# FireFly Solar / 日輪蛍

**Nichirin Hotaru**  
*A small solar-powered Morse firefly for the outdoors.*

**太陽を宿し、夜にモールスを灯す蛍。**  
*Taiyō o yadoshi, yoru ni Mōrusu o tomosu hotaru.*  
*A firefly carrying the sun, lighting Morse in the night.*

## Overview

FireFly Solar is a small outdoor LED blinker inspired by the idea of an LED throwie, but powered by sunlight instead of a disposable coin cell.

During the day, the solar cell charges the energy storage.  
At night, the circuit wakes up and sends a simple Morse-style light signal.

## 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

- Build a solar-powered outdoor Morse blinker
- Keep the hardware simple and robust
- Improve practical low-power design skills
- 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:

**FireFly Solar**

For blog posts, documentation, and presentation, the project also uses the Japanese name:

**日輪蛍 / Nichirin Hotaru**

The name roughly means **sun-disc firefly**. It keeps the solar theme, adds a slightly mythic tone, and still fits a small piece of outdoor hardware.

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## Contributing

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.

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## License

Default license: [CC-BY-NC-4.0](https://creativecommons.org/licenses/by-nc/4.0/)