Contact Closure Schematic

Schematic diagram which shows how to connect up to 4 soil moisture probes to a set of 4 relays. This circuit is useful for connecting the probe to sprinkler systems, or moisture sensing alarms.

The circuit can be used with analog probes that produce a voltage proportional to soil moisture such as the vegetronix.com VG400 probe. The circuit uses op-amps as unity gain buffers to buffer the output of the probes. This buffered output is then fed to a comparator with hysteresis, which compares the value to a voltage set by a variable resistor. If the high threshold is surpassed the relay energizes. Likewise if the low threshold is surpassed the relay releases.