I built one of those "powerbank keep-alive" circuits to draw ~150mA for ~2 secs every ~12secs. Seems to be working to keep a USB wall wart on for a few hours whilte the feeder is in standby - we'll see if it's still on tomorrow. If so I can ditch the D-cells for a rechargeable power bank and run the camera and feeder from one dual-port 5V 2.2A USB charger.

I think a 20K mAh power bank would keep the camera and the feeder going for about a day. Feeder alone should last 6-8 weeks. It lasted about a month on D-cells with 10-20 deposit/dispense interactions per day. Using USB power banks might also make solar charging easier to add.

And I get to use yet another 555! :)

If it works I'll add it to the schematic and sanity check the rest for any errors I missed - I know there are some.

UPDATE

Still not 100% on 5V DC wall chargers - for some reason the feeder does not work properly after  a day or so. I reduced the period of the keep-alive from ~12/2 to ~7/1 secs and that seemed to extend the running time a bit, but didn't fix the problem. Unplugging and replugging seems to fix it. Using 6V works fine over time, but 5V does not. I may have to solicit help from a friend in Texas that repairs power supplies for a living as I'm out of ideas.

The keep-alive does work with USB power banks so far. It's been running strong on one for a day with no issues. I think I can tweak the keep-alive to make it more power-efficient (shorter pulse, longer cycle) while still keeping the power bank alive.

Since the power bank works, I was thinking of putting two in the enclosure. Run off one of them and solar charge the other. I have some random solar cells I tried out and found two 3V 210mA cells that fit perfectly in a CD jewel case and put out ~6V at ~170mA when wired in series. Along with a small circuit to regulate the solar voltage to 5.1V it should run for ~4 weeks on one 10K mAh UBS power bank while the other charges. If there is an average/equivalent of 2-3 hours of usable sunlight per day, the second bank should be charged by the time the first one needs recharging.

UPDATE - USB working, forget solar for now

A 150mA keep alive pulse of ~1 sec every ~10 sec has been working on a 20K mAh power bank for a couple of days now with 20-40 interactions per day. I'm cheking it periodically to see when that first status light goes out - I'm hoping it's no less than ~10 days.

Solar is another issue that I will get to later. I want to focus on finalizing the electronics for a V1 for this thing and making some PCBs.