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An MCU-Based Irrigation Control System

Tom describes a PIC-based prototype design for a wireless water control system for EarthBoxes， which are gardening boxes with water reservoirs and fertilizer strips for cultivating plants and vegetables. The scalable design can accommodate various multiple-box configurations.

In this article， I will describe a prototype for a wireless water control system for EarthBoxes （see Photo 1）。 The design is scalable and can accommodate small to large quantities of EarthBoxes.



I first described EarthBoxes in "Java-Based EarthBox Watering System" （Circuit Cellar 201， 2007）。 EarthBoxes are advanced gardening boxes with self-contained water reservoirs and fertilizer strips that create a near-perfect environment for growing plants and vegetables in a compact 2.5′ long by 15″ wide growing area （see Figure 1）。 Because of its compact， self-contained configuration， the EarthBox enables productive gardening in small spaces.
点击查看Figure 1

My family and I are strong proponents of the EarthBox. We‘ve used it successfully for several years now to grow vegetables. As a side note， because of its productivity and low maintenance，the EarthBox is beginning to make serious headway as an alternative in some large commercial-scale farming activities.

When using EarthBoxes， you must make sure the internal water reservoirs are replenished on a daily basis， especially during the height of the growing season. A manual watering regimen could be my "back to nature" mantra； however， I get a lot more satisfaction using an automation process where microcontrollers （especially ones that I have designed） do the work. My articles on this subject document my attempts to automate that process. In one sense， the design I cover in this article can be viewed as "second generation"-that is，it is an updated version of the first design that features new technologies.

IMPLEMENTATION OVERVIEW
A representation of this system is shown in Figure 1. This configuration should not be conscribed as a design limitation， rather it is configured for my gardening needs. The design covers two sites that are on the south side of a large deck in my back yard. Each site is separated by about 50′ to 70′ to allow for gardening maneuverability and harvesting. The system， as implemented， can water eight EarthBoxes. I found that six to eight EarthBoxes can easily meet the fresh vegetable needs of my immediate family （six people）。

Let‘s review the major components. The first is the portable hand-held control unit （on the left-hand side）。 It is used to remotely determine the level of the water reservoir within each EarthBox and to initiate， under your control， a watering action for that EarthBox， if necessary. Another component is the remote-controlled monitor and water unit. There is one for every four EarthBoxes. This component is controlled by the hand-held unit and continuously monitors the water level for each of its associated Earth- Boxes. It also controls the water valves， which are used to supply water to the reservoirs in the EarthBoxes. The final component is the valve box. The boxes are a useful way to organize distributed valves using a manifold system to help minimize water hose connections. Both the hand-held and remote units are Microchip Technology PIC24F-based microcontroller systems. Communication is accomplished with MaxStream （Digi International） 802.15 ZigBee modules.

The hand-held control unit uses a Multilabs 160 × 80 monochrome touchscreen LCD. This approach offers a flexible way of dealing with the operator interface with virtual touch buttons and graphic and text displays. The product is easy to configure （via a vendor command set over a TTL serial port） and reasonably priced. Newer versions have greater pixel capability and support controlled backlighting. Photo 2 shows the user interface constructed for the present design. One of my design goals was simple representation and straightforward operation. I wanted a novice user，with some reasonable notion of the overall system configuration，to be able to effectively operate the system. In other words， I did not want the watering responsibility all the time！

点击查看Photo 2

The main screen （defaults at power-up） enables you to select the site of interest， determine the reservoir water status， and initiate watering， if necessary. Once the site is selected， the handheld presents the site‘s EarthBox water level and valve activation status. The status can be refreshed at any time by pressing the status button. You can then return to the main screen to select another site. To provide audio feedback for button depressions against the touchscreen， a simple 3.3-V buzzer is incorporated. From the status panel， you can enter the control panel by pressing the control button. At the control panel， you can initiate a watering. Here， only one box valve can be activated at a time by selecting the appropriate button or all of the box valves can be turned off simultaneously. Once a box valve button is selected， the control panel automatically exits to the status panel where all status information for that site is updated automatically.

