I plan to be away from home for 3-4 weeks, and had bought citrus plants for my lean-to. So I created a very low cost waterer (about £8. including demi-john). It consists of a demi-john part filled with water, inlet and outlet pipes through a non-porous bung. The outlet and inlet are controlled by non-return valves (pet fish anti-syphon valves). As the days temperature increases water is pumped by air expansion from the demi-john towards the plant. As the day cools in the evening water is sucked from a reservoir by the air contracting. I've a number now, with delivery from 50 to 300ml per day.
A nice feature is that on cool days less is pumped and the night gives time for drainage to occur. By altering the volume of air trapped in the demi-john, you alter the volume delivered per day.
I use it indoors and outdoors

I noticed a few postings about absent watering and so for me, this is my solution.