Designs

dAuratus

The dAuratus board is named after dendrobates auratus (aka the green and black poison dart frog) which is currently acknowledged to be the smartest of all frogs. This is my AVR MCU board that serves as the platform for most of my home hobby projects.

A VDD of 3.3V is provided by an ADP7104 regulator able to accept a VIN of up to 20V. Includes a switch for turning VDD on/off which doubles as the hardware reset mechanism for the MCU.
Pins for interfacing to external devices:

Pins	Count	Notes
PA0-PA7	8	The USART and SPI default pins.
PC0-PC3	4	Timer, CCL default pins. USART/SPI secondary pins.
PD0-PD7	8	Comparator, ADC, DAC default pins.
VDD	2
GND	2

Co-locate communication pins for easy connector design:
- Serial terminal communications requires USART TxD, RxD and GND.
- SPI requires MOSI, MISO, SCK and one I/O pin per device for slave select.
- VDD, GND and UPDI for programming (keep same order as SNAP). Place on 3 pin dedicated vertical male header.
External device header layout

Pin	J1	J3	J2
1	PA7 (SS)	PC0 (MOSI)	GND
2	PA6 (SS)	PC1 (MISO)	PD1
3	PA5	PC2 (SCK)	PD2
4	PA4	PC3 (SS)	PD3
5	PA3	GND	PD4
6	PA2	VDD	PD5
7	PA1 (RxD)	VDD	PD6
8	PA0 (TxD)	GND	PD7

Power traces are speced for 500~mA (the max regulator current) and signal traces are speced for 50~mA (the max sink/source current of the AVR). For a 1oz copper trace, this corresponds to pretty small traces (0.115mm for power) but I set my signal and power traces to 0.3mm and 0.5mm, the same as the Phil’s Lab KiCad tutorial.

Lithobot

Named after the Northern Leopard frog, lithobates pipiens; a frog that can help keep gardens healthy by eating pests and is currently threatened in Alberta.

Implemented using a dAuratus board with the following functionality:

Soil moisture sensor(s).
Photocell sunlight sensor.
Temperature sensor.
eRibbit wireless link.

bbBufo

The central bbb interface to the home network. Named after Bufo marinus, the largest known toad.

eRibbit

A wireless protocol built on top of LoRa.

Ignis Filesystem

A simple filesystem for the external FLASH chips used on the dAuratus board. A play on will-o-the-wisps or ignis fatuus which were flashes in swamps scientifically attributed to the spontaneous combustion of marsh gas.

Fan Controller

A controller for 12VDC fans to adjust their speed and gust pattern. Used to create air movement in a grow box to strengthen the plants and prevent fungus and/or rot.

The speed of the fans is adjusted by treating each one as a load for a TSM4NB60 MOSFET.
Gate voltage is adjusted by RC lowpass filtering the pulse width modulated outputs of the three 8 bit TCB timers on the AVR128DB28 MCU. The RC time constant equals (10kOhm)*(1uF) = 10ms.
Fan observations:
- The fans benefit from a brief stretch at a 100% duty cycle (max speed) to spin them up. Start up remains inconsistent so the duration of startup time may need to be adjusted.
- Fan speed is non-linear with the fan very quickly going from a medium speed to off (approximately over 90% to 87% duty cycle). A non-linear step pattern through this range would give the fans a more linear behavior.
- When fans driven too slowly, they enter an oscillatory state where they alternate between gusting and off.