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Once upon a time, rain gauges were as much a part of the Aussie backyard as the outhouse. Both started disappearing about the same time – possibly because both required periodic manual emptying! Here’s a PIC-powered, fully automatic rain gauge that reads the rainfall, empties itself, stores the reading and remembers up to two months of data – without you leaving the comfort of your home. By JOHN CLARKE PIC-Powered Rain Gauge Features * Self-clearing rain sensor. * Remote monitoring. * Rainfall shown on a 3-digit display. * Max. reading of 254mm (10") per day. * Stores 61 days of rainfall data. * Settable end of day empty time. * Hold feature to prevent incorrect readings when cleaning sensor. * Reset facility to clear previous readings. * Valid data indication for previous days readings. * Battery backup for operation in a blackout. * Tamper-proof against setting changes. 14  Silicon Chip T his fully automatic rain gauge does not require emptying and will store 61 days of rainfall readings. Rainfall is shown on a 3-digit LED display and you can monitor this without going outside. A traditional rain gauge consists of a container with a measurement scale down the side wall. The idea is that the rain gauge is mounted on a post right away from the influences of buildings (fence posts were typically used but paling fences alter wind patterns too much). Rain falls, a certain amount enters the container, it starts to fill and you read the result off the scale in either millimetres or inches and points. Those old enough to remember inches and points will also remember that there are 100 points to the inch – sort of a pre-decimal decimal, if you like. A point of rain isn’t much – just a short shower, really, while an inch of rain is usually several hours of steady rain. (Sydney’s annual rainfall is about fifty inches or so; the recent outback floods were reported to result from 8-10 inches in a couple of days). These days we use millimetres and, of course, there are 25.4mm to the inch. However, we digress: back to our old-fashioned rain gauge. While accurate and reliable, it suffers from the disadvantage of requiring emptying on a daily basis and manual recording of the rainfall if you want a record. A better mousetrap rain gauge? A more useful rain gauge would be one which did not require the constant daily maintenance and which could be monitored remotely. Also it would be ideal if the rain gauge could log, or remember, previous days rainfall up to several weeks in the past. This would allow the unit to be left unattended for extended periods without loss of the rainfall information. The SILICON CHIP Rain Gauge has these features and more. Ours uses a microcontroller to not only record the rainfall each day but to remember it. The rain sensor mechanism itself is housed in a short length of 90mm PVC stormwater pipe. The rainfall measurement from this is brought via suitably long wiring to the display unit which is housed in a small plastic case. The display unit comprises a 3-digit display for the readings and a row of rectangular LEDs to indicate the current display function or mode. Three pushbutton switches access the many features of the rain gauge unit. It is powered from a DC plugpack and has a battery backup to ensure operation in a blackout. The normal setting of the rain gauge displays the current day’s rainfall and this is indicated by the “Rainfall Today” LED. This reading is updated as more rain falls. You can access the previous day’s rainfall by pressing the Down switch. This will now be indicated by the “Previous Days” LED and the 3-digit display will alternate between showing the day selected and the current day’s rainfall. The display switches at a 0.78s rate giving you sufficient time to read the values. The next previous day will be shown at the second pressing of the Down switch and up to 60 days past can be selected. The Up switch provides the means to return to the “Rainfall Today” display. Indication of the previous days is referenced to the current day (today) so that a -1 on the display means the day before today (ie, yesterday). Similarly a -2 means two days before today. The -60 indication is therefore 60 days before today. It provides us with a total of 61 days of rainfall information. This rain gauge would have been useful for Noah. He would have been able to keep a record of the forty days and forty nights rainfall during the flood, all while he was sitting back watching videos and surfing the ’net The “works” end of the Rain Gauge shown in cutaway format. Rain enters through the mesh at top and is funnelled (literally) into the pivoted container. When the weight of the water in