An RF Controlled Irrigation System

8/2/2019 An RF Controlled Irrigation System

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An RF-ControlledIrrigation System

With access t o a

st eady wat er s upply,

Br ians gar den

should f lourish in

even the driest of

times. Having caught

wireless fever, he

set out t o use an

AVR and some RF

products t o man the

pump and close t he

valves. Now, water-

ing only takes a

press of the green

thumb.

When I sat down to write this article last fall, the leaves on the trees had notyet turned their autumn colors, but the beauty of the flowers in our gardenbeds was certainly on the wane. It was a dry summer, particularly punishingfor farmers, and our gardens werent particularly splendid last year. Not that Ididnt try to keep them well watered, its just that its hard to beat a steadydose of rainwater.

Were fortunate to have built a home on a large lake. Twelve years ago, wechose the lot based mainly on recreational concerns swimming, canoeing,and such. I became seriously interested in gardening about five years back,and decided to install an irrigation system to make use of the unlimited sup-ply of free water.

Our lot is about 25 feet above the lakes level. As any mechanical engineerwill tell you, its a lot easier to push water than it is to pull it, so I installeda 0.75-hp jet pump at the waters edge. I decided against using a pressuretank and switch, as the water would be needed only when the pump wasswitched on, and the maximum continuous flow rate was desirable.

Because most of the rough landscaping had been done when the house wasbuilt, I decided it would be too much effort and expense to bury irrigation linesthroughout the 0.75 acre of lawn and gardens that I have. Instead, I ran 1.5 plastic pipe on the surface, along the side border of my property. Sixvalves/ garden hose fit tings are spaced along the 400 foot length.

For a number of years, I was content to run down to the electrical panel in thebasement to switch on the pump when I wanted to do some watering. Besides

being inconvenient, occasionally Id shut off the water valves when finished

a n dthen for-get to return to thebasement to turn off the pump. Oneyear I damaged the pump by leaving it on for sev-eral days! Also I was gett ing lazy; I didn t like the troubleof hooking up a hose, unraveling 100 feet of it into the desired positioattaching a sprinkler head, and then having to walk all of the way back to thother end to turn on the water valve.

I decided what I needed was a controller that allows me to program specifwatering times and durations. Units like this are commercially available, course, but I also wanted to be able to control the water using a small keyfotransmitter while I puttered around in the gardens.

In my last article, I described a wireless MP3 player, which used low-cost UHtransmitt er/ receiver modules from Abacom Technologies ( ListEverywhere, Circuit Cellar 134) . I was pleased with their performance atechnical support from Abacom, so I decided to check out Abacoms producagain.

I wanted the transmitter to fit in a keyfob, so I chose the AT-MT1-418 Atransmitter module, which is about the size of a penny. I also chose Abacomkeyfob transmitter case, which comes in various switch cutout configurationI decided to use a sensitive receiver because I anticipated a low transmitte

signal level given such a small transmit ter. The QMR1 Quasi AM/ FM superhreceiver module fit my needs. I particularly like this module because its

square-inch SIP mounts easily on a circuit board by pins on 0.1 ceters. I like one-stop shopping, so of course I was pleased to be able get Holtek encoder/ decoder chips from Abacom, as well. I ll descrithe chips in more detail later in the article.

Controller/ ReceiverIf youve read my recent articles, it should come as no surprise thaused an Atmel AVR controller chip, the AT90S853 5-8PC (40 -pin Dpackage), for this project. This device contains four 8-bit ports, eight 1bit ADC channels, 8 KB of flash memory, and 512 bytes each of daEEPROM and RAM. Like most AVR devices, this one is easily serially prgrammable in-circuit. You may want to refer to my arti cle, M

fAVRorite Family of Micros (Circuit Cellar 133) for an overview of thfamily, along with the details of a free ISP programmer for these chip

I must admit up front that I probably could have done this project witthe smaller AT90S2313 by multiplexing some of t he I/ O pins and wring the program in assembly language. I decided it was more produtive for me to spend the extra dollars (Can $) on the 8535, whoslarger flash memory would allow me to program in BASIC, using tBASCOM AVR compiler.

With access t o a

st eady wat er s upply,

Br ians gar den

should f lourish in

even the dr iest of

t imes. Having cau ght

wireless fever, he

set out t o use an

AVR and some RF

products t o man the

pump and close the

valves. Now, water-

ing only takes a

press of the green

thumb.

Authors Note: I want to

thank John Barclay ofAbacom Technologies for the

support and samples thathelped out significantly whileI was putting this article

together.

