth, logging and meter mode are saved to the SD card the moment you change them. A sweep kit that forgets its setup is worse than no persistence at all.

🧠 How it works

Every powered-on NFC reader continuously pings the air with a 13.56 MHz carrier, waiting for a card to wake up. You can't see it, but the Flipper's NFC chip can: the ST25R3916 has a hardware external-field detector (the same circuit that lets the Flipper emulate a card and know when a reader is talking to it). Specter parks the chip in detect-only mode and samples that "field present?" bit ~500 times a second — without ever switching on its own carrier.

That single bit, sampled fast enough, carries two independent signals:

flowchart LR
    R["🔍 Hidden 13.56 MHz reader / skimmer<br/>(constantly polling its field)"] -- "RF carrier" --> A

    subgraph FLIP["Flipper Zero — Specter.fap"]
      A["ST25R3916 NFC chip<br/>external-field detector"] --> S["Sampler thread<br/>~500 samples/s"]
      S --> M["<b>How much?</b><br/>carrier duty-cycle<br/>peak · average · contacts"]
      S --> C["<b>What rhythm?</b><br/>burst / gap edge timing<br/>period · jitter"]
      M --> G["EMF gauge · waveform<br/>geiger clicks · LED · vibe"]
      M --> V["Survey verdict<br/>CLEAN / TRACE / ACTIVE"]
      C --> K["Emitter class<br/>CONTINUOUS / POLLING /<br/>INTERMITTENT + pulse train"]
      V --> L["📖 SD logbook"]
      K --> L
    end

How much → strength. Over a short window Specter measures what fraction of the time a carrier is present and smooths it into the FIELD %. A reader sitting right on top of the Flipper pegs the meter; a weaker or further one nudges it. That drives the needle, the proximity word and the click rate.

What rhythm → identity. Separately, every transition of that bit is timed. A reader that wakes for 20 ms every 200 ms produces a burst/gap pattern with almost no jitter, because its polling loop is driven by a crystal-timed state machine. Hand movement and RF noise are not that steady. That difference is what separates POLLING from INTERMITTENT.

The decision layers are tested on a real computer

The places where Specter turns numbers into a claim — "this is a polling reader", "this room is clean", "this is what the needle should read" — are pure C with no hardware dependencies, and they're pinned down by host tests rather than discovered on the device:

make -C test     # 284 checks: classifier, survey verdict, meter scaling

🚀 Install

No devboard, no firmware to flash — it's a single .fap.

Option A — prebuilt .fap (easiest)

1. Grab specter.fap from the Releases page.
2. Open qFlipper, drag the file onto SD Card / apps / NFC /.
3. On the Flipper: Apps → NFC → Specter.

Option B — build it yourself with ufbt

# one-time
python3 -m pip install --upgrade ufbt

# from the repo root, with your Flipper plugged in over USB:
ufbt            # build specter.fap into ./dist
ufbt launch     # build, upload to the Flipper and open it
make -C test    # run the host tests for the pure logic

The .fap lands in dist/specter.fap; ufbt launch copies it to apps/NFC/ and starts it for you.

Icons in icons/, screenshots and the banner in images/ are generated — regenerate with python3 tools_gen_icons.py, python3 tools_gen_mockups.py and python3 tools_gen_banner.py (needs pillow).

🎮 Using it

A sweep, end to end:

1. Launch Specter. On the Sweep screen, press LEFT and hold still for 3 seconds to calibrate to the room's noise floor.
2. Hold the Flipper flat and move it slowly across the thing you're checking — a card terminal, a door reader, the underside of an ATM lip, a parcel, a desk.
   • Quiet / flat waveform → no active reader in range.
   • Needle climbing, clicks speeding up → you're approaching an emitter. Keep going toward the peak.
   • ● ACTIVE READER + alarm border → an active reader is right here.
3. Found something? Back out and open Fingerprint. Hold the Flipper still against it and let the cadence settle — a few seconds is usually enough. Press OK to save the finding.
4. Clearing a whole room instead? Open Site Survey, walk the space until the bar fills, and read the verdict.
5. Leaving the area? Drop the Flipper in Watch Mode and it'll stand guard, waking and sounding off if a reader turns up while you're gone.
6. Check Logbook for everything you've saved, timestamped — or pull logbook.csv off the card into a spreadsheet.

💡 Sweep a known-good reader first (your own phone doing NFC, or a contactless terminal you trust) to see what a strong, legitimate field looks like on the meter — and what its fingerprint reads as. Then go hunting.