the container is high enough it flops over and a tongue attached to the container interrupts a phototransistor circuit, registering a pulse. The opposite half of the container then starts to fill in the same manner. The emptied rain then drains away through the bottom of the unit. inside the ark. OK, that’s a fib: all the video stores were flooded out! At the end of each day, the “today’s rainfall” tally is transferred to the previous day’s log (today -1) and the same goes for the rainfall information for all the other days past. For example the today -1 rainfall is transferred to today -2 with the today -2 information transferred to today -3. The today -60 information is lost since the today -59 rainfall tally is JUNE 2000  15 Fig.1 (left): the PIC microcontroller does all the work in the Rain Gauge. It accepts input pulses from sensor 1, processes the information and drives the LED displays. moved into that day’s location. Note that during the first 61 days of use, past days rainfall information may not be correct since the rainfall may not have been recorded for that day. In fact, when you first install 16  Silicon Chip the Rain Gauge, all the previous days readings from today -1 up to today -60 will not have been logged. After each day the next previous day’s readings will become valid as they are transferred to that day’s log. The Rain Gauge includes an “Invalid Data” LED indicator to circumvent any confusion over which data has been recorded and which data has not been counted by the rain gauge. The Rain Gauge includes a clock and an empty time setting facility. The clock is set at the current time and the empty time setting is selected for when you want the day’s reading to be stored. For example, if you would normally check a traditional rain gauge at say 7am then you can set the empty time to this. Alternatively, you can set the empty time to midnight so that a true daily reading is obtained. Or you could choose any other time. The rainfall will be counted over a 24-hour period, starting and finishing at the empty time. The empty time represents the start and finish of the day, so far as the rain gauge is concerned. The clock and the empty time are 24 hour types and only show the hours and tens of minutes. Thus the time and empty time can only be set in 10 minute increments. The display will show 033 for 3:30am, 120 for 12:00 (midday) and 121 for 12:10. Midnight is indicated as 240 for 24:00. The clock and empty time are set by pressing the Mode switch to select the required function, while the up and down switches are used to set the time. The indicator LEDs for “Set Time” and for “Set Empty Time” show which particular display is selected. The Mode switch also selects the Hold/Reset function. This is indicated by the associated Hold/Reset LED. With this selection any rainfall detected by the rain sensor will not be counted. This feature is useful for when the rain gauge sensor is being cleaned or if a sprinkler is placed near the sensor, causing false rainfall detection, or for any other reason. A Reset can be made when the Up switch is pressed and the Hold/Reset function is selected. The reset clears the current days rainfall tally and resets the “invalid data” indicator function so that it shows for previous days starting from today -1. Previous days’ rainfall values are not cleared Fig.2: follow this diagram when building the PC board. You could substitute burglar alarm or other cable for the link between the main unit and the remote sensor. The wiring to the DC socket suits positive-tocentre plugpacks. Reverse this wiring if your plugpack is negative-to-centre. but only indicated as invalid. You will need to reset the rain gauge after it has been fully tested and before commissioning it in use. The setting functions are tamper-proof meaning that it is not possible to change them unless the Mode switch is pressed, which can only be done using a small probe inserted into the case. The design has been optimised to make sure that rainfall data is not lost easily. As mentioned before, the rain gauge has battery backup so that it keeps operating in a blackout. However, if the battery backup is not used or the batteries go flat, the most data you can lose is the blackout time plus up to 10 minutes depending on when the blackout occurs. This is because the current time, the empty time, today’s rainfall and the previous days’ rainfall are stored in a permanent memory which is not lost on power down. The time and today’s rainfall count are updated into this type of memory every 10 minutes while the previous days’ data and the invalid data counter are updated at the empty time. 3 are tied together. The LEDs within DISP4 are tied to the “b”, “a”, “f”, “g”, “e” and “d” segments respectively To drive one of these displays one of the RA0, RA1, RA2 or RA3 lines is brought low. If RA0 is brought low, for example, transistor Q4 will be switch­ed on and allow power to the common anode connection of the LEDs in DISP4. Any low outputs on RB1-RB7 will light the corresponding LED in the display, DISP4. After this display is lit for a short time, the RA0 output is taken high and the RA2 line is brought low to drive Q1 and display DISP1. The new 7-segment data on the RB1-RB7 outputs is presented to this display. Similarly, the RA3 and RA1 lines are brought low to drive DISP2 and DISP3 respectively. The Mode, Down and Up switches (S1, S2 & S3) are monitored at the RA4 input. These switches also connect to the RA2, RA1 & RA3 outputs respectively. Normally the RA4 input is held high via the 10kΩ resistor connecting to the 5V supply. When a switch is closed, it will pull the RA4 input low. IC1 can test which switch is closed by knowing that if RA4 is low when RA2 is low then it is the Mode switch that is closed. A closed Down switch will show a low on RA4 when RA1 is low and a closed Up switch will Circuit details Fig.1 shows the Rain Gauge circuit. IC1 is the microcontroller which forms the basis of the circuit with the displays, switches and rain sensor input attached to it. The LED displays, DISP1-DISP4, are driven directly from the RB1-RB7 outputs of IC1 via 150Ω limiting resistors. All of the segments on DISP1, 2 and The completed PC board sitting inside its case. Note that the vertical PC board guides have been filed away to a depth of 13mm from the top – this allows the board to sit in position without screws. The four tapped spacers stop the board from moving when the lid is in place. JUNE 2000  17 Parts List For Rain Gauge 1 PC board, code 04105001,107 x 62mm 1 Rain Gauge front panel label, 124.5 x 62mm 1 plastic case, 130 x 67 x 44mm 1 3mm transparent red Perspex or Acrylic sheet, 56 x 18mm 2 AA cells (alkaline or NiCd/NiMH) 1 2 x AA cell holder 1 DC panel socket 1 9V DC 300mA plugpack 1 SPST tactile switch (S1) (Jaycar Cat. SP-0730 or equiv.) 2 PC-mount snap-action keyboard switches (S2,S3) 1 3.2768MHz parallel resonant crystal (X1) 1 18-pin DIL socket 4 M3 x 9mm tapped brass standoffs 5 M3 x 6mm screws 1 M3 nut 1 small rubber grommet 7 PC stakes 1 100mm length of 0.8mm tinned copper wire 2 50mm lengths of medium duty hookup wire 1 10m length of 3-way (or 4-way) cable Semiconductors 1 PIC16F84P microcontroller programmed with RAINA.HEX* (IC1) 3 LTS542A 7-segment common anode LED displays (DISP1-DISP3) 1 DIL 10-LED (red) bargraph (DISP4) (Jaycar Cat. ZD-1704 or equiv.) 1 photo-interrupter (sensor 1) (Jaycar Cat. Z-1901 or equiv.) 1 7805 5V 1 A regulator (REG1) 4 1N4004 1A diodes (D1-D4) 4 BC328 PNP transistors (Q1-Q4) Capacitors 1 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 2 0.1µF MKT polyester 2 15pF NP0 ceramic Resistors (0.25W, 1%) 1 100kΩ 1 10kΩ 1 1kΩ 4 680Ω 1 220Ω 7 150Ω Mechanicals The rain detector unit itself is relatively simple to make up – follow the photographs and the drawings and you should have no problems. Most of the mechanical parts are made by cutting up a standard (83 x 54 x 31mm) jiffy box and using offcuts from a 90mm stormwater pipe and end caps. The rain water enters the unit via a funnel which directs it into a divided water container mounted on a pivot. When one side of the container fills, the weight of the water causes it to tip, emptying its water load in the process. The other side now fills and tips the container back again. A lug attached to the container passes through a photo interrupter every time the unit tips and this is recorded as 1mm of rain. The unit is calibrated with a simple screw adjustment to set the water container tip angle. The entire assembly is housed inside 90mm stormwater pipe with drilled-out end caps on each end. The ends are covered in flyscreen wire to prevent spiders entering and fouling the mechanism with their webs. Rain sensor * If you wish to program your own PIC, raina.hex and raina.asm are available to download from the SILICON CHIP website, www.siliconchip.com.au Parts For The Rain Sensor 1 180mm length of 90mm PVC stormwater pipe 2 90mm UPVC endcaps 1 plastic jiffy box, 83 x 54 x 31mm 4 M3 x 25mm brass screws 2 M3 x 25mm Nylon screws 2 M3 x 12mm Nylon screws 4 M3 x 6mm brass screws 12 M3 brass screws 10 M3 Nylon or brass washers 4 untapped 4BA brass spacers, 6mm long 1 plastic funnel, 86mm diameter (or 90 x 180mm galvanised sheet and 12.5mm diameter x 15mm copper tubing) 2 90mm diameter aluminium or brass flyscreen wire 1 small rubber grommet 1 100mm long cable tie 1 neutral cure Silicone sealant (roof and gutter type) 1 tube super glue 