Just wanted to describe a circuit I built to automate blowing out a
multi-zone irrigation system using a small home compressor. But first
let me give some background.

For the past 5 years, I have successfully blown out my irrigation
system each fall using a 10 gallon compressor that I bought for $99
from Home Depot (and that therefore paid for itself the first year
that I didn't need to hire a service).

Until this year, I have used the following painstaking manual
process. Specifically, for each of the nine zones on my system, I have
let the compressor fill & empty about 6 times waiting about 4 minutes
between attempts so as to keep the duty cycle down and prevent the
compressor from overheating (I also aim a small fan at the compressor
fins to further aid cooling). The multiple times per zone is required
since 10 gallons of air is not sufficient to blow out all the
tubing. The visible water is blown out within the first 2-3 cycles and
I add another 3 cycles or so just to be sure and to get out the last
drops that show up as a light mist. This painstaking process, however,
requires me running back and forth to my garage about 54 times and
wastes the better part of a day. Last year, I was able to save some
time by "cleverly" programming and rewiring my controller so that
every other zone was disconnected, allowing the compressor to recharge
on the intervening disconnected zones - this reduced my manual
interventions to about 12 trips to the garage.

This year, I built a simple circuit based around a 555 timer and a
relay that runs off the 28VAC transformer (using a bridge rectifier,
regulator and capacitors to reduce and convert the voltage to
12VDC). The timer turns the relay on for 1 minute and then off for 4
minutes (though both times are adjustable with a pair of
potentiometers). Two leads connect to the VAC and the other two leads
go across the rain sensor terminals (with the rain sensor temporarily
disconnected). Then I can automatically blow out the whole system by
setting my irrigation controller to run sequentially through all nine
zones with 30 minutes or so per zone (which gets me 6 cycles per
zone).

If anybody is interested in the schematic, I would be happy to share
it along with photos of the completed project. Note I also added two
LED's (red & green) to show the relay state, an off/switch, and a nice
box to package it all up.