I have a project in mind where I would like to use an Arduino board connected to a moisture sensor (similar in function to a thermistor for temp) where resistance varies based on moisture content to turn on/off a 24v AC power supply. The 24v AC power supply is connected to a lawn sprinkler valve solenoid. The sprinkler valve would be turned on to deliver water to a garden when the moisture content is below a target.
I'm looking for guidance on the moisture sensor. Is anyone aware of a moisture sensor similar to a thermistor that could be used in this application?
Appreciate any help!
Sandy
A Google of Soil moisture sensor Arduino will bring up plenty of hits and examples. This is a good example but moisture sensors suffer from problems.The major problem is corrosion as a result of electrolysis. After a short time the sensors are shot and no longer function.
You can likely use a sensor but only apply power briefly every several hours, take maybe 10 quick samples, average them and remove power till next time. There are other ways around corrosion but they get more complex than the basic sensors.
You can use a Digital Out when you want to call for water and have the D/O drive an AC SSR (Solid State Relay). You want one with a 3 ~ 30 VDC control input which will handle 24 VAC at whatever your solenoid valve current is. You may also want to place a snubber across the SSR. Most AC SSRs will switch 24 VAC without a problem, just make sure it is rated that low.
You may also do well to well insulate your moisture sensor and bury it maybe a few inches deep where the roots live. 
Ron