18  Silicon Chip show a low on RA4 when RA3 is low. The rain sensor (sensor 1) consists of an infrared LED and phototransistor housed in the one package. There is a slot in the package to allow a vane to pass and block the light beam to the phototransistor. Normally, the vane is out of the slot and the light from the LED passes to the phototransistor, switching it on. This means that the voltage at IC1’s RB0 input is low. As the weight of water causes the container to flip over, the vane enters the slot and the light is blocked. This turns the phototransistor off and the RB0 input goes high via the 100kΩ pullup resistor. The 0.1µF capacitor suppresses noise on this input. The transition from a low to a high is acknowledged by IC1 as a count from the sensor. The vane quickly leaves the slot as the container continues to flip over, allowing the light from the LED to turn the phototransistor back on again and pulling the RB0 input low once again. It stays in this state until the container is once again flipped over. Crystal X1 provides the oscillator component for IC1 which runs at 3.2768MHz. This frequency is divided down four times by the microprocessor for its internal operation. Another internal counter further divides this by 512, resulting in a frequency of 1600Hz which multiplexes the displays. Further division provides us with a pulse once every ten minutes which updates the system clock. Power Power for the circuit is derived from a 9V DC plugpack which supplies the 5V regulator REG1 via a reverse-polarity protection diode D1. The 100µF capacitor at the input to REG1 decouples the supply, while the 10µF capacitor at the output provides protection against oscillation of the regulator. The normal regulator output of 5V is increased by about 0.6V due to diode D2 between the regulator’s ground terminal and ground. This increase in voltage at REG1’s output is brought back down again by diode D3. This diode isolates the regulator output from the 3V battery supply, while yet another diode (D4) isolates the 5V supply from the 3V battery. If the regulator is powered, D3 conducts and supplies power to IC1. Diode D4 will be reverse biased due to the higher voltage on its cathode compared to its anode and so the 3V battery will not supply current. If you wish to use rechargeable batteries (2 x NiCd or NiMH giving 2.4V), the 220Ω resistor can be used to provide a trickle charging current. If power to the plugpack fails due to a blackout, D4 will be forward biased and the 3V battery supplies IC1 with standby power. Construction We’ll start the construction with the electronic section of the Rain Gauge. This is built onto a PC board coded 04105001 and housed in a plastic The front panel (above) can be photocopied and used as is and/or used as a drilling template for the top of the case. Use the PC board pattern (below) to check commercial boards or to photographically make your own board. case measuring 130 x 67 x 44mm. A front-panel label is glued to the lid of the case and the LED displays are visible through a transparent red Perspex or Acrylic window in the case lid. Begin construction by checking the PC board for shorts between tracks or any breaks in the copper connections. Compare the PC pattern with the published artwork to be sure it is correct. Now check the hole sizes. The corner mounting holes and regulator tab mounting hole should be 3mm in dia­meter. The PC stakes should be a Resistor Colour Codes       No.   Value  1 100kΩ  1   10kΩ  1    1kΩ  4   680Ω  1   220Ω  7   150Ω 4-Band Code (1%) brown black yellow brown brown black orange brown brown black red brown blue grey brown brown red red brown brown brown green brown brown 5-Band Code (1%) brown black black orange brown brown black black red brown brown black black brown brown blue grey black black brown red red black black brown brown green black black brown tight fit into their respective holes. Install the PC stakes first, followed by the resistors. Use the resistor colour code table as a guide to their value. Alternatively use a digital multimeter to measure each one. Note that the 220Ω resistor should only be installed if you intend to use rechargeable cells for the battery backup. Insert and solder in the diodes next, making sure that they are oriented correctly. The 7-segment LED displays must be installed with the decimal point on DISP1-DISP3 facing toward the switches. DISP4 should be installed with the label side towards IC1. You can now install the IC socket with its pin 1 oriented as shown. Don’t install the microcontroller just yet. Capacitor Codes   Value IEC Code EIA Code   0.1µF   100n    104     15pF   15pF    15 JUNE 2000  19 The capacitors can go in next, using the capacitor code