Soil Moisture Sensor is used to control the soil moisture, content for plants. The circuit of soil moisture control unit described here helps the people to look after their garden in their absence. When the soil is dry, this electronic circuit automatically switches on a pump motor and waters the plants. Thus, plants are protected from getting dry. Conducting rods are inserted into the soil. The electrical conductivity of the soil is directly proportional to the water present in it. This phenomenon makes this unit to switch on or off automatically according to the moisture content in it.
About the Circuit :

• The circuit uses two timer ICs acting as multivibrators and one inverter IC. Out of the two probes, one is connected to ground and other is connected to the input pin1 of IC1 through R2. When soil is dry, resistance between the probes is very high. Hence input voltage at pin1 is high. This electrical signal gets inverted at pin2. As voltage at trigger pin2 of IC2 is below 1/3Vcc, IC2 gets triggered and gives logic high at output pin3. IC2 drives IC3, which is wired as an astable multivibrator. The output from IC3 turns transistor T1 on and thus it drives the relay.
• When soil is wet, the output of IC1 at pin2 is high. As the voltage at trigger pin2 of IC2 is greater than 1/3Vcc, it does not get trigger. The output from IC3 makes T1 off and in turn relay becomes off. LED D2 gives the visual indication of on/off motor. LED D1 is used to indicate the output status of IC1.
• Calibration :  The level of moisture content, at which the motor becomes on, can be set by preset P1.

Soil Moisture Sensor is used to control the soil moisture, content for plants. The circuit of soil moisture control unit described here helps the people to look after their garden in their absence. When the soil is dry, this electronic circuit automatically switches on a pump motor and waters the plants. Thus, plants are protected from getting dry. Conducting rods are inserted into the soil. The electrical conductivity of the soil is directly proportional to the water present in it. This phenomenon makes this unit to switch on or off automatically according to the moisture content in it.
About the Circuit :

• The circuit uses two timer ICs acting as multivibrators and one inverter IC. Out of the two probes, one is connected to ground and other is connected to the input pin1 of IC1 through R2. When soil is dry, resistance between the probes is very high. Hence input voltage at pin1 is high. This electrical signal gets inverted at pin2. As voltage at trigger pin2 of IC2 is below 1/3Vcc, IC2 gets triggered and gives logic high at output pin3. IC2 drives IC3, which is wired as an astable multivibrator. The output from IC3 turns transistor T1 on and thus it drives the relay.
• When soil is wet, the output of IC1 at pin2 is high. As the voltage at trigger pin2 of IC2 is greater than 1/3Vcc, it does not get trigger. The output from IC3 makes T1 off and in turn relay becomes off. LED D2 gives the visual indication of on/off motor. LED D1 is used to indicate the output status of IC1.
• Calibration :  The level of moisture content, at which the motor becomes on, can be set by preset P1.