bernie
Bought: RaspberryPi 5, 2GB version running Raspberry OS Lite, with a passive heatsink to give the lightest load. Set with parameter "POWER_OFF_ON_HALT=1" to reduce load during off phase.
From RPi docs, for RPI5, to enable low power mode and wakeup with the Real Time Clock:
$ sudo -E rpi-eeprom-config --edit
and edit the following line
POWER_OFF_ON_HALT=1
test with
$ echo +600 | sudo tee /sys/class/rtc/rtc0/wakealarm
$ sudo halt
This sets on time to ten minutes, then powers the Pi off, after 10mins it should auto power on again.
Current status
Python program written to set the next required on time into the /sys/class/rtc/rtc0/wakealarm location and then power off. Currently during the testing phase the program, during the mid-day on time, simply saves a file containing a timestamp string rather than taking a photo. This tested, and powers up automatically at the right times, then powers down. The timestamp files are saved regularly. The Pi on-switch can be used to turn the Pi on at any time, which gives five minutes before the Pi auto shuts down again. So the user has five minutes to connect, and if required abort the shutdown.
The Python files are on this site, as power.py and also a systemd service file, power.service which is used to ensure power.py is run on boot up.
Obtained: two 2W, 6V solar panels, batteries, power management unit, pi plus camera module3.
Next step, change python file to take photos, wire all together and test it works. Then build box and stand for the whole lot.
tomtibbetts
Xiaoj329
rifowler
Patrick Van Oosterwijck
Power budget:
Note: the 2GB version of the Pi5 has a tested current of less than 1A when running with lite OS, though possibly with a peak current higher than this on startup.
Assume on-time of 20 mins, energy used by Pi:
5V * 1A * 0.33h = 1.7 watt hours (Wh)
Assuming 0.05 Watt for standby during rest of day 0.05W * 24h = 1.2 Wh. (ignoring 20 mins less)
Total energy required for 24hr = 3.0 Wh
Waveshare power management unit, has 3x18650 Li-ion, and can give 3A output - so the possible higher startup current of the Pi is covered.
Batteries are rated 3.7V, 2500mAh
Max battery energy capacity 3 x 3.7V x 2.5Ah = 27.75 Wh
So fully charged batteries have plenty of capacity.
Assuming 6V 2.0W Solar panel available from PiHut.
Assume UK weather only gets 1W, then after four hours of daylight:
1.0(W) x 4(h) = 4.0 Wh
So should get the 3Wh energy required per 24hr.
However after further consideration: UK winter sun may produce less than 1W. This solar panel is advertised as 6V. If panel output drops below 6V, then the management unit may fail to work as it is advertised for an input of 6V and above. So decided to get two 6V solar panels and wire them in series to get a nominal 12V, together with the increased wattage output.