table as a guide to their values. The electrolytic types, which are positioned on their sides as shown in the photographs, must be oriented correctly, with the positive lead placed as shown on the wiring diagram. Similarly, the crystal is placed on its side and is secured at its free end using a short length of tinned copper wire soldered to the PC board and crystal body. CI 20  Silicon Chip A D Now install the PC stakes at the external wiring points. The switches can then go in, taking care to ensure that the flat sides of S2 and S3 are oriented as shown. S1 must be installed so that the leads are oriented as shown. This switch should normally be an open circuit between the bottom and top pins. Transistors Q1-Q4 can be inserted with their height level with the top of the displays. REG1 is mounted Fig.3: these drawings, in conjunction with the photographs, show how the various components are fashioned from a “Jiffy” box. Start with the box (without lid) and carefully make three cuts with a hacksaw where shown. Most of piece “A” becomes the water container itself while that end of the box (“C”) becomes the vane which triggers the photo­ transistor. The pillars and guides which need to be removed can be either broken off and filed neat or, if you are particularly careful, melted away with a soldering iron and then filed neat. Note that on the “A” piece, only one set of guides and one pair of pillars are removed – the rest are used! Similarly, on the support stands (“B” pieces), one pillar portion remains – this forms the support for the bearing shaft through which the water container pivots on Nylon screws inside untapped spacers. horizontally with the leads bent down 90° so they can be inserted into their respective holes. The tab is secured with a small M3 screw and nut. The corner mounting holes are used to mount the 9mm tapped standoffs above the PC board and are secured with M3 screws. The PC board is mounted in the case by cutting the integral guides on either side of the case so that their top edges are 13mm from the top. This will allow the PC A C BII E Fig.4: the Jiffy box lid is not wasted – most of it (“D”) becomes the vertical divider between the two halves of the water-measuring container (shown below glued in place with the two adjustment screws in position). One of the two offcuts (“E”) becomes the mount for the LED/ phototransistor assembly. board to slide into the case and be held in place by the standoffs when the lid is attached (see photograph). Drill a hole in the end of the case for the rubber grommet required for the rain sensor lead and at the other end for the DC socket. The leads from the PC board to the DC socket, the battery holder and to the rain sensor can now be run as shown in Fig.2. We used three wires from a length of 4-wire telephone cable for the connection between the sensor unit and the electronics. Other suitable cable would be alarm cable or twin conductor shielded cable with the shield being used for the earth connection. Use the front panel artwork as a guide to drilling the holes for the switches and display cutout. This cutout is drilled and filed so that the red Perspex or Acrylic window is a tight fit. A tiny drop of super glue may help hold it in place if it is not a tight fit. Attach the front panel label and cut out the holes in this with a sharp knife. Figs.5a & 5b: how the various pieces go together to make the tipping bucket. These two drawings show the same assembly – the top view shows the assembly side-on, while the bottom drawing is a sectional view (ie, rotated 90°). Testing Connect the DC plugpack and test that there is a nominal 5V supply between pins 5 and 14 of IC1’s socket. If the voltage is between 4.5V and 5.5V, the plugpack can be removed and IC1 installed. Apply power again and check that the display lights and shows 0 with the Rainfall Today LED lit. Press the Down switch and you should obtain a display which alternates between a -1 and a 1. The -1 refers to the day selected and the 1 is the initial preset rainfall data. The Previous Days LED should now be alight as well as the “Invalid Data” LED. Pressing the Down switch again will have the display show a -2 and a 2. Press the switch repeatedly to check that you can access all the previous 60 days (-60). Each day should have rainfall data equal to the selected day: day -59, for JUNE 2000  21 BII D A A BI C BII E LED/Phototransistor Assembly 90mm Pipe Cap Here’s how the completed bucket/sensor assembly should look. Again, we have labelled the various components to agree with Figs.3 & 4 to make life easy. Note how the vane (C) swings through the LED/phototransistor without any restriction. example, will have rainfall data of 59. Now press