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It’s cheap & simple to build, operates completely unatt Simple DataWeather Sta 58  Silicon Chip siliconchip.com.au ended & will run for years on a set of AA batteries If you need to record weather data at a remote location, there are very nice professional logging weather stations out there that do the lot, with solar panels for power and the ability to record rainfall, temperature, humidity, barometric pressure, wind speed and direction, sunlight hours etc. While it would be nice to have all that capability, I had a need that was a lot simpler. Like many people, I only wanted to record rainfall and temperature. More importantly, I couldn’t justify the cost of the professional systems, which typically run to four or five figures. There are plenty of hobbyist weather stations out there too – and at much better prices. They appear very capable but none can log data unattended for an extended period (well, I did find one but even it was well over a thousand dollars). SILICON CHIP has published weather station projects in the past, including a PICAXE-based system that recorded temperature and humidity (December 2004). There was also an electronic rain gauge but again, neither project was suitable for unattended remote logging for months at a time. A bit of research convinced me that it wouldn’t be too hard to build my own, including a suitable rain sensor. So that’s just what I did! -Logging ation siliconchip.com.au Part 1 – by Glenn Pure September 2007  59 O N THE ELECTRONICS SIDE, a low power microcontroller was the way to go. With the right device and a bit of care in design, current consumption has been kept down to an average of around 10mA, meaning a set of three AA batteries should last for years – virtually their shelf life, in fact. In terms of logging capability, with half-hourly readings, it is capable of storing just under a year’s worth of rainfall and temperature records, utilising the 64 kilobytes of on-board EEPROM memory. The firmware can easily be modified for reading at more frequent intervals. With sixminute logging frequency it has over two months capacity. At the other extreme, with hourly recording, it will store almost two years of data. The data is accessed through an on-board RS232 interface, enabling easy downloading straight to a laptop or desktop computer. If, like me, you don’t own a laptop, there is a simple solution. The controller is cheap and easy enough to build that you can make two and simply swap one out and take it home to dump the data at your leisure. In fact, the most time-consuming part about the project isn’t the electronics – it’s the hardware. Building the rain sensor will probably take the most time and effort. But if you don’t have the time or inclination, at modest cost you can even solve that little problem too. While unsuccessfully looking for a suitable commercial weather station, I found a good quality rain sensor for a few hundred dollars that will interface with the weather station. More on this later. Circuit description & operation As mentioned, the circuit is based around a microcontroller (IC1). Since low power consumption and simplicity were paramount, I chose a PIC16F88 “nanowatt” microcontroller. Here’s a close-up view of the data-logging weather station. The rain gauge is at top right, while the temperature measurement housing is at bottom left. The box containing the “works” (shown above) is housed in the lower right container. 60  Silicon Chip This has pretty-much all the peripheral interfaces needed already integrated into the device, including an on-board oscillator, a serial interface driver and A/D converters. While the A/D converter was used in an earlier version of the design for temperature sensing, it’s not actually needed in the final design since analog temperature sensing was abandoned. Instead, sensing is done by a Dallas DS1621 digital sensor (IC5). This greatly simplified the circuit, which previously required an accurate voltage reference for the A/D converter and a circuit to switch this on and off. Better still, the DS1621 is an I2C bus device (like the two 24C256 serial EEPROMs – IC3 & IC4), which further simplified design and software development. The DS1621 has a low-power standby mode when not in use, helping further to save power. A brief comment on the I2C devices is warranted. These devices require two lines for communication – a clock line and a data line. The data line