idea & prototype
After experimenting with blockchain implementations for IoT devices I decided to use a Raspberry Pi Zero W for a first iteration on my solar project for energy: a physical pay-for-a-smile interface for the blockchain.
basic circuit setup
Pi Zero W + Camera, micro servo, 5V booster, 3.7 V 1200mAh LiPO, charging/load circuit, solar panel
I re-measured the power consumption for the project and averaged the values over time/runs. The Pi Zero showed a significantly lower power consumption compared to the Pi 3 (in mA).
I am still contemplating on how to create a deep sleep mode that fits the concept of the project. At the moment, the Pi is listening constantly on changes in the blockchain, then reacts by turning the servo when a user sends money into the account. Conceptually the project would be online forever (so that users can continuously trigger the smiley face), the camera would need to stream constantly as well (something still not finished in pi sketch, at the moment I am using my laptop camera). This is not possible with the current circuit as it will be down after 4.5 hours if running exclusively on battery.
LiPO specifics: 3.7 V, 1200mAh
LiPO + booster: 5 V, 890mAh (rounded)
runtime for continuous sketch (ignoring the servo peaks): 4.5 hours (rounded)
This would mean it would inevitably stop at nighttime when there is no daylight to charge the LiPo from the solar panel. Assuming the physical interface would be mounted outside, it would make sense to power it off at night - as the camera would not be able to film the interaction anyway. This would as well emphasize the connection of the piece to a real environment - which a physical web-interface tries to evoke anyway.
Nevertheless, an external deep sleep circuit is necessary. I am currently waiting for an external hat for the pi that will manage the power with an automatic shutdown/wakeup schedule.
Hopefully the sun will come out in the next few days so that I can run the sketch at least for one or two days to test the performance of the circuit over time and alter it if necessary.
I want to experiment in the next few days with writing data to the blockchain directly with the Pi Zero W to store environmental data from a temperature sensor in the network permanently with solar power.