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John A. Cline writes: I have a research experiment in progress evaluating various emitters and schedules for irrigating apple trees. For one of the treatments I would like to moisture soil moisture levels and activate the irrigation when soil moisture falls below a pre-defined set point (ie, 50% Water holding capacity). The system needs to be relatively low cost and stand-alone (ie, not reliant on a intermediate datalogger). I had tensiometers in use last year, but found them be unreliable. My application is to connect two moisture sensors in parallel and connect them to a programmable Gardena garden water timer. The Gardena water timer will be programmed to apply water 6 times a day for 60 minutes each time IF either of the moisture sensors indicates soil moisture levels are falling below the set point (50% water holding capacity of the soil. The Gardena water time operates on a NORMALLY OPEN basis - that is if the circuit between the two inputs leads is not closed, the timer will supply water as programmed. If the circuit is closed, the watering schedule will be bypassed. The soil moisture switch/circuit must be NORMALLY CLOSED when there is adequate soil moisture, and OPEN when the soil moisture levels fall below the set point . Any information on appropriate sensors that meet my requirement would be appreciated. Gardena Timer - specifications Power Supply- 9VDC Input - switched open/closed circuit Output - none Sensor Specifications Power Supply - 12 vdc max (if any) Input - max 9-12 vdc, non regulated Output - switched open/closed (normally closed when soil moisture is above set point) _________________________________________ ____________________________________________ John A. Cline, Research Scientist Department of Plant Agriculture University of Guelph Horticultural Research Institute of Ontario Blueline and Hwy #3 Box 587, Simcoe, Ontario Canada N3Y 4N5 Tel: 519-426-7127 Ext. 331 Fax: 519-426-1225 Email: JCline@uoguelph.ca http://www.oac.uoguelph.ca/hrio/clin.htm#topp