is normally held high by a 10kW pull-up resistor. An active device pulls the data line low when it needs to during transmission. Hence, if two devices attempt to transmit at the same time, the worst that can happen is that they can pull the shared data line low. This is unlikely to present any risk of damage but could lead to unpredictable power consumption in some cases. So there are 390W resistors in series between the PIC (pin 7) and each of the data lines to the three I2C devices. The PIC actually has a synchronous serial port for I2C siliconchip.com.au +4.5V 100nF 220k RAIN SENSOR 6 S3 RB0 CLOCK CORRECT RA0 +4.5V 10k 8 Vdd 3 10 A2 5 SDA 2 A1 IC3 1 A0 24C256 6 SCL 7 WP Vss 4 390 S2 RESET S1 10k 6 7 SC 2007 A2 A1 A0 IC5 DS1621 SDA Vss 4 SCL 1 2 RB4 1 F 7 9 3 RB1 RB5 RB3 RB6 RA4 1k A RA1 RB2 6 RB7 18 8 11 10k 10k 10k 3 4 IC2 MAX232 1 F 1 F 5 TO PC CON1 8 9 12 13 11 14 1 2 3 4  LED1 5 K bus interfacing but this hasn’t been used here since it has more limitations than benefits. Instead, the I2C interface is implemented fully in the firmware of the weather station. The asynchronous (RS232) serial port on the PIC is connected through a standard MAX232 serial interface driver (IC2), providing suitable voltage levels for serial communication. The MAX232 part of the circuit is manually switched on and off by the user (using S4 ) when a data dump is needed. Getting this part of the circuit to work proved more difficult than it might appear because even when switched off, the MAX232 would sometimes stay in a partially running state. It appeared to be drawing power parasitically through its three I/O connections to the PIC. Resistors (10kW) between the PIC and each of these I/O lines solved that particular problem. The RS232 interface is set up for 2-way communication but only transmission from the PIC is built into the firmware since this is all that is needed. However, the capability is there for the device to receive serial communication for anyone who wanted to extend the capabilities of the design. Interfacing the rain sensor is simple. The rain gauge is a tipping bucket type and operates by closing a switch momentarily each time the bucket empties. The PIC detects this through an interrupt and increments an internal rain counter by one. The rain sensor input on the PIC is normally held high 6 7 8 9 5 12 13 DB9F 15 X1 32.768kHz Vss DATA LOGGING WEATHER STATION siliconchip.com.au 1 F 16 1 RA2 IC1 PIC16F88 390 10k 1 F 10k 3 390 A2 5 SDA 2 A1 IC4 1 A0 24C256 6 SCL 7 WP Vss 4 5 17 2 1 8 Vdd 8 Vdd 'DUMP' S4 14 Vdd LED 33pF 33pF K A Fig.1: there are just five ICs and a handful of other components in the Weather Station circuit. by a 220kW resistor when the switch is not closed. A high value was used for this resistor because there is a small risk that the tipping bucket could stick in the centre position and keep the switch closed. If this occurs, the battery would The control box from the rear, showing the battery pack (three AA cells) and the 5-pin DIN connector, along with the hanger bracket at the top. September 2007  61 17090140 IC1 PIC16F88 SW 33pF RST 33pF P11 32.768kHz X1 CLKADJ K 27090140 + +V 10K 10k 10k 100nF P12 quickly drain if a smaller (say 10kW) pull-up resistor had been used instead. There are two extra features included in the circuit. One is a small pushbutton switch (S2) on the PC board that is only accessible when the case is open. This is used to calibrate the clock in the controller. You may wonder why this is needed. To achieve low power consumption, the PIC spends most of its time “sleeping”. Even though the 16F88 has an on-board oscillator that could potentially run a real time clock, this shuts down when the device sleeps. Hence, an external crystal oscillator, using a 32.768kHz “watch” crystal was necessary. The PIC keeps driving this crystal even when it is sleeping. Even though these crystals are pretty accurate, they aren’t perfect and can be out by maybe five seconds a day. In the worst case, over a year, this can add up to an error of half an hour. Details on using switch S2 can be found later in this article. A second pushbutton switch (S1) is accessible from the front panel. This is used to reset the weather station. “Reset” in this case does not mean a hardware reset of the PIC. Instead, the reset button is used to zero the address pointer for the EEPROM memory. The user would normally do a reset after data is dumped so that all the memory in the device becomes available again for