the Mode switch and select the Set Time function as indicated by its associated LED. It should show 120 for 12:00 midday. It may show a later time if you have left the rain gauge on for more than 10 minutes. Press the Mode switch and the Set Empty LED should light and the display will show 240 for 24:00 midnight. Adjust this time with the down switch so that it shows the same time as the Set Time display. Now press the Mode switch once to obtain the Hold/ Reset LED and “- - r” display and press it again to obtain the Rainfall Today display. Now press the Down switch and the -1 day should now have 0 as its rainfall. The “Invalid Data” LED will not be lit. The -2 day should have a 1 for its rainfall. The “Invalid Data” will show for this and remaining days. Each other previous day should have a rainfall that is a value one less than the day’s absolute value. For example, the 22  Silicon Chip -60 day should have 59 as its rainfall. This demonstrates the end of day transfer of data from one day to the previous day. The input counter can be checked by momentarily contacting the GND (E and K connections of sensor 1) to the collector terminal (C for sensor 1). The Rainfall Today display should show 1, then 2, etc for each contact, incrementing from 0 to 254. The next count will be three dashes, indicating overrange. You can clear this data by returning to the Hold/Reset mode and then pressing the Up switch to reset. The display will show “rES” indicating that it has reset. Returning to the Rainfall Today Mode will show a 0 in the display. Return to the Hold/Reset mode and trigger the counter as before. Return to the Rainfall Today selection to check that the rainfall display is still at 0. The Hold feature therefore prevents any rainfall counting. When setting the time, the 10 minute counter is reset whenever the Up or BI D E Another view of the complete assembly looking almost straight down. The double-sided water container is perfectly balanced, brought that way by adjusting the screws at the top in and out as required. Down switch is pressed. This means that the time begins from the time set and it will be a full 10-minutes before the time increments. So to obtain the correct time you must press the switch at the time when your reference clock shows a 10-increment. In practice, this means that the time should be set when the minutes on the clock you are setting it against changes from either a 9 to a 10; eg 19 to 20, 39 to 40, etc. This does not apply to setting the empty time which can be set without regard for the current time. The current time is compared with this empty time and when they are equal, the microcontroller moves the daily rainfall data along by one day. Rain detector As mentioned, many of the parts for the rain detector are made from parts cut from a plastic case measuring 83 x 54 x 31mm. Part of the base of the case is used as the water container, with the lid providing the divider between the two sides. An end of the case is used to make a vane for the sensor 1 detector, while the other end of the case makes up two support stands for the water container pivot. One end of the lid makes a mounting plate for the photodetector (sensor 1). Fig.3 and Fig.4 show how the parts are cut from the lid and base of the case. The base is cut so that some of the integral slots in the side become the centre of the water container. The end marked (C) is cut off as shown and the pillars removed on the water container section. File the sawn edges Rain enters the top of the 90mm pipe via a protective insect screen. This is thin enough to be gripped by the cap, shown removed here. You can make your own funnel, as we did, or simply cut the top off a small funnel so it forms a tight fit in the pipe. Secure it in place with silicone sealant. to a smooth finish. The support stands are made by cutting the (B) side of the box into the sizes shown. Remove one of the pillars from each support stand (B’ and B’’). The vane (C’) can be cut to shape as shown. Cut the lid to size and remove the small flanges on the divider section (D). The section marked (E) is to mount Sensor 1. You will need to file the two edges of the divider (D) to 50mm wide so that it slides neatly into the side slots of the water container. We rounded the bottom section of the divider so that the base of the container will be curved slightly. This is not strictly necessary and can be left straight. The divider is glued into the water container with super glue. To do this, first slide the divider in place and run the glue around the edges to secure it. The glue sets very fast on this plastic so do not run glue in place before inserting the divider or the slots will become clogged with set glue. The vane