05 · 12 min
125 kHz RFID
Low-frequency proximity tags, what a UID actually is, and how to practice on fobs and pet chips you already have rights to.
125 kHz (and nearby 134.2 kHz animal) RFID is inductive. The reader energizes a coil; the tag load-modulates that field. Range is centimeters, not rooms. That is a feature: LF is messy around metal and water, so it stayed on cheap access fobs, parking tokens, and livestock / pet chips.
Families worth knowing
| Family | What you get | Write? |
|---|---|---|
| EM4100 / EM4102 | A public ID, often 40 bits, endlessly repeated | The original chip is not a writable credential |
| T5577 and similar | A blank you can program to look like a simple LF tag | Yes — on blanks you bought |
| HID Prox / Indala | Older US proximity formats | Proprietary; not a toy and not your office door |
| ISO 11784/85 FDX-B | Pet and livestock transponders ~134.2 kHz | Read the ID; do not re-encode a living animal |
UID is a name tag, not a password
If a door controller only checks 'did I hear this ID,' then copying the ID is copying the door. That is a broken design, not a clever trick. Modern systems do not stop at LF UID. Your lab is: read a tag you own, write down the ID, confirm Flipper and a USB reader agree. Your lab is not: the gym, the office, or a rental car.
Emulation versus writing
- Read: the Flipper energizes the tag and demodulates the ID.
- Save: you store that ID on the SD card.
- Emulate: the Flipper coil plays back a tag waveform. Use this only for tags that are yours, on a reader that is yours.
- Write: only onto a writable blank (T5577 etc.) that you purchased as a blank.
Pet microchips are meant to be read. If you have a dog or cat with an implant, reading the number and confirming it matches the registry paperwork is a legitimate, gentle first 134.2 kHz lab. Do not try to rewrite an implanted chip.
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