logging. If a reset is not done via this button, the weather station will keep logging from where it last left off. This will happen even if the device is powered down or re-boots itself due, for example, to a fault condition. There is no way to wipe the EEPROM memory in the weather station. This has been done deliberately to enable data to be recovered even if the address counter has become corrupted. If a data dump is performed just after a reset, the entire contents of the EEPROMs will be dumped – all 64kB or 16,384 records (four bytes per record). Normally, only the records up to the last one recorded will be transmitted through the serial port during a data dump. The way the data is recorded also enables breaks in the recording to be detected if a full data dump needs to be done – but more on that later. The weather station is very reliable and I’ve never had a need to do a full data dump (except for testing) but the feature is there just in case. Finally, there is a LED on the front panel to indicate status. This flashes very briefly every four seconds during normal operation. It comes on permanently during a data dump and it quickly flashes three times when a reset is performed by the user. A high-intensity LED is used to improve visibility since it is only on for about three milliseconds each flash – again, this was done to help keep power consumption down. 62  Silicon Chip -V P9 1k 390 IC3 24C256 IC4 24C256 390 +V 220k 1 F P12 10k P11 10k SW 10k A LED1 1 F –V + 390 Rain 1 F MAX232 IC2 1 F + +V 10K –V SDA SCL + Fig.2: two PC boards are used: (1) a main board, containing the PIC16F88 (IC1) and the two 24C256 serial EEPROMs (IC3 & IC4); and (2) an RS232 interface board which holds the MAX232 (IC2). The DS1621, is not mounted on a PC board but is housed inside the temperature measurement container. + 1F P9 Pin 1 TO S4 Four I/O pins on the PIC are not used at all, including an analog input for the A/D converter. Hence, there is scope to expand the capability of the weather station for those who may need additional sensing. The following table summarises the I/O pin usage on the PIC. Pin I/O port,bit 1 Port A,2 2 Port A,3 3 Port A,4 4 Port A,5 6 Port B,0 7 Port B,1 8 Port B,2 9 Port B,3 10 Port B,4 11 Port B,5 12 Port B,6 13 Port B,7 15 Port A,6 16 Port A,7 17 Port A,0 18 Port A,1 Allocated to… ‘Reset’ switch input (unallocated analog input or digital I/O) LED output (unallocated, digital I/O) Rain sensor switch input I2C bus data line (SDA) RS232 port receive (input) I2C bus clock output (SCL) Clock calibration switch input RS232 port transmit (output) Clock crystal Clock crystal (unallocated, digital I/O) (unallocated, digital I/O) Data ‘dump’ request input RS232 ‘communication ready’ input Putting the controller together The project is assembled in a small plastic utility box (second smallest size is used). Looking first at the externally visible parts, the front panel of the box has holes for the LED and the Reset switch, plus a larger cutout for the DB9 female serial port connector. There is also a single-pole, single-throw slide switch (S2), used for powering up the MAX232 when preparing for a data dump. The battery holder (3 x AA) is stuck to the back of the box with double-sided tape and the wires from this run through two small holes in the box. The only battery holder I could find for three AA cells was one with a plastic cover and an on-off switch. Unfortunately, the case opens on the opposite side to the switch. Hence the switch is inaccessible when the case is stuck to the utility box – and in fact the switch actuator had to be cut flush with the surface of the battery case to enable mounting. Since the switch is now inaccessible, to minimise the risk of failure, I broke open the battery case behind the switch and soldered a link across the terminals to bypass it (so the switch is effectively permanently on). Of course, a 4 x AA flat battery holder could also be used with either a dummy cell or shorting wire replacing one of siliconchip.com.au And here are those two PC boards, shown slightly over-size for clarity, which match the diagrams at left. Note that there are also connections underneath the boards – the underside of the main PC board is shown below. the four cell positions. If you use this method, don’t forget which cell you’ve replaced or you could end up putting one into the shorted position! One end of the utility box has a socket for connecting the temperature and rain sensors. The