Fullscreen HTML5Med Res (??MB)HTML4

SPRINKLER CONTROLLER Multi-sector sprinkler controllers don’t have to be difficult to drive. This unit controls up to six sectors, has an easy-to-set clock and is programmed using simple switches and knobs. It’s also based on a PIC microcontroller and that means relatively few parts. By NED STOJADINOVIC 14  Silicon Chip T HEY SAY THAT necessity is the mother of invention but this invention was necessitated by my mother. Although the sprinkler timers currently available are wonders of modern technology, they can be rather formidable to operate. Alternate cycles, independently programmable sectors, the ability to set times months in advance and the like are all excellent features for those who want them. However, the extra complication can be a serious barrier to those who simply want to regularly water the lawn and a vegie patch a couple of times a day. This completely new design is the answer to this problem. It’s a timer that avoids programming as much as possible and is controlled by old fashioned switches and knobs, just the way mum and people of her generation (and some of mine) like it. Despite this, the timer is capable of controlling six independent solenoids (or sectors). You can individually set the watering period for each sector, turn individual sectors on or off and set which days watering takes place. The design uses a fairly new and quite high-powered piece of technology in the form of a PIC16C74A micro­ con­ troller. This device packs in a tremendous amount of complexity where nobody ever has to see it and greatly simplifies the external circuitry. And that allows us to keep the cost down. Operation Naturally, there is some programming to be done and this involves first setting the clock and the watering start times. You can set two watering start times per day, typically one for early morning and one for late afternoon. This is very easy to do, as we shall see later on. If you can set the time on a digital clock, you will have no problems because the operation is self-evident. The watering duration is controlled by a row of six knobs, each corresponding to a sector. For those not familiar with the terminology, a sector is an area controlled by an electrically-operated water valve, commonly called a “solenoid”. Sector 1 might be your front lawn, sector 2 a garden bed, sector 3 the vegie patch and so on. Each knob can set the watering duration of its sector from a few min- It’s pretty much self-evident how you drive this Sprinkler Controller. Once the two watering start times have been set (on the LCD clock), you use the knobs to set the watering duration for each sector (0-60 minutes) and the toggle switches to set the days of the week that watering takes place. utes to about one hour. Furthermore, turning the knob fully anticlockwise means that the corresponding sector will be off and no watering will take place. Similarly, turning it fully on (clockwise) turns that solenoid on continuously. The seven toggle switches (one for every day of the week) allow you to choose the days that watering takes place. This is handy if you only want to water on alternate days, for example, or to comply with any council regulations which may restrict watering to certain days of the week. Flicking a switch off means that there will be no watering at all on that day. An important point to note is that the sectors operate sequentially; ie, only one sector is on at a given Main Features • • • • Controls up to six 24V AC water solenoids (ie, six sectors). • • • Individual sectors can be turned fully off or on. Easy-to-set LCD clock with two watering start times per day. Toggle switches for day of week selection. Sector times independently variable from 0-60 minutes using rotary controls. Sectors are turned on sequentially to ensure adequate water pressure. Backup battery maintains settings during short-term power interruptions. FFEBRUARY ebruary 2000  15 16  Silicon Chip time. In operation, Sector 1 starts at the preset time(s) and completes its watering period before switching off and allowing Sector 2 to start. Sector 2 then completes its watering period, after which Sector 3 starts and so on until all sectors have been stepped through. In practice, this means that if all six sectors have been set to 30 minutes (say), the total watering time will be three hours. The reason this has been done is that, depending on the installation, there may not be enough capacity in the water lines to run all sectors simultaneously. Operating them one at a time ensures that each sector operates with good water pressure. Where To Buy A Kit Of Parts Parts for this design are available from the author, as follows: (1) PC board plus all on-board parts (includes LCD module, programmed microcontroller and switches but not the battery, optional fuseholder or optional reset switch) .....................................................................