automatic watering system

8/12/2019 automatic watering system

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Chapter 3

Methodology

3.1 System Overview

Solar

Battery

Moisture

Sensor

Microcontroller

Transmitter

Solar

Battery

Moisture

Sensor

Microcontroller

Transmitter

Solar

Battery

Moisture

Sensor

Microcontroller

Transmitter

AREA 1 AREA 3AREA 2

MICROCONTROLLER

RECEIVER

GATE VALVE for AREA 1



Figure 3-1 Block diagram of microcontroller based automatic watering sprinkler with moisture sensor.



The project Microntroller based automatic watering system is divided into two

main parts: (1) Sensor Module, (2) Channel (3) Controller Module node as shown in

Figure 3-1.

The sensor module is composed of the following: (1) Solar Panel, (2) Battery, (3)

Moisture Sensor, (4) the PIC16F689, and the (5) RF Transmitter. The battery will supply

the power for the microcontroller, transmitter and the moisture sensor and the solar panel

will do the charging of these batteries for an uninterruptible working system. The solar

panel is also equipped with control switch to avoid overcharging of the batteries. The

moisture sensor measure the water change through the changes in its resistance and these

values will be fed to the microcontroller. The PIC16F688 is the main controller of the

Sensor Module with the following functions: (1) accept the input of the moisture sensor

and analyze the data if the moisture content has reached its critical point (2) send a data

to open a valve and a data to close the valve to the RF transmitter. The RF transmitter

processes the data from the microcontroller and modulates the data suitable for

transmission. The transmitter acts as a link between the sensor and the controller modules

which will have a range of more than 100m distance separation. This allows ease

transmission of serial data between two locations wirelessly.

The channel is a medium that uses Radio Frequency allowing transmittal of

frequencies within the radio wave propagation in the electromagnetic spectrum.

The third block is the Controller Module which comprises the following: (1) the

RF Receiver, (2) the PIC16F688, and (3) Gate Valves. The RF Receiver accepts the

received data from the transmitter module. It is in this block that the received data is

checked for its consistency and if it is error free before it will be fed to the

microcontroller. The PIC16F688 is the main controller of the Receiver module that

performs the following: (1) check data’s reliability and accept input from the RF

Receiver (2) analyze the data and identify from what area it came from, (3) trigger the

opening of the gate valve if the critical level is reached and closing it if the received data

shows soil stability on the specified area.

3.2 Description of the Design Process and Implementation



The main objective of this project is to automatically open the valve when the

moisture content is at its critical level and close it when the moisture content is reached.

Thus, the input of this overall system is the moisture content and the output is the

opening and closing of the valve on the receiving side.

In the preceding section, the overview of the system is discussed. It is composed

of three major blocks, the Sensor Module, the Channel and the Controller Module. The

Sensor Module sends a controlling data for gate valves via Radio Frequency Channel and

the Controller Module accepts what has been sent. The discussion that follow highlights

the design process and implementation of the Sensor module and Controller module. It is

discussed in separate sections. Since each module comprises of different parts, these

sections are subdivided into subsections tackling each of the subcomponents.

3.2.1 Sensor Module

The sensor module functions as a sensing mechanism if the soil moisture has

reached its critical level and send command data to the controller side through radio

frequency. It powered by battery charged by the solar panel. Below is the thorough

discussion of the components of sensor module.

3.2.1.1 Moisture Sensor

The moisture sensor can read the amount of moisture present in the soil

surrounding it. A manufactured soil moisture sensor is used for this project shown in

VCC

GND

DO

AO

PROBE

CONNECTING

WIRE

Figure 3-2 Soil Moisture Sensor [20]



Figure 3-2. This sensor has a working voltage of 3.3 V - 5 V and uses the two probes to

pass current through the soil, and then it reads that resistance to get the moisture level.

More water makes the soil conduct electricity more easily (less resistance), while dry soil

conducts electricity poorly (more resistance).

The control board consists of four pins: (1) A0 for analog output, (2) D0 for

digital output, (3) VCC for voltage of 3.3V- 5V and (4) GND for ground. The board also

includes comparator LM393 chip, adjustable sensitivity, and has an analog and digital

output which enable to read the specific soil moisture information for analog signal

output or the over-wet or over-dry soil information according to the threshold for digital

signal output. The adjustable potentiometer is used to set the moisture threshold. This

control board can get the moisture value or threshold in the soil via analog or digital pins.