temperature sensor requires four connections (Vcc, ground, data & clock), while the rain sensor has a 2-wire connection (ground & signal). A 5-pin DIN audio connector was chosen for the task, with the ground connection shared between the temperature and rain sensors. A range of other socket types would be suitable, including separate sockets for the temperature and rain sensor if this is desirable. The main consideration should be ensuring a reliable connection. Inside the box, there are two PC boards, on which all components are mounted except the slide switch for dumping data and the rain and temperature sensors. The PC boards slide into the mounting slots provided in the utility box, with the component side of both facing towards the socket that connects the temperature and rain sensors. I’ve included solder pins on the PC boards for the interconnections that are needed. Those pins on the main controller (PIC) board that are needed for connection to the MAX232 board should be mounted on the copper side of the board so they point towards that board, enabling easier connection. Six connections are needed between the two boards (including +V and ground). The overlay of both boards (Fig.2) makes it clear where the interconnections should occur (‘SW’ to ‘SW’, ‘P9’ to ‘P9’ and so on). There are two sets of positive and negative connection In the prototype, the on/off switch on the battery pack was shorted (see enlargement) because the switch was on the wrong side of the pack. You could use a 4 x AA pack with one cell shorted out. CLOSEUP OF BATTERY PACK WITH SHORTED SWITCH siliconchip.com.au points on each board. One of the sets on the MAX232 board (which should face out from the copper side) is for connection to the battery pack, while the second set connect power to the PIC board. The second set of power connection pins on the PIC board is for the temperature and rain sensor socket. The MAX232 board also has two pins marked “to switch” on the overlay which need to be run to the “dump” switch. Assembly is straightforward. As usual, watch for correct orientation of polarised components – besides the ICs, the only ones are the five electrolytic capacitors on the MAX232 board and the LED on the main board. There is one PC board link – sort of, anyway. The in-line DB9 socket solders directly on to the MAX232 board, with the edge of the board pushed between the two rows of pins The DS1621 temperature sensor chip is soldered to the end of a four-wire lead as shown at left and in the photo below. If using telephone or alarm cable, it makes sense to use red for +ve, black for -ve and the blue and white wires for data. September 2007  63 Parts list – Data Logging Weather Station 1 PC board, 63 x 37mm, code 04109071 1 PC board, 63 x 32mm code 04109072 1 130 x 68 x 43mm plastic utility box (UB3) 1 ~500mm length of 100mm diameter PVC sewer pipe (150mm length for rain sensor and 200mm length to house the controller) 3 PVC end caps to fit 100mm sewer pipe 1 180 x 360mm piece of 0.4mm galvanised steel sheet (for primary funnel) 1 260 x 15mm piece of 0.4mm galvanised steel sheet (for secondary funnel bracket) 1 80 x 125mm piece of 0.6 to 0.8mm thick aluminium sheet (for tipping bucket) 1 100 x 50mm piece of 0.6 to 0.8mm thick aluminium sheet (for secondary funnel) 1 95 x 25mm piece of 0.6 to 0.8mm thick aluminium sheet (for tipping bucket bracket) 2 M4 x 20mm machine screws and nuts, corrosion resistant 4 M4 x 12mm M4 machine screws (corrosion resistant) plus 1 nut 1 small piece of fine wire gauze (for primary funnel; also used on the discharge holes below the tipping bucket) 2 100 x 8mm galvanised steel bolts, plus nuts and washers for each (to make mounting brackets for rain sensor and controller housing) 1 steel strip, 70 x 25 x 3mm, for rain sensor mounting bracket 1 20 x 8mm galvanised steel bolt, plus nut and washers to suit (for rain sensor mounting bracket) 1 length of stainless or galvanised steel wire, 50mm long 1-2mm diameter Assorted pop rivets 1 AA battery clip (for three AA batteries) 1 1m length single-core shielded audio cable 1 1m length 4-core alarm cable 1 3 x 2mm disc-shaped rare earth magnet (or two 3 x 1mm magnets) 1 DB9 female socket (in-line solder type) 1 5-pin panel mounting DIN socket and line plug to match (plus mounting screws for socket) 1 right-angle PC-mount momentary close pushbutton switch (mini tactile) (S1) 1 PC-mount momentary close pushbutton switch (mini tactile) (S2) 1 glass-encapsulated