$125.00 (2) Programmed microcontroller .......................................................$45 (3) Plastic case and front panel .........................................................$50 All prices include postage. Payment by cheque or money order only to: Ned Stojadinovic, 23 Harricks Crescent, Monash, ACT 2904. Email: vladimir<at>u030.aone.net.au Note 1: 24V AC plugpack power supplies are available from garden supply shops or from Altronics (Cat. M9714). Note 2: copyright for the PC board and microcontroller program associated with this design is retained by the author. Circuit description Fig.1 shows the circuit details of the Programmable Sprinkler Controller. It’s all built around the PIC16C74A microcontroller (IC1). The PIC16C74A is a very capable chip which allows the elimination of a great deal of support circuitry such as A/D converters, serial transmitter/receivers, clock generators and buffers, etc. Indeed, there is so much packed into it that you might like to download the data on this device from Arizona Microchip and study it carefully when reading this article. Don’t be too dismayed at the seeming complexity of the chip. It’s true that there are so many functions that the pins are almost all multiplexed but once the desired function is programmed into the appropriate registers, they all work the same way as simpler devices. A/D converter The first really useful function is the 8-channel A/D converter. In this design, six channels are used to read the voltage on pots VR1-VR6. The microcontroller converts each voltage to a number ranging between 0 and 255 and the values from 0-240 are Fig.1 (left): a PIC16C74A microcontroller forms the basis of the circuit. This takes its inputs from the sector pots and the day switches and sequentially activates power Triacs via MOC3021 optoisolators. The PIC microcontroller also drives a 2-line LCD which displays the time and the watering start times. then divided by four to give values of 0-60 which are loaded into a minutes counter. The values between 240 and 254 are used as a buffer zone, as 255 tells the microcontroller to turn that sector on continuously. Note that I didn’t have to do anything similar at the zero time end as I found that all pots apparently drop to zero resistance long before they get to the end of their travel. By the way, the data sheet shows that there is only one A/D converter in the PIC16C74A and this is multi­ plexed eight ways by appropriately selecting the converter’s control register. The speed of the micro­controller means that we effectively have eight converters but it does have the limitation that you cannot do all eight conversions at once, such as might be required when doing high speed data processing. Port B internal pull-ups PIC processors are CMOS devices and so have a high resistance looking into their input pins. This means that stray static electricity can switch the pins rapidly from high to low and back again, which can cause the inputs to overheat. To counteract this problem, it’s standard practice to “tie” any unused inputs to either ground or Vcc (in this case +5V) via a reasonably large resistor; eg, 10kΩ. However, the PICs can do this internally and configuring the Port B pins as inputs ties each to +5V via its own 200kΩ resistor. Switches S1-S7 take advantage of these internal pull-ups by simply isolating the Port B input pins when the switches are open, leaving the corresponding inputs at +5V (logic high). Closing each switch grounds the pins through a common 250Ω resistor, forcing them to a logic low. Why include the 250Ω resistor? Well, it’s like this: the ports on a PIC can be configured as either inputs or outputs. As inputs, they look like high resistances to ground but as outputs they can supply up to about 20mA of current (per pin) to the outside world and not much more. If you configure a port as an output (either accidentally or otherwise) and it shorts directly to ground, that port will be destroyed and possibly the entire micro­ con­ troller as well. Another “gotcha” is that noise can cause the pin to reconfigure itself as an output in mid-program. In this case, the 250Ω resistor will limit the current to a safe level until the port settings are revised by the running program. Pull-down resistors The Set (SET) and Increment (INC) inputs at pins 30 & 29 both require pull-down resistors. For convenience and to allow for later expansion, these resistors are part of a resistor array package (RP1). This handy little component contains five 10kΩ resistors, all connected at one end to a single pin (in this case, pin 1). In this design, pin 1 is grounded, while the resistors at pins 3 & 4 go to FEBRUARY 2000  17 Fig.2: install the parts on the PC board exactly as shown in this wiring diagram. Note that the Reset switch (S10) is optional and won't be needed in most cases. The panel mount fuseholder is also optional. pins 29 & 30 of IC1, respectively. This means that pins 29 & 30 are normally pulled low via the 10kΩ resistors in RP1. Pressing the Set and Increment switches pulls these inputs high via a 250Ω resistor (R15). In a similar vein, pin 6 is open collector and is normally pulled high via R16. In this case, however, pin 6 functions as an output. A high output results in the pin remaining high resistance, allowing R16 to pull it to +5V. Conversely, a low effectively 18  Silicon Chip shorts the pin to ground. Pin 1 (MCLR) is also normally pulled high, in this case via R13. Switch S10 resets the microcontroller by pulling pin 1 low. This clears the time settings and restarts the program. The clock If you’ve dabbled before with micro­ controllers, you’ll know that they accept a variety of clock signals. Crystals, ceramic resonators and resistor/capacitor timing can all be used, depending on how accurate the clock has to be. For example, serial data transmission and reception requires good clock accuracy and so a ceramic resonator (at least) is necessary, or even a crystal for high baud rates. A look at the circuit diagram reveals a crystal lurking between pins 15 and 16 but this crystal has nothing to do with the microcontroller’s clock. Instead, the microcontroller’s clock is based on a simple RC timer consisting of R7 and C1 (pin 13). Such a rudimentary timer is quite sufficient for such simple functions as switching solenoids and updating registers, etc. However, it’s not good enough for the real time clock, which is where the crystal oscillator comes in. The crystal oscillator operates at 32.768kHz and the resulting square wave is fed to an internal counter which divides by 216 to give a frequency of 1Hz. This signal triggers an interrupt routine that updates the seconds, minutes and hours counters. Serial ports The more sophisticated PICs, including the 16C74A used here, all have hardware serial receiver/ transmitters, commonly referred to as USARTs (universal synchronous asynchronous receiver transmitters). The most common application is as an asynchronous receiver/transmitter which is the protocol that the average modem uses. The ability to do serial communication in hardware is enormously useful. Although it’s possible to write software that performs this function, it’s difficult because the timing of the individual bits needs to be very precise, especially at high baud rates. Not only that but the time between bits can get very short at high rates and the software has to constantly hover, waiting for the next bit to have its turn, making it difficult to do anything else. By contrast, a hardware USART allows you to simply dump in the byte to be transmitted and set the “send” bit. Similarly, reception of a complete byte causes a “message received” byte to be set and this in turn can trigger an interrupt. The receive buffer is three layers deep so two complete bytes can be received before the buffer needs to be unloaded. Having said all that, the USART is not used in this project. However, if there is sufficient interest in the Sprinkler Controller, a future expansion that uses serial transmission is planned. The display 16 x 2 LCD displays are quite cheap these days and go a long way towards making the operation of electronic equipment nearly foolproof. In this case, the LCD is used to show the time and day and to guide the operator when setting the watering start times. The data is shifted into the LCD in two 4-bit chunks via inputs D7-D4. This saves four pins on the micro­ controller but is a fraction slower and makes it a tad more difficult to program. Note also that I have not used a trimpot to set the contrast of the display. Instead, a fixed contrast voltage of about 0.25V is used and this is set by the resistive divider formed by R8 and R9 on pin 3 (VEE). Triac switching A complicating factor in sprinkler timer design is that the systems run off 24V AC, which is necessary to avoid corrosion in the lines to the solenoids. Consequently, there