Threshold voltage is a voltage for comparison. When the soil moisture value read

by the sensor is above the threshold value, a low level (0V) will be digitally output; when

the soil moisture value read by the sensor is below the threshold value, a high level (3.3V

or 5V) will be digitally output. In this way, the digital pin can be used directly to read the

current soil moisture value to see if it is above the threshold or not. The threshold voltage

can be regulated by simply twisting the potentiometer which is shown in Figure 3-3, and

it increases by rotating to left side and decreases by rotating to right side. In this system

there will be using two moisture sensors that is separated by some distance for an

efficient moisture measurement detection.

Figure 3-3 Adjustable Sensitivity on the Control Board of Soil Moisture Sensor [19]



3.2.1.2 PIC Microcontroller

The PIC16F688 serves as the main controller for the Sensor Module, this same

transmitter module will be used in all designated AREAS. As already stated, this

controller performs two functions: (1) check the digital output of the moisture sensor if it

is “1” or “0” and (2) send data that would open or close the valve in the receiver module

side. It has a total of 14 ports, 12 ports of which are configurable as input/output pins.

More than enough for the system design. Below is the figure of a PIC16F688.

The PIC microcontroller is set to run through its internal 20 Mhz oscillator. As

stated above, we will be using two sensors in each area so two ports will be used. Pins 9

and 10 are used as an input port for the digital output of the soil moisture sensor. The

transmitter is connected to its pin 6 where it is supported of UART Com suitable for

transmission. If this pin has a transmit enable bit then the sensor module will send data to

the receiver side.

Figure 3-4 Pin diagram of PIC16F88



Yes

No

The schematic diagram shows the algorithm of the microcontroller shown in Figure 3-6.

Initialize I/O Ports

Send Open Gate

Valve Data Frame

Receive Data from

Sensor

Is data = ‘1’?

START

Initialize UART Com

Figure 3-5 PIC16F688 connection (Sensor Module)



No

Yes

At start-up, the MCU initializes the I/O and UART ports including the pin

assignment of the line circuit. After initializing all the ports, the MCU accepts the

feedback from the sensor about the soil moisture measurement. It then examine if the

data from the sensor signifies that the soil already reached its critical level. If the input is

“0”, the sensor will just continue to measure the moisture content until such time that the

digital output of the sensor is “1”. When an input “1” is given, the microcontroller will

give an output data that would trigger the transmitter to send data to open the gate valve

data frame to the receiver module. After sending, the microcontroller will again receive

data coming from the sensor, it will analyze if the data is “0”. If the reading is “0” the

MCU will again trigger the transmitter to send data but this time, it will be a close gate

valve data frame. After transmitting the close data gate valve, it will start again from

where it accepts data after initializing all the MCU ports.

3.2.1.3 RF Transmitter

The output from the MCU is used as an input to the RF Transmitter Module. This

modules functions as follows: (1) accepts the input from the microcontroller and (2) send

this data serially to the RF Channel. HopeRF’s RFM22B is the transmitter that will be

used for the system for its wireless transmission. Shown in Fig. 3-7 below is the

transmitter used for the system.

Is data = ‘0’?

Receive Data from

Sensor

Figure 3-6 Algorithm for Sensor Module

Send Close Gate Valve

Data Frame



It is highly integrated and low cost wireless ISM transceiver module. The low

receive sensitivity coupled with industry leading +20dBm output power ensures extended

range and improved link performance. Built-in antenna diversity and support for

frequency hopping can be used to further extend range and improved link performance.

The direct digital transmit modulation and automatic power ramping ensure precise

transmit modulation and reduced spectral spreading ensuring compliance with global

regulations. An easy-to-use calculator is provided to quickly configure the radio settings.

The RFM22B has a wide operating voltage range of 1.8-3.6 V and low current

consumption makes an ideal solution for battery powered applications. The project will

use FSK as a modulation technique. A standard 4-pin serial peripheral interface (SPI) bus

is used to communicate with an external microcontroller. Three configurable general

purpose I/Os are available. Figure 3-8 below shows the pin diagram of the transmitter

that will be used.

Figure 3-7 HopeRF RFM22B Transceiver [18]



3.2.1.4 Solar Panel

A solar panel is used to charge the battery in each area. A single cell can produce

½ volt by converting the sun’s energy into an electrical signal. A 4.5AH, 6V lead acid

battery will using for this system. To charge the batteries with this rating, building

multiple cells into a single substrate in order to yield the convenience of higher output

voltage from a single package will be necessary. 24 cells will be connected in series to

provide the necessary voltage. Figure 3-9 below shows a configuration of the battery

charging from the solar panel.