is glued to the underside of the water container central to and at right angles to the divider. Draw a pencil line down the centre of the underside of the container to mark the position for the vane. Now run some super glue along the 45mm long edge of the vane and attach it to the container in position. Hold it in position until the glue sets. The pivots for the water container need to be 12mm down from the sides as shown in Fig.5 and in the centre of the divider. Drill holes so that the 6mm long spacers insert as a tight fit with about 0.5mm of spacer protruding at each side of the water container. You will need to seal the edges of the divider and ends of the spacers inside the water container with a smear of silicone sealant to prevent water from escaping. The photodetector, sensor 1, can be mounted onto the (E) mounting plate using two Nylon screws and brass nuts. The sensor is mounted central to the mounting plate. Secure this mounting plate in the centre of a 90mm stormwater end cap using brass screws and nuts. Drill mounting holes in the support stands B’ and B” as shown in Fig.3. Drill out the pillars so that the M3 Nylon screws will form a shallow thread when screwed in place. Mount the support stands 69mm apart along the same centre line as the sensor 1 mounting plate using brass screws and nuts. Screw in the Nylon screws through the pillars in the support stands and cut them to protrude by 5mm from the inside edge of the support stands. Drill holes for the M3 x 25mm screws on the top of the divider. These are best drilled slightly undersize so that the screw will cut a thread in the hole. Place the 6mm spacers in position as shown using nuts to hold Fig.6: here’s how we fashioned the funnel from a small piece of thin galvanised iron. Alternatively, you could use a suitable plastic funnel and cut the top off so it measured 86mm across. Similar to the top, the bottom is protected against spiders and other insects by a screen. Make sure the holes are big enough to allow the water to drain away immediately. them in place. The second screw does not have the spacers. Assemble the unit with the water container pivoting on the Nylon screws by gently prising the support stands outward to allow the screws to be inserted into the water container’s bushings. Check that the vane passes through the sensor slot without fouling. You may need to trim this piece for best clearance. Note how many washers you will need on each side of the pivots so that the vane swings through the centre of sensor 1’s gap. Install these in place. Adjust the top screws on the divider so that the tendency of the water container to fall to either side from upright is not biased in one direction or the other. Mark out where the end stop screws need to be installed in the end cap so that the top of the screw end will catch the underside of the water container for each side. Note that these screws will need to be offset from centre to prevent fouling the vane. Drill the holes for these end stop screws slightly undersize so that the thread will be cut into the plastic. This will allow easier adjustment. The nuts are simply used to lock the screw in place after the adjustment is set. Also drill holes for the rubber grommet for the wire entry and large holes to allow the water to flow out. Cut the 90mm-diameter stormwater pipe 180mm long and cut out the inside of one of the end caps so that it has an 86mm diameter hole. The 86mm diameter is important in the JUNE 2000  23 calibration process – it must be peramount of water. because of the water inertia rather fectly round and it must be exactly How do you get exactly 5.8ml of than its weight. The calibration can 86mm. be checked by counting the number water? By far the easiest way is to use This endcap becomes a cover to a 10ml syringe (hypodermic) without of tilts. hold on the wire mesh over the end the needle, of course. Your local pharFor example, one litre (1000ml) of of the pipe. macist or doctor might be able to give water should tip the water container you one once he or she has disposed 172 times (1000/5.8 = 172). It is probA funnel can be modified by cutably easier to wire up the sensor to the ting a slightly larger plastic funnel to of the pointy bit in their sharps bin. circuit and attach the 90mm pipe and 86mm outside diameter or you can These syringes are graduated to fashion your own using galvanised 0.1ml so you can get the right amount funnel assembly, so that the number iron sheet and a 15mm length of easily – and it’s easy to put the water of tilts can be counted on the display. 