SW to v0.4, timing only via watchdog, filled a little bit README.MD

README.md

@@ -14,7 +14,16 @@ FireFly Solar is a small outdoor LED blinker inspired by the idea of an LED thro
 During the day, the solar cell charges the energy storage.  
 At night, the circuit wakes up and sends a simple Morse-style light signal.
 
-The project is partly a small garden object, partly a low-power electronics experiment, and partly a design exercise for future DIY kits.
+## About this Project
+
+FireFly Solar is my **technology playground**. It is a small garden object, but its real purpose is to learn by building something complete. With it I learn about:
+
+- **Circuit design at the edge**: in this case, as low power as possible
+- **PCB design**
+- **Weatherproof casing**
+- **ATtiny programming**
+
+Possible next technology step: everything about DIY SMD design and soldering.
 
 ## Project Goals
 
@@ -24,6 +33,40 @@ The project is partly a small garden object, partly a low-power electronics expe
 - Test mechanical concepts for outdoor 3D-printed parts
 - Learn what is needed to turn a small electronics project into a possible DIY kit
 
+## Software
+
+The firmware runs on an **ATtiny85** at 1 MHz (internal clock, ATTinyCore: `clock=1internal`, `millis=disabled`). The LED itself is used as the light sensor. When it is dark, the firefly sends a short Morse text ("TGO 26") about once a minute.
+
+### Current Firmware version: v0.4 (2026-11-08)
+
+Goal: lowest possible average current.
+
+Changes vs v0.3:
+
+- No `delay()` / `millis()`; Timer0 and Timer1 are gated off in PRR.
+- The watchdog (interrupt mode) is the only timebase: Morse units (~96 ms), the sensing window and the 8 s sleeps. The CPU sleeps in power-down in between.
+- Darkness sensing: the CPU sleeps in 16 ms steps while the LED junction discharges, instead of a busy `digitalRead()` loop. Bright light exits after the first step. `darknessThreshold` is replaced by `darkSteps`.
+- The ADC is disabled before it is gated off in PRR; pins are set up before PRR.
+- After a power-on or external reset, no blinking for `RECOVER_CYCLES` (about 16 min), so the supercap can recharge.
+- `DEBUG_TEST` switch for bench testing: 8 s cycle, no recovery wait, short flash when the sketch decides "bright". Set it to 0 for the field.
+- Optional `BOD_FUSE_ENABLED` (BOD off during sleep); unused with the BOD fuse off.
+
+Tested: compiles with ATTinyCore and runs on hardware. Current draw is not yet measured.
+
+## PCB
+
+- Finished: a batch of 10 is assembled, tested and working.
+- Latest adjustment: fine-tuned the resistors for the LED, and the 3.9 V Zener diode is confirmed safe avoid overcharging 4V supercap.
+- Next version: the PCB should be a little shorter. The wiring space is a bit too tight now.
+
+## Casing
+
+Works basically as intended. Lessons for the next version:
+
+- Better locking of the solar modules.
+- The rope structure works, but building it is too complicated and takes a lot of nerves.
+- Using material tension to secure the modules via rope was basically a good idea. 
+
 ## Naming
 
 The English project name remains:
@@ -39,9 +82,12 @@ The name roughly means **sun-disc firefly**. It keeps the solar theme, adds a sl
 ---
 
 ## Contributing
-PRs and issues welcome. Keep it simple.
+
+PRs and issues welcome. Keep it simple.  
 Photos of your build you send me go in `Documentation/gallery/` with a short caption and license notice.
 
 ---
 
-## Default license**: [CC-BY-NC-4.0](https://creativecommons.org/licenses/by-nc/4.0/)
+## License
+
+Default license: [CC-BY-NC-4.0](https://creativecommons.org/licenses/by-nc/4.0/)