MCU & TOOLS
The PIC24F is a 16-bit， general-purpose，3.3-V based microcontroller. Microchip conveniently offers the chip in a 28-pin skinny DIP package（PIC24F64GA002）， which allows for easy assembly in a solderless breadboard setup. The processor has ample program flash memory （64 KB）， 8 KB of RAM， and 16-MIPS operation with no need for an external oscillator， low-power operation，and a host of on-chip peripherals. In fact，it contains more internal peripherals than there are pins available. Microchip has gotten around this problem by introducing a peripheral pin select feature，which enables you to map up to 16 （pins RP1 to RP16） of the 28 pins to internal peripherals. The days when designers had to face up to fixed-pin configuration for peripherals are gone. This was a handy feature in this design， where in one case，two UARTs are required for the handheld unit （one for the touch panel and the other for the MaxStream radio）。While in the monitoring and control unit， a single UART is still required for the radio， but all other pins are needed for ADC inputs or relay control. The total available peripheral capability of the PIC24F64GA002 is impressive： 10 channels，10 bits， 500-kbps ADC， dual UARTS， dual SPI， dual I2C， dual analog comparators， a parallel master port， five 16-bit timers， five capture modules， five compare/PWM modules， a real-time clock calendar RTCC with an alarm function，as well as just 19 digital I/O. In this design， I hardly scratched the surface but there is good use of the digital I/O， the UARTs， a SPI， ADC channels， and RTCC.

The PIC24F64GA002 supports onboard debug and in-circuit programming using Microchip in-circuit serial programming （ICSP）。 This interface requires a simple six-pin interface that is easily accommodated with simple six-pin 0.1″ headers on the solderless breadboard. The ICSP appears as a common interface on all of the prototype breadboards for obvious reasons. Microchip also supplies the MPLAB integrated development environment（IDE） and C30 compiler （the C compiler for this part） for free. （C was used in these designs， but MPLAB also supports assembly.） To complete the picture for debug and programming， Microchip recently introduced a low-cost version    of a real-time debugger/programmer that works with PIC24F microcontrollers.

The PICkit2 Debug Express costs less than $50 for this design implementation. The entire tool set was used. The PICkit2 plugs nicely into the on-board ICSP， which enables quick insertion for program updates and debug without any prototype disassembly. When I assembled the ICSP on the breadboards，I ensured that the PICkit2 could be easily mechanically inserted and removed.

HAND-HELD UNIT
This design is a mixed 5- and 3.3-V design （see Figure 2）。 The graphics display touchscreen requires 5 V. This voltage is derived from a SparkFun Electronics regulator adapter board. The 3.3 V is derived from a separate 3.3-V three-pin regulator connected to the output of the SparkFun board. Fortunately for 5-V digital I/O， the PIC24F， which is a 3.3-V part， can accommodate this need. Any nonanalog peripheral pin of the PIC24F can be configured as open-drain digital output that with a pull-up resistor to 5 V，can drive 5-V logic without issue. UART2 TX， connected to an LCD， is configured in this matter using C code：
__ODB5 =1； //set RB5 to be open drain

All input pins on the PIC24F64GA002 are 5-V tolerant. In this matter， the Multilab serial output，a 5-V output， is simply connected to the UART2 RX from the LCD. UART1 TX/RX is connected to the MaxStream ZigBee radio. The radio module uses a 3.3-V serial interface so no additional level shifting is needed. I directly plugged the radio module into the solderless breadboard with a carrier module that has 0.1″ spacing （from SparkFun Electronics）。 The PIC24F64GA002 real-time clock calendar requires external circuitry，which is accomplished by a simple 32-kHz clock crystal and dual 30-pF capacitors. The clock can be used to activate any needed automatic timebased operations on part of the handheld application （i.e.， polled remote status updates or data logging， if necessary）。There is also a 1-KB EEPROM interface supported by SPI1. The EEPROM is needed to establish application-specific power-up parameters. Finally， a RadioShack 3.3-V buzzer provides audio feedback for the graphics touchscreen. The list of RP pins and C code to modify them is in Listing 1. Note that a macro code is used to unlock and lock the RP pin configuration using a special-purpose register OSCCON.