is no easy way to use simple DC components such as transistors to drive the solenoids; the drivers have to handle AC and in this design we use Triacs to switch the power. In greater detail, the six sector outputs from IC1 appear at pins 23-28 and drive MOC3021 optically-coupled Triac drivers (OPT1-6) via 300Ω current limiting resistors. These in turn drive six power Triacs (Triacs1-6). The MOC3021s serve to isolate IC1 from the inductive voltage spikes generated when the solenoids switch on and off. When a sector output goes high, the LED inside the relevant MOC3021 turns on and this turns on its companion optically-triggered Triac. This in turn applies bias to the gate of a power Triac which then switches on and applies power to the solenoid. Power supply The transformers available for use with sprinkler timers are rated at 24V AC and this gives a nominal 34V DC after rectification and filtering. However, this creates a small problem because standard voltage regulators only operate safely up to 30V. The answer is to use a pre-regulator, in this case based on resistor R10 and zener diode ZD1. Bridge rectifier BR1 rectifies the incoming AC and feeds the resulting DC voltage to R10 and ZD1, which provide a regulated +10V rail. This rail is filtered using C5 & C6 and fed to 3-terminal regulator REG1 which provides a +5V rail for IC1 and the LCD. Note that it is good practice to use high-quality capacitors in power supplies such as this (remember they are on for 24 hours a day for years) and these are rather expensive. Because the settings are stored in volatile RAM (in IC1), the circuit requires battery backup so that the set Parts List 1 PC board (available from author) 1 plastic electrical case to suit 7 SPST PC-mount toggle switches (S1-S7) 2 momentary contact pushbutton switches (S8,S9) 1 MF-R050 polyswitch 1 V100ZA3 metal oxide varistor (MOV1) 1 32.768kHz crystal (X1) 2 M205 fuseholder clips plus 100mA fuse (F1) 2 6-way PC-mount screw terminal blocks 1 battery snap connector 6 10kΩ PC-mount miniature potentiometers (VR1-VR6) (Farnell Cat. 697-990) 1 16 x 2 LCD module 1 6-pin SIL 10kΩ resistor array, pin 1 common (RP1) Semiconductors 1 PIC16C74A programmed microcontroller (IC1) 1 78L05 5V regulator (REG1) 6 MOC3021 optoisolated Triac drivers (Opto1-6) 6 2N6075B 600V 4A Triacs (Triac1-6) 1 DIL 200V diode bridge (BR1) 1 1N4740 10V 1W zener diode (ZD1) 1 1N4001 silicon diode D1 Capacitors 2 1000µF 16VW electrolytics (C4,C6) 4 0.1µF monolithic (C5,C7-C9) 1 15pF ceramic (C1) 2 12pF ceramic (C2,C3) Resistors (0.25W, 1%) 3 10kΩ 6 300Ω 1 2.2kΩ 2 250Ω 1 470Ω 5W 1 68Ω 6 390Ω times are not lost during blackouts. This is provided by a 9V battery via diode D1. Normally, D1 is reverse biased and no power is drawn from the battery. However, if mains power fails, D1 becomes forward biased and the battery supplies power to regulator REG1. Note, however, that the battery backup is only intended to cater for short interruptions to the power supply. FEBRUARY 2000  19 A conventional fuse could also be used here and indeed the circuit shows a 750mA slow blow type (F2) wired in series with the polyswitch. In most cases, this fuse won’t be necessary and can be replaced with a wire link. Construction The front panel is secured to the PC board by placing it over the switches and pot shafts and doing up the switch nuts. Take care with your soldering to ensure that adjacent tracks or IC pads aren't bridged. Varistor MOV1 across the AC power input is there to protect the diode bridge from switching spikes generated by the solenoids. This device acts like a high resistance to the “normal” voltages from the 24V AC power supply but breaks down at about 100V. As a result, switching spikes from the solenoids are effectively clamped to 100V and this protects the bridge rectifier (BR1) which is rated at 200V. As a further precaution, fuse F1 is included to protect against short circuits and other faults in the electronic circuitry. The solenoids and Triacs are separately protected using a polyswitch (or self-resetting fuse). These devices use a conductive polymer that melts internally and becomes a high resistance when too much current passes through them and then returns to normal when the overload is removed. The reasons for using a polyswitch are mainly to do with reliability. Many people use sprinkler controllers to keep their plants alive during holiday periods and if an intermittent problem develops in a solenoid, a conventional fuse would bring the whole system down. A polyswitch can recover from such problems so that the owners don’t return to a desert. However, it