12.5mm copper pipe. It is folded so the exactly where you want it, too. After wiring the sensor you will two straight edges are placed together need to cover the connections with Adjust the end stop screws so and the 5mm flange is soldered to the silicone sealant. This will prevent that the water tips both sides of the back of the funnel. The copper tubing container with the 5.8ml quantity of corrosion and also prevent the water is soldered to make an outlet at the water. Screw the end stops higher making contact between collector funnel base. and emitter of the detector transis(clockwise) so that the amount of tilt Overall height is about 85mm. The is less if you need more than the 5.8ml tor, which may prevent the sensor outlet from a plastic funnel may need of water to tip it. Screw the end stop from detecting the swing of the vane to be cut shorter to prevent it catching screws anticlockwise to lower the reliably. the divider. endstops if you require less than the Secure the end stop screws with 5.8ml of water to tip. the lock nuts and insert the 90mm The funnel is secured inside the diameter flyscreen inside the bottom tube with some silicone sealant apThe top weights on the divider will plied around the inside top edge of need to be changed if you cannot ob- end cap. Secure with some dobs of the funnel and pipe. Place the 90mm tain an adjustment with the end stops silicone sealant. diameter flyscreen on top of the pipe that calibrates the tipping correctly. Installation and place the opened end cap in position. Wait for the The rain gauge sensor can sealant to cure. be supported using standard 90mm downpipe fittings or Current consumption: 30-60mA with a 12V DC input, Calibration even with heavy-duty cable 3mA when powered by 3V battery. ties or galvanised wire and First of all, we need to 2 2 Rain gauge collection area: 5808mm (or 5.8cm ). attached to a free standing determine how much water Volume collected per mm of rain: 5.80884ml (cc). post or one protruding above (ie, rain!) entering the gauge a fence. represents 1mm. You will Measurement resolution: 1mm. recall we said it was imporIt will need to be located Rainfall accuracy: Depends on calibration adjustment tant to get the end-cap cutout away from trees and similar (can be set to within 1% plus 1 digit). exactly an 86mm dia­ meter rain obstructions for the best Clock accuracy: ±10.5 minutes per year unadjusted. circle. This is the figure we accuracy. It should also be use to calibrate the gauge. several metres away from walls and solid fences to Rainfall is measured in If the water container tips too early prevent it from being in a rain shadow millimetres which simply means the depth of the rain which gathers (ie, before the full quantity of water or even a rain funnel. has been poured), then the weights in a specific area where there is no The clock accuracy depends on are not sufficient and you will need run-off or no run-in. To work out the the actual crystal frequency. These rainfall, all you need to know is the to add more weight. have a tolerance of ±20ppm which Try adding two more 6mm spacers area. Our area is of course an 86mm means that the clock could be some on the second screw and secure with 10 minutes fast or slow at the end of diameter circle. two nuts in the same way as the first one year. This should be adequate The area of a circle is represented screw. If the amount of water required for the rain gauge, however, it could by π times the radius squared (πr2). be readjusted each year if necessary. The radius is, of course, half the dia­ to tip the water container is more than the 5.8ml, you will need to reduce meter so that is 43mm. Therefore the Alternatively you can use a 22pF collection area is πr2 or 3.14159 x the weight. trimmer capacitor in place of the Try removing both of the 6mm 432 = 5808 square mm. For 1mm of 15pF fixed capacitor between pin 16 spacers. Make sure that you use the of IC1 and ground. The crystal can rain, the volume is 5808 cubic mm same weight on either side of the di- then be trimmed to 3.2768MHz using or 5.8ml. vider to maintain the balance of the a frequency meter or by trial and error You can initially calibrate the rain tilting action. testing over a period of time. sensor by slowly pouring in 5.8ml Best calibration results can now be (or cc) of water directly into one side Note that a probe on the oscillator obtained by again slowly pouring in of the water container without the pins will affect the crystal frequency a large quantity of water. Note that – it is best to place a low capacitance 90mm pipe and funnel installed over if you pour the water in too fast you (10:1) probe on pin 15 (OSC2 input) the unit. Check that each side of the SC water container tips at exactly this will cause the water container to tilt for least frequency change. • • • • • • 24  Silicon Chip Specifications