Irrigation System

Run a motor-driven pump

In this section we will set up a simple irrigation system that waters from top. The parts included in the kit allows you to build a watering system matching to the size of greenhouse.

System Overview

Pump Controlling

To water from top, a pump is needed to lift water from the tank. The following illustration shows a simple structured motor driven pump, with a series of blades that spins rapidly to move the water. As the impeller rotates, it creates flow in the water, causing it to be pumped through the piping system.

To fully automate the watering system, we need to have the pump controlled electronically rather than manually turning it on and off using a power button.

We utilize a MOSFET to achieve fully automate the watering system. In this circumstance, the MOSFET acts as an electronic switch, regulating the flow of electrical current in the circuit.

Detecting Soil Moisture

"Before the system can function automatically, we need to gather information. Soil moisture is crucial for plant growth, and this value can be detected using either a resistive sensor or a capacitive sensor.A

In this experiment, we are going to use a resistive sensor. A resistive soil moisture sensor typically consists of two metal plates that to be buried into the soil. When a voltage is applied across the two plates, current may pass through the soil between the plates and therefore the resistance of the soil can be measured. As the soil becomes wetter, its electrical resistance decreases.


How to decide R1

From a sensorโ€™s perspective, R1 should match to the resistance of the sensor when it is in normal state. For example, if the sensor resistance is around 10kฮฉ when the soil moisture is moderate, then we can use R1 = 10kฮฉ. Matching the resistance allows maximization of the sensitivity. For example, for R1 = 1ฮฉ, the equation becomes to:

The consequence is that regardless of the sensor resistance change, you may hardly observe any voltage changes at the output.

Pump Regulating

The motor circuit starts pumping water by sending roughly 3.25V to the MOSFET gate. Note that is result is only for IRF640 MOSFET used in this project, if you use other MOSFET this number is very likely different.

If the circuit operates at a 5V reference voltage (VCC), its output voltage will vary from 0V to 5V depending on the soil's moisture level. However, it's essential to limit the gate voltage to ~3.3V since exceeding this limit could cause the motor to run too fast and pump an excessive amount of water. Therefore, the main concern is how to regulate the sensor output so that it doesn't exceed 3.3V.

A simple and low cost solution is to use a Zener diode. A Zener diode is a type of diode designed to operate in the "reverse breakdown" region, which means the current may flow from Cathode to Anode. A Zener diode exhibits the same characteristics as a general-purpose diode in the forward bias region, but has a โ€œZenerโ€ breakdown voltage at reverse region, as shown in the following chart.

This breakdown voltage is called Zener voltage, which is a key parameter specified by the manufacturer. It typically ranges from a few volts to several hundred volts. This voltage is the point at which the diode begins to conduct in the reverse direction and is a constant value that does not change with current flow.

By integrating soil moisture detector, Zener diode voltage reference circuit and MOSFET motor driver together, we can build an automatic watering system, which is capable to activate the pump for dry soil and stops the pump when soil is wet.

Complete Circuit

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