OK, I'll be specific. The main circuit is the watering timer controller I have been working on the last month (on and off). I'm building it for some friends who live out in the country, and have a big polytunnel greenhouse which needs watering once or twice a day from a tank that fills off the roof of one of the buildings. They don't have mains power - just a windmill / battery / inverter system which isn't always reliable, so I'm trying to build it as a standalone solar powered system. It's meant to be as automatic as possible, so they can mostly just leave it running and forget about it. E.g. if they want to go away for a few weeks. I'm also trying to build it so the design can be used by anyone else in a similar situation, and as cheaply as possible - i.e. use cheap, standard parts and simple construction rather than exotic components.
The main circuit triggers a 0.5s 12A pulse through an electromagnet when the light goes from day to night or vice versa. This is all working and tested on breadboard. The circuit uses about 0.7-0.8 mA when it's waiting to trigger, and 12A during the pulses, which makes:
24*0.7mA=17 mAh/day for the controller 12*.5*2/3600=3.3 mAh/day for the electromagnet = 20 mAh/day total.I've kept the current as low as possible, because the solar panel will be the most expensive single component - I've found someone who does a 1 W (rated) panel for about 10 UKP, and these panels only reliably provide 70 mAh/day in uk winters, 250 mAh/day in the summer. The battery will be a
1.2 or 2.1 Ah yuasa NP battery.The battery protection circuit is probably icing on the cake. There ought to be enough charge in it to cover a long run of dull days, but I wanted something like that in there to stop battery damage if something goes wrong. Also, there are other similar projects I'm thinking about which would need this more.
The important thing is that when the circuit is switched off, the current drain should be well under the drain in normal use, so the solar panel can recharge the battery. Say no more than 2 mAh/day, which would make 80 uA average current. And during normal use, it shouldn't add to much to the total current consumption - say 200 uA max. A reset button would be an annoyance - there's no way to tell if it's working properly without waiting for the circuit to trigger, or maybe pressing a button to activate a power test led, but this would mean there would be something else to do every day to make sure the system was working, which would spoil the point of making it automatic.
Like I said, I'll have a proper look at all the designs people have posted, and then build something based on them. Thanks to everyone who has replied - I reckon I know enough now to get the thing built, so I'll probably leave off posting about it until it's finished and written up on the web, and then post a link.