magnetic reed switch (Jaycar SM1002 or equivalent) (S3) 1 SPST slide switch and mounting screws (S4) 21 PC solder pins 1 18-pin IC socket 1 16-pin IC socket 2 8-pin IC sockets 1 32.768kHz watch crystal (X1) Semiconductors 1 PIC16F88 microcontroller (IC1) programmed with “weather station.hex” 1 MAX232 serial (RS232) interface driver (IC2) 2 24C256 or 24LC256 serial EEPROMs (IC3, IC4) 1 DS1621 temperature sensor (IC5) 1 5mm super bright red LED Capacitors 2 33pF ceramic (C1, C2) 1 100nF ceramic (C3) 5 1mF electrolytic (C4-C8) (code 33 or 33p) (code 104 or 100n) Resistors (0.5W, 5%) 3 390W (colour code orange white brown gold [5%] 1 1kW (colour code brown black red gold [5%] 7 10kW (colour code brown black orange gold [5%] 1 220W (colour code red red yellow gold [5%] or orange white black black brown [1%]) or brown black black brown brown [1%]) or brown black black red brown [1%]) or red red black orange brown [1%]) Optional parts 1 steel star picket (1.2m long) Aluminium and galvanised (or Colorbond) steel sheet to make a louvred housing for temperature sensor 1 galvanised steel bolt, 100 x 8mm (and two nuts and washers to suit) for mounting the louvred housing 1 DB9 serial communication cable for computer connection See part II of this project (next month) for more details on materials for the separate temperature housing 64  Silicon Chip siliconchip.com.au on the socket. Pads are provided on the solder side of this board for pins 1-5 of the socket. Pins 6, 7 & 8, which sit on the component side, also need to be connected. A single pad and hole in the PC board is provided for this, just near pin 6 of the DB9. A wire link should be soldered into this pad and, on the component side, bent and soldered to pins 6, 7 & 8 (see photo). Don’t connect pin 9 of the socket. Wiring the DS1621 temperature sensor The DS1621 temperature sensor comes in an 8-pin DIP package. For use with this project, it is mounted on the end of a cable, so it can be placed in a housing or other suitable location where the temperature is to be measured. There are a few possible cable choices, including a length of 4-core alarm cable, telephone cable or Ethernet LAN cable. A length of about a metre was used for the prototype and this worked well. It’s likely to be feasible to extend the length but no testing has been done on longer lengths. The cable only needs to handle a digital signal at about 60kHz, so it shouldn’t be too demanding. The wires on one end of the cable are simply soldered Fig.3: it’s up to you which software you use for data logging – there’s a mountain of it out there, a lot of it freeware. This screen grab shows the “Eltima” RS232 software which the author uses. More on this next month. directly to the appropriate pins on the DS1621. Follow the wiring diagram and the photograph, which shows the underside of the DS1621. All DS1621 pins except pins 4 & 5 are trimmed before soldering so that they can be bent flat onto the back of the device without touching one another. Bending them back like this gives a more compact final result. After soldering, coat the DS1621 and the end of the cable in 2-part epoxy. Try to keep the amount of epoxy to a minimum – the more there is, the more bulk that has to heat up or cool down each time the temperature changes, thereby reducing responsiveness. Protecting the DS1621 like this should be fine for most uses. But be warned: experience has shown that it won’t tolerate extended immersion or prolonged exposure to wet or damp environments. If high water-resistance is needed, pot the DS1621 in silicone sealant (again, minimising the amount used) then use a short length of adhesive lined heatshrink tubing over this. After heating the heatshrink (and while the adhesive is still melted), pinch the open end closed until the adhesive re-hardens (use gloves or you could burn yourself!). It’s a good idea to apply white paint to the coated sensor to reflect any radiant heat that may reach it. If you don’t do this, you may measure heat from sources other than the surrounding air. The temperature sensor is accurate to 0.5° Celsius and SC is not adjustable. We’re getting a bit ahead of ourselves (mechanical details will be presented next month) but this shot shows how the control box is “hung” inside its PVC pipe with the hanger bracket riveted to the PVC pipe cap “lid”. siliconchip.com.au NEXT MONTH: Full construction details for the rain gauge and temperature measurement housing September 2007  65