---
title: Built an RF fingerprint door lock
date: 2025-06-05
layout: post
project: true
thumbnail: thumb_sm.jpeg
---

Wanted to unlock the door with fingerprint, wirelessly to avoid drilling.

2024-11: Started with basic 433MHz RF modules and two Arduinos. Connected data
lines of the transceivers to UART RXD/TXDs of the MCUs. Unreliable—constant
packet loss.

2025-01: Switched to RFM69 modules. Complete ball-ache to program. Followed the
datasheet as well as I could, audited the code multiple times, cross-checked
with RadioHead and RFM69 drivers. No luck. 

The ATmega328P runs at 5V and the RFM69 at 3.3V. Suspect logic-level converter
(LLC) issues. Not enough swing?

2025-04: Ditched the RFM69s. Switched to NRF24L01+ modules. Data pins 5V
tolerant, no LLC required. Spent six weekends writing a clean-room driver.
Finally some success. 

Implemented a simple XOR cipher for the RF channel—good enough for my threat
model. The Key is cycled to resist replay attacks:

```
static inline void keygen(char *buf, uint8_t n)
{
    int i, imax;
    uint8_t sreg;
    uint16_t seed; 
    
    sreg = SREG;
    cli();
    seed = TCNT1; 
    SREG = sreg;

    for (i = 0, imax = n - 1; i < imax; i++, seed++)
        buf[i] = tab[(seed % tablen)];
    buf[imax] = '\0';
}
```

Protocol: FPM sends SYN. Servo responds with a session key. Both xor-ed with a
static key. Session key is used thereafter. Command set is kept private—serves
as an authentication layer for the endpoints.

2025-05: Wrote drivers for R503 and FPM10A sensors. UART RX sequence was
tricky—adopted the Adafruit C++ implementation to C. Chose the R503 for the
lock due to its built-in LEDs and better form factor.

2025-06: Two PCBs for FPM (front) and servo (back) controllers. 

<table style="border: none; width: 100%">
  <tr style="border: none;">
    <td style="border: none; width: 49.5%; vertical-align: top; text-align: center;">
      <img src="front_design.jpeg" alt="Design (front)" style="width: 100%">
      <p>Footprint (front)</p>
    </td>
    <td style="border: none; vertical-align: top; text-align: center;">
      <img src="front.jpeg" alt="PCB (front)" style="width: 100%">
      <p>PCB (front)</p>
    </td>
  </tr>
  <tr style="border: none;">
    <td style="border: none; width: 49.5%; vertical-align: top; text-align: center;">
      <img src="back_design.jpeg" alt="Design (back)" style="width: 100%">
      <p>Footprint (back)</p>
    </td>
    <td style="border: none; vertical-align: top; text-align: center;">
      <img src="back.jpeg" alt="PCB (back)" style="width: 100%">
      <p>PCB (back)</p>
    </td>
  </tr>
</table>

PCB specs: 2-layer, 1oz copper, 0.3mm traces (0.5mm for power). Ground plane.

2025-06: NRF24L01+ stopped working after mounting on PCB. Too close to the PWM
line. Soldering a large 47uF (16V) electrolytic capacitor between VCC and
ground fixed it.

Power problems became clear. Linear regulators dissipated too much heat. The
sensor and the servo drew 13.8mA and 4.6mA quiescent currents—unacceptable for
a battery-powered device. Servo inrush current exceeds 1A. 0.3mm tracks cuts it
too close.

Verdict: Functional but not practical. Battery died in under 24 hours. Led to
[redesign](../fpm-door-lock-lp/) with proper power management.

Commit: <a
href="https://git.asciimx.com/smart-home/commit/?id=f4b0b734a595919cf451ab9448b06274c8e609a4"
class="external" target="_blank" rel="noopener noreferrer">f4b0b73</a> |
Gerber: [gerber_back.zip](gerber_back.zip),
[gerber_front.zip](gerber_front.zip)