Solar Powered Automatic Garden Watering Idea

One PCB many uses:

Moisture / light sensor (solar standalone, RF, or wired 4-20mA/232/USB),
sprinkler controller (, or )
- multi-unit master:
  - w/slave ag sensors via wire/232/RF and valves via wired solenoids
  - slave ag controllers via wire/232/RF
- single: RF or standalone w/ big solar/battery, or wired

Starting from http://www.xs4all.nl/~sbolt/e-groeneVinger.html Solar powered "green thumb" monitors plant water level. Can we place a larger load on the "dry" LED output so that a significant increase in current is required when it flashes, thereby dropping the supply voltage and signaling the condition of the soil back to a master unit which is supplying power and ground on a two lead cable? The power supply would run through a resistor so that the voltage would drop as the load increased. The master unit would need a resistor divider and would then use the A2D to monitor the relative voltage supplied, or a circuit that triggers an input when the supply drops below a preset voltage could be used. Ala: aout

Add a sprinkler controller solenoid driver chip, and a jacks to use other units as moisture sensors.

Needs a good relay driver to operate a sprinkler valve. Orbit Model 57460 ($10.46 from Home Depot) which is rated 24VAC requires 5.2VDC (166mA) to pull in and holds to 0.5VDC (20mA). At 3V, 100mA. 32 Ohm Coil. A standard Lawn Genie 54000 valve pulls 0.4A at 24VAC to open and 0.2A to hold.

http://books.google.com/books?id=e_X2-LqfKaMC&printsec=frontcover on page 202 (search for solenoid) says you should hold at 25% PWM, but lists a discontinued driver chip.
http://www.mondo-technology.com/mmg.html pic controlled charge pump with sudden release

Solar Panels: Must have battery to manage high current requirements for sprinkler valve. Will charge keep up with use? Need a large panel...

Solar cells and batteries included in yard lights... how much do they produce? About 50mA in full sun
$7.50 for a 200mA at 6V panel
http://www.siliconsolar.com/6v-200ma-amorphous-solar-panel-p-17854.html

Use servo (with power cut via FET when not moving) to open / close a valve instead. Could leave /partially/ open and just adjust slightly more or less open.