“Motivation: I wanted to use battery powered Raspberry Pi camera to take once-a-day photos outdoors to create long term time-lapse videos. My particular application is to record ground cover plant growth this coming spring and summer.
Challenge: Design low current Raspberry Pi power control to ensure long battery life.
My Solution: I use a hacked alarm clock, Attiny85 circuit & Pimoroni OnOff shim to completely cut power to Raspberry Pi when not in use. While the Attiny85 and alarm clock continues running in standby mode, current draw is only 5 microAmps. Two AAA batteries power both Attiny and alarm clock, while a USB power bank powers the Pi.
Basic Operation: When to alarm clock goes off it wakes a sleeping Attiny circuit, which then signals Pimoroni OnOff shim to apply power from USB power bank to Raspberry Pi. The Pi executes a run-at-boot script (take a photograph). After sufficient time has passed (60 seconds in my application), the Attiny circuit again signals Pimoroni OnOff shim and then the Attiny enters sleep mode. Based on the signal from Attiny, the Pimoroni OnOff shim executes Pi shutdown command, and after Pi shutdown process completes, cuts power from USB power bank to Raspberry Pi.”