can’t prevent the unit from shutting down if a solenoid or the wiring to it develops a permanent short. Because of the simplicity of the circuit, the construction is very straightforward. Virtually all the parts, including the LCD, mount on a single PC board measuring 152 x 123mm and this is housed in a waterproof electrical instrument case. The main exceptions are the Set and Increment switches which are mounted on the front panel. The Reset switch and panel-mount fuseholder (both optional) can also be mounted on the front panel. Fig.2 shows the assembly details. Begin by installing all the wire links and resistors, followed by the power supply circuitry (at bottom right) including the fuses, battery snap connector, varistor and polyswitch. Watch the polarity of the electrolytic capacitors and note that the resistor array must be installed with its dot towards the optoisolators (Opto 1-6). Next, install a socket for IC1 and fit the screw terminal blocks along the bottom edge of the board. Don’t install IC1, the optoisolators or the LCD at this stage – that step comes later after you’ve tested the power supply and confirmed that it works correctly. As mentioned above, the panel-mount fuse (F2) is optional. Install a wire link across the fuseholder pads on the board if you don’t intend to include this fuse. At this stage you should have a fully functioning power supply and this should now be tested before installing any more parts. To do this, connect the leads from your 24V AC plugpack supply to the relevant screw Resistor Colour Codes         No.   3   1   1   6   6   2   1 20  Silicon Chip Value 10kΩ 2.2kΩ 470Ω 5W 390Ω 300Ω 250Ω 68Ω 4-Band Code (1%) brown black orange brown red red red brown not applicable orange white brown brown orange black brown brown red green brown brown blue grey black brown 5-Band Code (1%) brown black black red brown red red black brown brown not applicable orange white black black brown orange black black black brown red green black black brown blue grey black gold brown If the unit is to be moved about, it would be a good idea to fit a couple of stand-offs between the main PC board and the LCD module, so that the header pins don’t lift the copper pads on the PC board. terminal block and switch on. This done, switch your multimeter to a low voltage range and connect the negative lead to the negative side of one of the 1000µF capacitors (either C6 or C4). The main supply rails can now be checked by probing with the positive lead. The righthand lead of the 470Ω 5W resistor should have +10V on it and this is the voltage across zener diode ZD1. Similarly, the positive lead of C4 should be at +5V which represents the output from REG1. Pins 11 & 32 of the microcontroller socket should also be at +5V, while pins 12 & 31 should be at ground (ie, 0V). Pins 17, 18, 29 & 30 are pulled down to ground by the resistor array and so these pins should also be at 0V. Pin 1, the reset pin, should be at +5V. The LCD has two unused pins on the righthand side (labelled A & K) and then it’s ground, +5V and contrast in that order. Because the LCD is not yet installed, it’s easier to carefully flip the board over and check for the required voltages. The contrast pin should have a fairly low voltage on it – around 0.25V. Checking the Triac circuitry If everything checks out so far, check the remaining pins of the micro­ controller socket. These should all be at 0V and the same goes for the LCD. Assuming everything is OK, switch off and install the MOC3021 opto­ isolators and the Triacs, taking care of their orientation. This done, reapply power and connect a couple of solenoids to the lefthand screw terminal block (CON1) and also a flying lead to the positive side of C4; ie, the +5V power supply rail. Now touch this flying lead to pins 23-28 of the microcontroller socket in turn. Provided you have a solenoid hooked up to the appropriate output, you should hear a satisfying click as the solenoid switches on. Note that pin 23 controls solenoid 1, pin 24 controls solenoid 2 and so on. If you only have a couple of solenoids, just move them to successive positions on CON1 so that you can test all the Triac drive circuits. Assuming that the circuit passes all these tests, switch off and install the microcontroller, the LCD, crystal X1, toggle switches S1-S7 and the six pots (VR1-VR6). Note that the micro­ controller is static sensitive and will need to be treated carefully. You will find that the pins of the microcontroller need to be bent slightly inwards before it can be inserted into its socket. This is best done by holding the device