Photo 2b shows the hand-held control unit in a makeshift case. The main screen is active. It also shows the handheld unit disassembled with major components highlighted. If it is not clear by now， I am keen on minimal assembly efforts and rapid prototyping techniques. In these early stages of design，it is prudent to use techniques that can accommodate quick modification and changes. Once you are through these early prototype validation stages， a more permanent configuration can be employed based on market value and customer needs.

MONITOR AND CONTROL UNIT
This unit is exclusively 3.3 V with the exception of 12 V used in relayswitched control of water valves （see Figure 3）。 UART1 TX/RX is connected to the MaxStream ZigBee radio， identical to the hand-held. I tried to employ as much commonality as I could across the two designs to maximize hardware and software reuse. I also used the PIC24F real-time clock calendar. Here the clock can activate necessary automatic time-based operations on part of the monitor application（i.e.， unsolicited status updates every predefined time）。 Finally， the 1-KB EEPROM is used， for the same reasons as the hand-held， to establish application- specific power-up parameters.
点击查看Figure 3

The use of the ADC across four unique channels for individual monitoring of each sensor input is new. This use of ADC is critical to the"wet test." The Microchip MP608 opamp is used exclusively for all sensor interfaces. Each MP608 is configured as a unity gain follower. The part is low-power and has sufficient gainbandwidth product to handle the sensor‘s 10-kHz input waveform. It also has rail-to-rail operation for maximum output swing. Each MP608 is capacitively coupled to the water medium. The input capacitor is then connected to a resistor bias network to ensure that the incoming level is properly DC-shifted to reduce clipping before ADC conversion. A single MP608 is used to drive the water media from the P24Fgenerated 10-kHz square wave. Again， a unity gain follower configuration is used. The sensor probes are close to each other in the water medium （less than 0.5″ separation）， so no gain is required.

The use of PIC24F digital I/O with interface electronics to switch power to the water valves using relays is also new. The relays used are DPDT capable of switching up to 2 A at 24 VDC. The coils can be activated with just 3.3 V. The 12 V is connected to one side of the DPDT wiper of each relay with the normally open side of the relay connected to the power input to the water valve. To drive the relay， each PIC24F digital I/O uses a TP120 Darlington pair transistor with its collector connected to one side of the coil while the other is connected to the 3.3-V supply.

Listing 2 includes the list of RP pins and the actual C code to modify them with the monitor and control unit. Photo 3 shows the disassembled monitor and control breadboard.

THE WET TEST
The key to ensuring successful operation is what I call the "wet test." This test is conducted at every sensor in the monitor and control unit to determine if water is present （see Figure 4）。

During the wet test， the PIC24F64GA002 continuously generates a 10-kHz 50% duty cycle square wave doing nothing other than bit-banging output pins. The output square wave is then applied to an MCP608 3.3-V， singlesupply，full-rail op-amp configured as a voltage follower. The op-amp then sources the signal to all of the reservoirs within the site associated with the monitor unit. On the other side of each reservoir is the sensor pick-up for that signal. A replica of the signal should appear on this side of the reservoir if water （as an electrical conductor） is present. Each pick-up feeds its own MCP608 op-amp that is capacitively coupled to the source and DC-biased to center the mid-voltage point of the original square wave to avoid clipping. The recovered signal， a copy of the original square wave， is somewhat "rounded" in its edges （because of inherent low-pass filtering）and then applied to an ADC channel of the PIC24F64GA002. Here‘s where the magic happens. The PIC24F64GA002 is used to generate the output waveform level. This level is first stabilized and then ADC sampled. This synchronization with ADC sampling is done in lockstep with all output waveform changes. Because you know the signal at origin and you can synchronically sample it as it crosses the water media， you should know the answer a priori. In essence， you perform a"digital correlation." If there is a failure in comparison between input and output at any time， this constitutes a lack of water between the output source and the input sensor. The only other implementation concern is to mechanically set the source and sensor probes within the water medium at the right level in the reservoir to establish a "full condition."

