A Small controller based on a Digispark ATTiny controller I have lying around and a ispare DHT11 combined temperature and humidity sensor.
Turns the Fan on and Off (and several speeds in between) depending on the temperature, humidity, user input and battery state (voltage).
- DigiSpark Based
- ATtiny85, plenty powerful enough for this application, and with a robust toolchain. It has 6 IO pins & very little extra cruft
- Has a onboard 5v regulator, I can supply it with the battery voltage, and I can power the DHT11 from the 5v line
- Somewhat tricky to flash/develop for, Lack of a uart is a total pain during debug since BitBang serial emulators eat half the memory+
- DHT11 Temperature and Humidity sensor
- Not ideal, something more accurate would be prefered, but this is what I have.
- A DHT22 would be an easy (plug and go) upgrade
- PWM fan control via a high-current logic-level NPN mosfet
- I have a quiet but powerful (Noctuna) 12V PC case fan to run from this
- Battery voltage sensing for powersave and shutdown
- A button.. to allow on/off/quiet overrides
Case design (scad) and model files are in the repo.
See the 'BoatFanControl' script in the 'Software' folder.
- It takes one reading every 20S and then decides how to set fan.
- Power Control
- Vmin->11.8V : Powersave mode; no activity, but still wake every 20s to take and process a voltage reading.
- 11.8->12.5V : Low Power Mode; fan is restricted to 30%
- 12.5->Vmax : Full power mode. fan can rise to 100%
- Fan Control
- Is done on a simple Trigger value for temperature and humidity (28c and 70% respectively), the fan will start at 30% PWM power, and rise to 100% over the next four degrees temperature or 20% humidity. In low power mode the speed will never rise and remains at 30%
- User Control
- The Button cycles: Full -> Quiet -> Off etc.. and flashes for feedback.
- Quiet mode is Low power mode.
- After three hours the mode will change to the next power level, the button is a 'sleep' function, not a on/off switch.
- Average readings; I keep five readings and discard outliers before averaging the three middle values.
- Testing is tricky. The code has an extensive debug mode for this.
- I'd like to use interrupts and have the controller sleep between readings/decisions, but for now I just delay(); power consumption is negligible like that, it does not register at all on the boats powermeter when the fan is off (and only 0.1A when the fan is full speed.)