Rainman John Has a Natural Moisturizer and an Excellent Duck

March 15, 2004

Seeing lawn spinklers constantly running in the rain, John thought about how silly and wasteful they are. So he developed a device that monitors the moisture content of the soil and automatically triggers a sprinkler system when required. The sensor is a pair of electrodes (based on bicycle spokes), while the controller for the device is based on a fluid detector powered by the nominal 24V ac most sprinkler systems use.

Water minder parts list
Amt Part Description Allied Part #
1 Relay, 24V dc 821-2212
1 Switch, SPDT Toggle 676-3000
1 Box Plastic 736-0063
1 Trim Pot, 50k 754-3212
1 2N3906 Transistor 950-3002
1 LED, Red, T1, .75 749-9819
1 Zener Diode, 24V dc 950-6043
2 Diode, 1N4001 266-0001
1 Cap, Elec, 220 mF/50V 852-7082
1 Resistor, 270V, 1W 296-6508
Additional parts required: Fluid detector, miscellaneous resistors and capacitors, and bicycle spokes

Additional information:

The LM1830 has a built-in oscillator, a comparator and output drive circuitry. When the sensed pin has a higher resistance (drier) than the reference, the output turns on. In this case the output cannot supply sufficient drive current for the signal LED and relay, so a transistor buffer is added. The dryness setpoint is set by the trimpot and a bypass switch is included for testing, etc. The relay specified can handle 10 amps - plenty for a water solenoid.

The sensor is a pair of electrodes that contact the soil at the desired level. They should be stainless steel to resist corrosion. Where's the best (cheapest) place to get these - bicycle spokes! I potted 2 of them in a 35mm film canister and put heatshrink on all but the lowest 1" of the spokes.

Camera ready artwork for the pcb and chassis cover are available from:

Sunspot Svcs,
115 Greenbrier Dr
Simpsonville, SC 29680 $2, check or money order plus S.A.S.E.

PDF diagram

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