Hey Friend!
When you are looking for some good products, amazon.com always helps you. It at least helps me every time and when i searched for an automatic water timer which works with low water pressure, i got a machine for you on same online store.
I think that one is exactly you are looking for and i am also adding link for you to visit the site and have a check yourself. :)
http://www.amazon.com/Melnor-3060-6-Cycle-Electronic-AquaTimer/dp/B00004SDVT

Thanks for the info, rosessecretgarden.

Unfortunately, the reviews on this thing are less than stellar for the cost.

It does have quite a lot of features and it does mention the low pressure - but I just wonder what it means by low pressure.

The other thing I do like is the two outputs - because I will have one spot where I will need to control the output for two separate areas of the garden (can't run both at the same time - just not enough water for that!).

But, I just don't like all the reviews about taking up battery life, it stopping after a year, the issues with it being in the sun, the device busting open, and the issue that folks mention about it keeping time correctly.

I'm just quite surprised that there doesn't seem to be a good alternative for timers that simply shut off or on the water supply - like a ball valve - but doesn't reduce the flow of water.

The only other way I would know how to do this would be to put a water pump on the line feeding into the timer - but then that means electrical wiring and buying an expensive pump.

Here is a link that might be useful: BsnTech Gardening Blog

It looks like this may be a really old posting, but I have been looking for a similar type of timer. Low pressure (rainbarrel source) and not "junk". I did find the Aquano Logica timer on the Drip Depot site: http://www.dripdepot.com/1429
Spec says it goes down to 1.45 psi and it looks really simple to use. Only disadvantage is you have to use one of the presets. Let me know if you find anything else.
D

For some timers it is possible to pull out the steel filter where you connect it to the hose. That will allow the water from your rain barrel to flow better through the timer. The water in your barrel should be clean if you set up your rain catchment system according to the American Rain Catchment System Association (ARCSA) standards, making the steel filter unnecessary for irrigation.

As far as looking into pumps you can easily just use an RV pump to add pressure to your line. They can range from $50 to $300.

RV pumps are also 12V so they can be easily connected to a solar panel. This will eliminate you needing to run power at all unless you need to position the panel away from the barrel. The only draw back is you will need to place the pump in some kind of housing to protect it from the elements. I personally wouldnt leave it out if it gets near freezing where you are.

Hope this helps

Here is a link that might be useful: OffGridSolutions