RF lock.

1 files changed, 36 insertions, 33 deletions

diff --git a/_log/fpm-door-lock-rf.md b/_log/fpm-door-lock-rf.md
index b677591..2643c2d 100644
--- a/_log/fpm-door-lock-rf.md
+++ b/_log/fpm-door-lock-rf.md
@@ -1,18 +1,30 @@
 ---
 title: Fingerprint door lock (RF)
-date: 2025-04-26
+date: 2025-06-05
 layout: post
 ---
 
-In 2023, I started tinkering with DIY electronics as a hobby. Until now, I've
-been using development boards like the Arduino Uno and ESP-32-WROOM so that I
-can focus on the software. Recently, I decided to step outside of my comfort
-zone and design a PCB from scratch for a door lock I'm working on.
+Wanted to unlock door with fingerprint, wirelessly to avoid drilling.
 
-The lock comprises two subsystems: a fingerprint sensor in front of the door
-and a servo connected to the physical lock behind the door. The fingerprint
-sensor authenticates the person and signals the servo behind the door to unlock
-the door over an encrypted RF channel.
+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. A complete ball-ache. Followed the
+datasheet to the letter, audited code more than 30 times, cross-checked with
+RadioHead and RFM69 OSS drivers. No luck. ATmega328P runs at 5V, RFM69 3.3V. I
+suspect the problem is with the logic-level converter (LLC). Not enough swing.
+
+2025-04: Ditched RFM69s. Switched to NRF24L01+ modules--5V tolerant, no LLC
+required. Spent six weekends writing driver from scratch--clean-room, using
+datasheet. Works perfectly.
+
+2025-05: Wrote FPM sensor drivers for R503 and FPM10A. UART RX sequence was
+tricky--took multiple attempts to get handshake working. R503 has built-in LEDs
+and better form factor--will use it for the lock.
+
+2025-06: First PCB design. Two boards: fingerprint sensor (front), servo
+controller (back). Encrypted RF link between them.
 
 <table style="border: none; width: 100%">
   <tr style="border: none;">
@@ -37,27 +49,18 @@ the door over an encrypted RF channel.
   </tr>
 </table>
 
-The PCBs have two layers. A copper region serves as the ground plane. The 0.3mm
-wide 1oz/ft<sup>2</sup> copper traces can carry up to 500mA (the tracks
-connecting the power source and the linear regulators have a width of 0.5mm).
-Both subsystems were functional. I was able to control the servo reliably using
-the fingerprint sensor.
-
-The designs aren't without flaws, however. The main shortcoming of the circuits
-is that they draw significant amounts of quiescent currents despite employing
-sleep modes. The linear regulators were a poor choice as they dissipate too
-much heat. The fingerprint sensor and the servo draw 13.8mA (3.3V) and 4.6mA
-(5V) respectively, as long as they are connected to the power supply. 
-
-Although the circuit didn't draw more than 200mA without a load, the servo
-under load could draw up to 600mA. I'm sailing too close to the wind with 0.3mm
-copper traces. Instead, 0.4mm wide 2oz/ft<sup>2</sup> traces would have been
-safer.
-
-I'm working on improving the design to reduce idle current consumption and
-extend the battery life. Despite its deficiencies, this was my first PCB
-design, and I'm glad that it worked as well as it did. Custom PCB design marks
-an important milestone in my DIY electronics journey.
-
-Files: [gerber_back.zip](gerber_back.zip), [gerber_front.zip](gerber_front.zip),
-       [source.tar.gz](source.tar.gz)
+PCB specs: 2-layer, 1oz copper, 0.3mm traces (0.5mm for power). Ground plane.
+Both subsystems worked—could control servo from sensor over RF.
+
+Power problems became clear. Linear regulators dissipated too much heat. Sensor
+and servo drew 13.8mA and 4.6mA quiescent—unacceptable for battery. Servo
+inrush current can exceed 1A. Trace width (0.3mm), especially for servo and
+power, cutting it close.
+
+Verdict: Functional but not practical. Battery dead in under 24 hours. Led to
+[complete redesign](../fpm-door-lock-lp/) with proper power management.
+
+Commit:
+[f4b0b73](https://git.asciimx.com/smart-home/commit/?id=f4b0b734a595919cf451ab9448b06274c8e609a4)
+| Gerber: [gerber_back.zip](gerber_back.zip),
+[gerber_front.zip](gerber_front.zip)