Charging

circuit

Figure 3-9 Power supply diagram for the transmitter module

Figure 3-8 Pin diagram of RFM22B [18]



The circuit shown in Figure 3-10 is a solar battery charger circuit which uses a 12

volt solar panel and IC LM 317 as its variable voltage regulator. LM317 is an adjustable

three terminal positive-voltage regulator capable of supplying more than 1.5A over an

output-voltage range of 1.25V to 37V. It has also a better standard than other regulators.

The charging current passes through diode D1 to the LM 317 and to regulate the output

voltage and current you need to tune the Adjust pin. A variable resistor (VR) is placed

between the adjust pin and ground to provide an output voltage of 9V to the battery. The

resistor (R3) controls the charging current and diode D2 prevents current discharge from

the battery. To stop the charging of battery when it is full, the transistor T1 and Zener

diode ZD act as cut off switch. Normally, T1 is off and battery is charging. When the

terminal voltage of the battery rises above 6.8 volts, Zener conducts and provides base

current to the transistor. It then turns on grounding the output of LM317 to stop charging.

3.2.2 Controller Module

In the controller module side, this part act as a receiver of the data command sent

by the sensor module. It will analyze those data and perform necessary actions with

respect to the sensor data. Below is the discussion of the elements present in thecontroller module.

3.2.2.1 RF Receiver

Figure 3-10 Schematic of solar battery charger



In building an RF system, the transmitter selection is accompanied by the

selection of the receiver. Hence, the Receiver Module design similar to that of the

transmitter side. The input is the serial data from the RF channel sent by the transmitter

module. This receiver is capable of accepting all the data sent by the transmitters in each

area. With this, the same type of module will be used as with the transmitters.

3.2.2.2 PIC Microcontroller

The design for the controller of the Receiver module is far more complicated to

that of the Transmitter Module. While the controller in the latter only checks the input

given by the sensor and analyzes it to trigger the transmitter to send an open or close gate

valve data to the receiver side. The receiver MCU performs the following functions: (1)checks if the data received is valid, (2) verify if the data received is area A, B or C, (3)

trigger the gate valve if an open valve data is received and (4) close the gate if the

transmitter send a close valve data. Figure 3-11 is the pin connection of the PIC16F688 in

the controller module.

Fig. 3-11. PIC16F688 connection (Controller Module)



The schematic diagram shows the algorithm of the microcontroller is shown in

figure 3-12. After initializing the ports, the MCU will check the validity of the data

received. It then verify if the data is from the transmitter of area A, B, or C. If for

example, the data is from area A, the MCU will trigger the pin to open the valve A. After

opening the valve, it will continue to accept data coming from designated or specific area

while ignoring others. If a command data is received to close the valve that is the time the

MCU will acknowledge and process data from other transmitters. Since the MCU will

only process the data received one at a time, each gate valve will have an open and close

transition.

3.2.2.3 Solenoid Valve

Solenoid Valve is an electromechanically operated valve. The valve is controlled

by an electric current through a solenoid. A solenoid is operated by opening and closing

an orifice in a valve body that permits and prevents flow through the valve. The orifice is

opened or closed through the use of a plunger that is raised or lowered within a sleeve

tube by energizing the coil.

Figure 3-13 shows a solenoid valve that will be used for this project. The valve is

made up of plastic having a port size of 12mm, both for inlet and outlet. It is a 2- way

Figure 3-13 Solenoid Valve [20]



solenoid valve having one inlet and one outlet. The valve is a normally closed type which

operates at 12V DC with a pressure range of 0.02- 0.8 MPa. A normally closed valve

prevents the water to flow through unless a current is applied.

In this project, three solenoid valves will be used since there are three plots or

irrigation fields needed for the prototype. This solenoid valve is triggered during the

critical level where soil moisture content is below the threshold value or the desired

moisture level. When the soil moisture content is below the threshold value, the solenoid

valve will automatically open to allow the flow of water, and closes when soil moisture

content is reached.

Each area or irrigation field has a circuit shown in Fig.3-3 to control a solenoid.

The circuit is connected to the output pins of the PIC16F688. The output pins used for the

circuit for area 1, area 2, and area 3 are RC0 (pin 10), RC1 (pin 9), and RC2 (pin 8)

respectively.

Many output devices such as relays and solenoid will require a transistor

switching circuit. In most cases a darlington pair formed from two transistors is ideal. In

the circuit in Fig.3-3, a transistor called BCX38B is used which looks like a single

transistor but is in fact two transistors set up as a Darlington Pair in a single package

which gives a high gain and high current capability. This discrete device has a collector-

current rating of 800mA that will easily drive the output devices. In addition to the

Figure 3-14 Schematic for the triggering circuit of the solenoid valve



circuit, the use of a back emf suppression diode across the relay contacts is to prevent

damage to the transistor when the relay switches off.

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