Moisture Sensor Circuit
The moisture sensor uses two metal probes which are inserted into the plants soil. If the soil is moist it will conduct a small current. The amount of current depends on the chemical composition of the soil. Dry soil will not conduct an appreciable amount of current.
The amount of current between the probes is not enough to cause the Gadget Master II input line to change state (0 to 1). However, the current is enough to trigger a transistor to turn on and connect the input line to ground which changes its state and can be detected by the computer. (In an earlier lesson you learned that a tiny current in the Base-Emitter circuit causes a much larger current to flow in the Collector-Emitter circuit.)
The resistors limit the current to prevent damage to the transistor and guard against a short circuit in the Gadget Master II. By making the value of the resistor(s) higher you can make the circuit less sensitive. A variable resistor substituted for one of the 470 ohm resistors would give the circuit variable sensitivity. This could be used to adjust for differing soil compositions.
The circuit shown next uses a large NPN transistor (TIP29A). The circuit can be soldered directly to the transistor or a small piece of "hobby" circuit board can be used to mount the components. The illustrations that follow show both techniques as well as the circuit schematics.