A couple of other points need to be made. The input samples through the ADC channel are 10-bit unsigned magnitude values. They need to be threshold set to determine the one or zero representation. The PIC24F can accomplish all of these activities in its 16-MIPS operation and sustain a 10-kHz waveform digital correlation process. The 10 kHz， its 3.3-V swing， and the probe material （steel）， tend to minimize any potential for corrosion build-up at the probes. Finally， the entire wet test runs over several hundred cycles at 10 kHz for each sensor to ensure that a wet or unwet condition is consistently stable before a determination is made and reported.

DEPLOYED SYSTEM
The valve box is shown in Photo 4. The solenoid irrigation valve‘s PVC manifold constructions are also in the box. For each manifold， a garden hose must be connected for the water supply （see Photo 1）。

I mounted a 0.5″ garden hose adapter to the water entry for each manifold. The solenoid relay I chose was a Lawn Genie 54048 0.75″ in-line valve， which is available at most hardware stores. I used crushed white rock to weigh down the remote box.

OPERATING SOFTWARE
No picture of a microcontroller-based design would be complete without an overview and description of the software. As I mentioned earlier， the design is written in C code using Microchip‘s C30 compiler. The design is multifile and the file structure is used commonly across all of the designs. What is unique to the design is constrained and captured in the WATER.C file. This is also the main function file for C. Again，WATER.C is different for the hand-held and the monitoring and control unit with all of the other files being identical.

The file structure captures interfaces via their .H and the actual functional code in their .C manifestations. There are seven file sets all together （see Figure 5）。 The EZDISPLAY file set is a driver set to communicate with a Multilab graphics display in its predefined manufacturer‘s protocol. This file set is active only in the hand-held unit； it is reliant on UART2 for its physical connectivity. The support code needed for its operation is UART2.H and CONU2.C. This set is a basic console with 9，600 bps， 8 bits， no parity， 1 stop bit initialization，and input and output routines for exercising UART2. This is required for EZDISPLAY.

The CONUART1 file set is a basic console with 9，600 bps，8 bits， no parity， 1 stop bit， initializing， and input and output for exercising UART1， which is connected to the MaxStream radio. This code enables UART1 for interrupts when a character is received over radio and made available to the PIC24F64GA002.

The ADC file set is the basic initializing and input driver for the PIC24F ADC. It is active only in the monitor and control unit. The EEPROM file set is the basic driver to read and write from the 25LC101A 1-KB EEPROM using SPI1. The RTCC file set is a basic driver for initialization， reading， writing to the PIC24F64GA002's clock， and setting the alarm. This code optionally enables the RTCC alarm to interrupt.

The MENU file set is a low-level console-based menu system that can be used to enter proper date/time arguments for clock， read/write ASCII to EEPROM， bit control， and exercising relays. This functions as a minitrapdoor to exercise the system using the MaxStream X-CTU terminal emulation code over a radio link. The receive character interrupt service is active in both units. The RTCC interrupt is also active in both designs with a TBD function for hand-held and an autostatus reporting capability for the monitor and control on alarm.

Figure 6 is indicative of the WATER.C code in the handheld unit. An executive routine drives the graphic display. During operator direction within the constraint of the graphic menu presentation， are the available buttons that the operator uses to operate the system. Note that the status display is a template initially. It requires incoming status messages to be received from the remote monitor and control units to complete the presentation. Characters are captured by interrupt from the radio， but they are processed for operational context by software one character at a time.