Brian Millier is an instrumen-tation engineer in the

Chemistry Department ofDalhousie University, Halifax,

Canada. He also runsComputer InterfaceConsultants. You may reach

him at [email protected].

www.atmel.com p a g e 2 8

A T M E L A P P L I C A T I O N S J O U R N A L

By Brian Millier

Photo 1 Heres the actual controller/ receiver sitting in my f amily room. Just visible in the

background is a glimpse of the l ake the source of water f or the gardens. Not visible is the

ACadapter used for power or t he power relay, which is located at the electrical panel in the

basement.


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Figure 1 is a schematic of the controller/ receiver. Lets start by look ing at theuser interface. The user interface consists of a 4 20 LCD and four push but-tons. The display is operated in the common 4-bit mode; in this case, becauseit saved some wiring, not because of a shortage of I/ O pins.

The four push-button swi tches are individually strobed by port pins PC0 3 andsensed by the INT1 input of the 8535. I hooked up the switches this waybecause I originally drove the LCD using the same four port Clines. I had been

saving the ADCinputs of port A for future use, but later changed my mind andswitched the LCD over to port A, leaving this switch circuit intact.

The four push-button switches operate this unit the same way that many smallelectronic devices work. There is a Menu button to scroll through severalmenus as well as a Select/ Cursor button. The buttons are used to position thecursor within a time field for adjustment purposes or to select a particular valuewhen finished changing it. Finally, there are plus sign and negative sign but-tons used to increment or decrement the current parameter.

I chose to implement the real-time clock in the software. One reason I initial-ly picked the 8535 over the slightly less expensive 8515 is because itincludes a third timer, which may be driven by a 32,768-Hz watch crystal. Imust say that my at tempts to implement the RTC using this feature gave mesome problems! Atmels datasheet for the 8535 advises you to merely con-

nect the 32,768-Hz watch crystal between the TOSC pins 1 and 2 wi th capacitors to ground. [1]

When I did this, I could see a reasonable 32,768-Hz sine wave signal on eithcrystal pin with my oscilloscope using a 10 probe. I soon discovered, thougthat my clock was losing about 1 min./ h. After troubleshooting, I f ound ththe crystal oscillator waveform contained serious glitches coinciding with LCscreen refreshes.

At that point, I was using the port pin adjacent to TOSC1 to drive the LCENABLE pin. Moving the LCD ENABLE pin over to port A eliminated the glitces, but the clock was still slow. This was odd because I could not see anythinwrong with the crystal waveform with my oscilloscope, and the built-in frquency counter in the oscilloscope indicated that the frequency was bang-onSo next, I contacted Mark at MCS Electronics to see if he had run into the prolem. He mentioned capacitors, which made me think that capacitance ground was probably needed (contrary to the datasheet). It turns out that moscilloscope was providing the necessary capacitance, but only when it wahooked up. Adding 22-pF capacitors to ground cured the problem, at least wthe particular crystal I was using. However, for this project, I decided to playsafe and implement the RTC using Timer0 of the 8535 clocked by t4.194304-MHz crystal of the CPU, which works perfectly. A side effect of thwas that I couldnt use BASCOMs intrinsic real-time clock function and insteahad to write my own routine.


Figure 1 The Atmel 8535 AVR controll er is at the center of the action of the irr igati on controll er. An Abacom QMR1 receiver takes care of the wireless reception f unc-

tions. The LCD operates in 4-bit mode.

Reprinted with permissionof Circuit Cellar -Issue 138January 2002


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My pump draws about 10 A when running (much more when starting), so Ichose a Potter & Brumfield T9AP5D52-12, which is inexpensive and rated for20-A continuous current. A small 2N3904 transistor is all that is needed tohandle the 200 mA that its coil requires. This sealed relay is small. I haventused it long enough to know how well it will hold up, so the jury is still out onthis component choice.

The controller/ receiver is powered by a 9-VDCadapter followed by a 78L05

regulator. The actual output of the adapter is closer to 12-V, and is enough tooperate the relay coil. Photo 1 shows the controller in place in my family room.The wireless part of the controller consists of an Abacom QMR1 receiver fol-lowed by a Holtek HT12D decoder chip. This receiver is one of the choices rec-ommended for use with the AT-MT1 AM transmitter that I use. The datasheetthat comes with the package (available soon on www.abacom-tech.com) callsthe QMR1 a quasi-AM/ FM receiver module. The datasheet doesnt spell out ifit also works with FM transmitters, but it sounds like it would.

In any AM transmitter/ receiver link, one thing for certain is that the receiverwill spit out a stream of noisy data during much of the time when its com-panion transmitter is not transmitting. The QMR1 is sensitive (RF sensitivityspecification is 110 dBm) and it has no squelch circuitry to suppress spuri-ous output signals arising from any RF interference that it might receive. Withcell phone towers cropping up all over the countryside, even my rural home isprobably not RF-quiet anymore. I definitely see lots of noise output from theQRM1 receiver module.