between its ends and pushing one row of pins against a metal ruler. This done, turn it over, do the other row and test to see if it will fit in the socket. If it doesn’t, just repeat the above procedure until it fits correctly. The LCD is mounted on a 16-pin header socket before it is installed on the PC board. Push the assembly down onto the PC board as far as it will go (ie, push the pins of the header socket all the way through the plastic) before soldering the pads. FEBRUARY 2000  21 The PC board assembly is housed in a plastic electrical case and this can be fitted with a lid for waterproofing. This lid prevents easy access to the front panel controls but that doesn’t matter if the settings are seldom changed. You can now complete the board assembly by wiring up the Set and Increment switches (S1 & S2). The Reset switch (S3) is optional. In most cases, it can be omitted but we’ve made provision for it in the unlikely event that severe electrical noise sometimes causes the microcontroller to malfunction – in which case, the switch can easily be added. That said, the circuit is designed to tolerate electrical noise, so you shouldn’t have any problems along these lines. There certainly haven’t been any such problems with the prototype to date. Final testing Once the assembly is complete, clip in a battery – you should immediately be rewarded with a display that says 12:00 am, Monday. If not, the first thing to check is whether the microcontroller is running. Try turning Pot 1 to the “On Now” position. This should immediately result in pin 23 of IC1 going to +5V (check this with 22  Silicon Chip your multimeter). If that works, then the problem is most likely in the LCD. When the timer starts up, there is a flurry of activity on the data lines to the LCD and so the next step is to look for that, preferably using a logic probe. Another possibility is the contrast setting on the LCD. If you suspect that this is a problem (or if the contrast is poor), remove resistors R8 and R9 and replace them with a 5kΩ pot. The wiper of the pot should go to the contrast pin on the LCD while the other two pins go to +5V and ground. By suitably adjusting the pot, the dots that make up the digits should become visible. If they do and there is only one line of digits, then the interface to the microcontroller is faulty. No display at all probably means that the LCD is either defective or has no supply rail. Operation The operation of the Programmable Sprinkler Controller is self-evident with the possible exception of setting the clock. To set the current time, press the Increment button until the clock setting cursor pops up (at the minutes digit) and hold the button down until the value is correct. Pressing the Set button then cycles the cursor to the next digit which is then adjusted using the Increment button and so on until the time setting is complete. The next press of the Set button then takes you to the day of the week field and this is again altered using the Increment button. By the way, if you want to change the “am” indicator to “pm” or vice versa, position the cursor to the right of the “<” sign and press the Increment switch to toggle it. Pressing the SET button sets the watering start times. The defaults are for a morning (8.00am) and evening (7.30pm) watering. If you only want to water once per day, make the two start times exactly the same. Once all the clock settings have been completed, use the toggle switches to select the days that watering is to take place and adjust the watering period for each sector using the pots. Final assembly The completed board assembly is attached to the front panel (the panel is fastened using the switch nuts) and secured inside the plastic case using self-tapping screws. Before doing this, you will have to drill a hole in the bottom of the case to take the leads for the solenoids. Fit a rubber grommet to this hole to prevent lead damage. If necessary, this hole can later be sealed with silicone sealant (after you’ve installed the leads) to make the assembly waterproof. Once everything is working, connect the solenoids. To test each sector, simply turn the appropriate knob to “On Now” and watch for the sprinklers to start operating. Just remember that only one sector can be turned on at a time, so turn off each sector before trying the next one. Similarly, remember that the sectors operate sequentially in automatic mode, so don’t expect to see them all come on simultaneously at the starting times. Instead, only one sector will come on and this will complete its watering cycle before the next SC solenoid switches on.