Controls

🔬 Honest limitations

• 13.56 MHz (HF) only. Specter senses the NFC band — the one used by contactless payment skimmers, most modern access readers, transit and hotel readers. It cannot see 125 kHz (LF) readers (older HID Prox / EM4100 door panels); the Flipper's LF path has no equivalent field-detect bit.
• It senses a reader's carrier, not what it reads. Specter tells you an active reader is here, roughly how close, and what rhythm it polls on — it does not decode, identify, or capture anything the reader does.
• Strength is relative, not calibrated. FIELD % and the proximity words are a comparative "warmer / colder" guide for sweeping, not a measured distance in cm. It's a scaled reading of carrier duty-cycle against a typical polling band (see Reading the meter); a reader that polls unusually sparsely will read lower at the same distance, and one in continuous-wave mode will peg from further away.
• The meter tops out. MAX means the carrier is up as much as this reader ever keeps it up — past that point, closing in genuinely cannot produce a bigger number. Use the PK peak-hold to compare positions instead.
• Cadence resolves to ~2 ms. That's the sampling period. Timings anywhere near it are shown with a ~ and the confidence is discounted accordingly — Specter would rather flag its own resolution floor than quote a precise-looking number it can't stand behind.
• CLEAN means clean at the sensitivity you chose. A dormant skimmer that only wakes on a real tap, or one that's shielded, stays invisible at any threshold. Absence of a reading isn't a guarantee of absence.
• One radio at a time. Specter takes over the NFC chip while sweeping, so close any other NFC app first (it'll say NFC unavailable if something else holds the radio).

⚖️ Legal & ethical

Specter is a defensive, listen-only tool — it never transmits, never powers a field, never touches the reader. Use it to sweep your own POS area, door, desk or belongings, or hardware you're explicitly authorised to assess. You are responsible for how you use it. Know your local laws.

🗺️ Roadmap

• Persist sensitivity / sound / vibe / LED across reboots
• "Logbook" of detections with timestamps to the SD card
• Background sweep with the screen off — stealth mode
• Adjustable threshold for finer range control — auto-calibration + Custom sensitivity
• Fingerprint an emitter by its polling cadence
• Timed site survey with a room verdict
• Export the logbook as CSV for reporting — written live alongside the .txt
• Long-run unattended watch mode with a wake-on-detection alarm
• Make the meter use its full range against real polling readers
• Investigate an LF (125 kHz) coil-based reader sense as a separate mode
• On-device logbook filtering by type

🗂️ Project layout

Specter-FlipperZero/
├── application.fam               # Flipper app manifest (category: NFC)
├── specter.c / specter_i.h       # app entry, wiring, alert feedback, stealth
├── helpers/
│   ├── field_detector.{c,h}      # worker thread: samples the field-present bit,
│   │                             #   duty-cycle + edge timing + calibration
│   ├── emitter_classify.{c,h}    # pure: cadence -> CONTINUOUS/POLLING/INTERMITTENT
│   ├── survey_verdict.{c,h}      # pure: survey stats -> CLEAN/TRACE/ACTIVE
│   ├── field_scale.{c,h}         # pure: raw carrier duty -> full-scale meter
│   ├── specter_settings.{c,h}    # persisted settings (saved_struct)
│   └── specter_log.{c,h}         # SD logbook, RTC-stamped .txt + live .csv
├── views/
│   ├── sweep_view.{c,h}          # the EMF gauge / waveform / alarm screen
│   ├── fingerprint_view.{c,h}    # classification card + pulse-train trace
│   ├── survey_view.{c,h}         # progress + verdict card
│   └── watch_view.{c,h}          # unattended guard: clock + count + alarm
├── scenes/                       # scene-manager navigation
├── test/                         # host tests for the pure decision layers
├── icons/                        # 1-bit Flipper icons (generated)
├── images/                       # banner + screen mockups (generated)
└── tools_gen_*.py                # regenerate icons / mockups / banner

🙏 Credits

• Built by at0m-b0mb.
• Part of a Flipper security-tool family: Argus (Wi-Fi deauth / evil-twin watchdog), GhostTag (BLE anti-stalking hunter) and Cerberus (Sub-GHz RF watchdog).
• Powered by the Flipper Zero firmware NFC HAL (furi_hal_nfc field detection) + ufbt.

📄 License

MIT © 2026 at0m-b0mb