My intent ion is to emphasize the need for some form of error detection/ dataformatting in any AM RF link. What I havent mentioned is that the circuitry inthe receiver that recovers the data from the RF signal (called the data slicer)is choosy about the form of data modulation that it will accept.

For example, most data slicers work reliably only if there is a roughly even dis-tribution of zeros and ones in the datastream, even within the short-term suchas the time taken to send 1 byte of data. This means that you cannot, forexample, just feed in the signal from a UARTto an AM transmitter, and expect to hook upa UART to the receiver output.

Instead, Manchester encoding is generallyused because it guarantees an equal numberof zeros and ones in the datastream, regard-less of the particular data being sent.Furthermore, it is good practice to send thesame data several times and check that itmatches when it comes out of the receiver. Afinal precaution could include some form ofchecksum or better still, a CRC byte in thedata packet to further verify the integrity ofthe received data.

Another concern is the amount of time it

takes the receiver to adjust itself to thestrength of the incoming signal or wake upfrom an idle state if that feature is present inyour receiver module. To allow for this, thetransmitter must send out a short stream ofknown data, called a preamble, to allow thereceiver to get ready for data reception, so tospeak.

This is a lot tougher than your average RS-232 serial data link! There are many books

that cover in depth the theory of reliable RF data communication; AIntroduction to Low Power Radio by Peter Birnie and John Fairall is a goostarting point for those of you starting out in this area. [2]

Encoder/ decoderTo address these concerns, it made sense to use the inexpensive line oencoder/ decoder devices from Holtek (HT12D/ E) rather than roll my owThese matching chips address the concerns, at least for applications that nee

only to transmit the status of a small number of switches.

There are a number of good reasons for choosing this device. The HT12encoder chip consumes only about 0.1 A in Standby mode, so it can be lepermanently connected across the small transmitter battery. It comes in small, 20-pin SOP and fits in a small transmitter case (the same could be safor the Atmel ATiny and smaller PIC processors) . To reduce parts count ancost, it uses a single resistor to set its internal RC clock. RC clocks are nknown for their frequency stabilit y; the design of this encoder/ decoder pallows the receiver to be able to lock onto the transmitters data clock frquency even though it may vary considerably over time or temperature. Refto Figure 2 for the schematic of the transmitter module.

Both the encoder and decoder sample eight lines (A0 through A7), which a

as device address inputs. That is to say, a given encoder/ decoder pair can set to operate at one of 2 56 discrete addresses. This strategy, f or example, pvents your neighbors remote control from operating your garage door opene

Addressing can be done with a dip switch, jumpers, or by cutting traces onPCB. Modern encoder/ decoder chipsets used in remote car starters use, bnecessity, a much more complex addressing scheme because theres a mucgreater chance of false triggering by other, unintended transmitters in the vicity. Obviously, this leads to worse repercussions.

The data packet sent by the HT12E consists of the 8-bit address followed by4-bit data field corresponding to the state of up to four switches connected

SOFTWARE

To download the code, go to

f tp .c i rcu i t ce l lar . com/ pub/ Ci rcu i t_

Cellar/ 2001/ 138/ .

REFERENCES

[1] Atmel Corp., 8-bit AVR

Microcontroller with 8K Bytes

In-System Programmable

Flash AT90S8535

AT90LS8535, rev. 1041GS,

September 2001 .

[ 2] P. Birnie and J. Fairall , An

Introduction to Low PowerRadio, Character Press Ltd., UK,

1999.

[ 3] Holtek Semiconductor Inc., 212 Series

of Decoders, July 12, 1999.

[4] , HT12A/ HT12E212 Series

of Encoders, April 11, 2000.


Figure 2 There isnt too much to the schematic diagram of the keyf ob transmitter. However, getting i t to f it i nto the

small keyfob was another matter!


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inputs D8D11. The datasheets for the HT12D/ E devices dont mention a pre-amble being sent before the data, nor do they mention a checksum nor CRCbytes for data checking. [3, 4]

In place of this, the data packet is transmitted three times for each switch clo-sure and then checked for equality by the receiver. Holding the switch downfor any more than an instant, will result in the repetition of the datastream.Presumably this is how the lack of a preamble is handled the receiver like-ly misses out on the first occurrence of the data packet, but catches subsequentones.

The Abacom AT-MT1 transmitter has a maximum data transmission rate of2400 bps. There-fore, I set the encoders oscillator of the HT12E to 2 kHz byusing a 1 .5-MW resistor across OSC1 and OSC2. [ 4]

The AT-MT1 t ransmitter is a two-wire device. It is not modulated per se; insteadit is powered up and down in step with the datastream. The SAW oscillatorused in this module is able to turn on and off quickly fast enough to handlethe maximum data rate. The output of an encoder chip is supposed to direct-ly power the AT-MT1, according to its datasheet. Although the data output pinof the HT12E is capable of sourcing up to 1.6 mA, the AT-MT1 requires up to9 mA at 12 V to operate. So, in this case, I had to add a 2N3904 emitter fol-lower to provide the necessary current boost.

