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---
title: Fingerprint door lock (RF)
date: 2025-06-05
layout: post
project: true
thumbnail: thumb_sm.jpeg
---
Wanted to unlock 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/TXD of an ATmega328P.
Unreliable--constant packet loss.
2025-01: Switched to RFM69 modules. Complete ball-ache. Followed datasheet to
the letter, audited code many times, cross-checked with RadioHead and RFM69
open-source drivers. No luck.
ATmega328P runs at 5V; RFM69 3.3V. Suspect logic-level converter (LLC)
issues. High resistance. Not enough swing.
2025-04: Ditched RFM69s. Switched to NRF24L01+ modules--data pins 5V tolerant,
no LLC required. Spent six weekends writing a clean-room driver from scratch.
Works like a charm.
Basic security via xor cipher–good enough for a door behind a guard post and
gate:
```
void xor(const char *k, const char *s, char *d, uint8_t n)
{
int i;
for (i = 0; i < n; i++)
d[i] = s[i] ^ k[i];
}
```
Resists replay attacks by cycling the key:
```
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 session key. Both xor-ed with
static key. Session key used thereafter. Private command set authenticates
endpoints.
2025-05: Wrote FPM drivers for R503 and FPM10A. UART RX sequence was
tricky--adopted Adafruit C++ FOSS implementation to C. R503 has built-in LEDs
and better form factor. Chose it for the lock.
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+ on the back stopped working after mounting on PCB. Too close
to servo's 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. Sensor
and servo drew 13.8mA and 4.6mA quiescent currents--unacceptable for battery.
Servo inrush current exceeds 1A. 0.3mm tracks cuts it too close.
Verdict: Functional but not practical. Battery dead 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)
|
