Film Canister (Scraper) Node

Firmware: https://github.com/jonsowman/ukhasnet-fc-node/tree/master/fc-node2/firmware

Hardware: https://github.com/jonsowman/ukhasnet-fc-node/tree/master/fc-node2/pcb

A super small, super low power transmit only solar node.

The idea behind this node design was to see how long a (transmit only/sensor) node could be made to last and how cheaply such a node could be made.

The power supply is a single 650mAh NiMH AAA cell (about 50p). Energy is collected via 40mA 5Voc solar panel that came from China for about £2. A footprint for a HTU21D temperature/humidity sensor is included. The microcontroller is an ATTINY44V (the -V version is required since operation down to 1.8V is part of the operation of this node).

The node uses the Microchip MCP1640 boost regulator to boost the (nominal) 1.2V from the NiMH cell to the 3.3V (nominal) operating voltage of the node.

A BU49 voltage detector is connected to the resevoir capacitor (regulator output) and its output is connected to the INT0 pin on the AVR.

The RFM69HW radio is used, as usual.

Working principle

On boot, the regulator turns on, since its EN pin is tied weakly to Vbatt.
The regulator charges up the 100uF resevoir capacitor, then the AVR boots and performs setup tasks, including setting up the RFM69 and sending an initial packet.
The RFM69 is put into sleep mode
The AVR enables an interrupt for a low level on INT0 from the voltage detector, then turns off all peripherals and all clocks. It sets the regulator EN pin low, thus disabling it, and goes into power down mode.*
Eventually the voltage on the reservoir capacitor falls below the detection threshold, and the BU49 pulls INT0 low.
The AVR wakes, and re-enables the regulator to charge the capacitor up again.

* This works because the AVR output latch states are retained even when the core is not running

The reason behind this is that the quiescent current of the boost regulator is not negligible, so we don't prefer to keep the regulator running all the time. Using the above method, the regulator “on-time” is around 0.1%.