I intended to use a Linx Splatch antenna, which is a small PCB containing a418-MHz antenna and ground plane. Unfortunately, this small antenna radi-ated much less signal than a quarter-wave whip antenna and would not pro-vide the range I wanted. However, it wasnt too great a loss because I washaving trouble fitting everything into the keyfob anyway. I ended up using a

6.25 piece of flexible wire as an antenna, which just hangs out of the key-fob case and doesnt mind being stuffed into my pocket.

Photo 2 is a close-up of the transmitter PCB, which has to fit in the case andline up with the switch cutouts. I included the PCB layout in PDF format alongwith the firmware files, because the design of the transmitter PCB is tedious.Choosing a battery for the transmitter wasnt difficult. There seems to be only

two choices in small batteries: 3.6-V coin cells and the 12-V alkaline batteriesused in many remote car starters. The HT12E encoder would have worked fineat 3.6 V, but the output power of the transmitter module would have beenlow. Thus, I chose the 12-V batteries.

The FirmwareOne of the reasons for choosing thAT90S8535 instead of one of its little broters, like the 2313, was to allow me the luury of programming the firmware in BASICFrom past experience, I thought there was nenough space in the 2-KB flash memory of th2313 for an application such as this usi

compiled BASIC.

I wrote the firmware using the MCS ElectroniBASCOM-AVR compiler. It took up more thahalf, 4800 bytes, of the 8192 bytes of flasprogram memory, confirming my fears thatwould not have fit into the memory of thsmaller 2313 device. Incidentally, the demversion of the BASCOM-AVR is available frfrom MCS Electronics, and is fully functionapart from the fact that its program size limis 2 KB.

As I mentioned earlier, problems I had usi

Timer2 ( designed for RTC purposes) of the 8535 prevented me from usithe built-in RTCroutines in the BASCOM-AVR. This had an upside: The RTCrotines needed by this application do not require week, month, or year, so theuse less memory space even though they were coded in BASIC (Note: TBASCOM intrinsic RTCf unction is done in assembly language) .

Most of the firmware takes care of the user interface. An LCD with four pusbuttons is easy to build, but takes up considerable program space to implment a friendly user interface. There is a routine that allows you to set thclock to the current time. Another routine enables you to enter up to six prgrams. Each program consists of a time, action (pump on/ off) , and a Daily Once-Only mode. And, a final menu item allows you to turn the pump on anoff immediately from the controller.

The six user-defined programs are stored in EEPROM, so that they survive power failure. However, because the CPU (and therefore the RTC) will stopthe power goes off, this is a moot point, unless I add a battery backup for thcontrollers CPU.

When a command comes in from the wireless transmitter, the valid transmsion (VT) line on the decoder will go high, and its four data output lines wreflect the state of the four buttons on the keyfob transmitter. The VT signalfed into the INT0 interrupt input of the 8535 (through RCfiltering to prevefalse triggering). An interrupt service routine checks the state of the decodefour outputs and turns the pump on or off accordingly. Although I fitted foubuttons into the transmitter and allowed for all four in the controller, thfirmware currently responds to only two switches pump on and pump offwill likely think of some other device to hook up to this in the future.

Times upTheres no doubt that its much less expensive to buy a remote control moduoff the shelf than it is to build your own, if you can find one that suits yoneeds. However, if your requirement is unique or you can combine a few funtions into one unit, then the satisfaction of designing your own unit makesall worthwhile. I find building these wireless gadgets addictive. In the back my mind, Im already thinking of my next project: a controller for air exchaner in my home using indoor/ outdoor temperature and humidity sensors andpower line modem.

Photo 2 The PCB that I fabr icated for the transmit ter sits below the keyf ob case. You can see a bit of the thi n black

wire, which forms the antenna, connected to the t iny tr ansmitter module.

SOURCES

AT-MT1-418 AM Transmitter module

Abacom Technologies

(416) 236-3858

Fax: (416) 236-8866

www.abacom-tech.com

AT90S8535-8PCMicrocontroller

Atmel Corp.

(714) 282-8080

Fax: (714) 282-0500

www.atmel.com

HT12D/ EDecoder chip

Holtek Semiconductor Inc.

(510) 252-9880

Fax: (510) 252-9885

www.holtek.com

BASCOM-AVR Compiler / progr ammer

MCS Electroni cs

31 75 6148799

Fax: 31 75 6144189www.mcselec.com


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