Figure 7 is indicative of the WATER.C code within the monitor and unit. An executive routine continuously performs the wet test on all the sensors while checking the radio communication for incoming characters between tests. After receiving a command from the hand-held， the monitor and control checks for its address condition. The address condition is valid only for a single command. The unit must be addressed for each new command.

MESH OR PEER-TO-PEER？
The radios are based on MaxStream OEM RF embedded modules that are compliant with the 802.15.4 standard and operate within the 2.4-GHz band. The MaxStream XBee type is used in this design. They have an outdoor range of up to 300′ and data rates of 250 kbps. The modems interface to a microcontroller through a 3.3-V logiclevel asynchronous serial port. By default， these modules operate in a"transparent mode." When operating in this mode， the modules act as a serial line replacement. All UART data received through the DI pin is queued for transmission. When RF data is received， the data is sent out of the DO pin. To modify or read RF module parameters， the module must first enter into Command mode-a state in which incoming characters are interpreted as commands. This is achieved by issuing a Command mode sequence， which looks like a Hayes modem command sequence. The manufacturer supplies PC-based software designated X-CTU that enables you to test and configure modules that are connected to an adapter board that uses a serial port or USB. Out of the box it is trivial to set up and test a point-to-point wireless network. I have expanded on this to quickly configure a peer-to-peer network. For my purposes， each module at the transparent level uses a unique address one character. In this case， it is a simple designator ASCII X for site A and ASCII Z for site B. I was able to do this because there is never an X or Z ASCII character used in any other communications within the system. In fact， most communication is restricted to messages with ASCII numbers， a slash， or a colon. The hand-held uses this address to designate which module a command is intended for. The addressed module responds with an echo of its address and results of the command operation. I tried to keep things simple because most commands are simply S （status），R （relay）， and M （menu）。 The Menu command is not used during normal operation； it‘s used only for setups using the X-CTU in terminal mode（thereby allowing me full PC keyboard and screen） to set the clock， configure the alarm， and modify the EEPROM.

The peer-to-peer configuration works well for my small-to-mediumsized system； however， for a larger system， a mesh will be required. I tried MaxStream XBEE_B modules that support mesh. The latencies of communication are longer than the peer to peer， but the range and extensibility of mesh shouldn't be underestimated because you may have several EarthBox sites spread over a wide area. In the mesh configuration， one of the units must function as a coordinator，while the others are router/end devices. The coordinator must be on first to be able to coordinate all of the other routers or end devices within the mesh.

START ASSEMBLING
I assembled a comprehensive watering system using a PIC24F64GA002 and commercial radio electronics. The system， as configured， represents an initial prototype that I can easily modify for greater range and capability. In small configurations， peer-to-peer radio setups are sufficient； however， as the system expands， the need for MESH becomes necessary.

Tom Kibalo （tkibalo@gmail.com） holds an M.S. in electrical engineering. He has been a practicing engineer for over 36 years in the Washington， D.C.， area. In addition to engineering， Tom is an adjunct faculty member and technical advisor at a local community college，where he teaches courses on microcontroller technology. He is also a Microchip academic partner. Tom enjoys reading，biking， gardening， and bonsai.

PROJECT FILES
To download code, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2009/223.

SOURCES
Darlington drivers, diodes, LEDs, relays, and solder
All Electronics Corp. 
www.allelectroincs.com

XBEE Radio modules
Digi International, Inc. (distributor)
www.digi.com

EarthBox Gardening system
EarthBox
www.earthbox.com

Capacitors, clock crystal, resistors, and solderless breadboard
Jameco Electronics
www.jameco.com

54048 In-line valve
Lawn Genie
www.lawngeniestore.com

MCP608 Op-amp, PIC24F64GA002 microcontroller, and PICkit2 Debug Express
Microchip Technology, Inc.
www.microchip.com

Graphic LCD
Multilabs
www.multilabs.net

Regulators board and XBEE carrier adapter
SparkFun Electronics
www.sparkfun.com