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Solar-powered wireless weather station Based on the popular PICAXE processor, this simple weather station will “wirelessly” transmit temperature and humidity data to a receiver up to 200m away. Build one for your backyard. By NENAD STOJADINOVIC T HE RADIO MODEMS presented in the November 2003 and May 2004 issues have proved to be very popular and it’s been an education to see some of the uses they have been put to. One of the most common FAQs concerned the monitoring of weather, so in response to what is clearly a widespread fascination, I’ve decided to produce a simple weather station design. Educational value was a major consideration, so I decided that the best arrangement would be to have a PICAXE-18A handling all of the smarts. This would be followed by a serial data modem (from the November 2003 issue) that simply relayed the 56  Silicon Chip various readings to a receiving station, where they could be displayed. The avid experimenter or student would then be able to poke around the sample code to their heart’s content, secure in the knowledge that they don’t have to worry about messing up a complex RF data link. A computer is one of the easiest ways to present the readings, so the results go into the COM port of an old IBM-compatible running Hyperterminal. Fig.1 shows the basic layout. And now the weather The PICAXE takes a reading from a Dallas DS18B20 temperature sensor and a humidity reading from a Honey- This view shows the completed Weather Station board (bottom) with its companion Radio Modem Transmitter board connected. well HIH-3610-001. The readings are processed and loaded into the modem transmitter, together with information to make it more easily readable by the user. In the example program listing, the PICAXE loads bytes that correspond to “T = ” before loading the temperature value and “RH% = ” before the humidity. The receiver will then simply print the received bytes onto the screen resulting in, say, T = 24, RH% = 64. As an added refinement, I have followed up the data with a separate transmission that sends an <enter> and <LF> siliconchip.com.au Fig.1: block diagram of the Weather Station. It uses a PICAXE microcontroller to monitor temperature and humidity sensors and this then drives a Radio Modem Transmitter (described in November 2003). The RF signal is picked up at the receiving station, decoded and fed into a PC which displays the data. (line feed) command so that you end up with a neat list of readings down the lefthand side of the screen. Why a separate transmission? Because the modem accepts only 16 bytes at a time and by the time you’ve loaded T<space> = <space>, etc, there isn’t room for the <enter> and <LF>. Speaking of refinements, I’ve designed in an elegant switchmode power supply from the clever people at Maxim. Based on the MAX757, it will accept any voltage between about 0.5V and 5V and turn it into a stable 5V. Not only can you use a 1.5V battery rather than an expensive 9V type, you can also use a solar cell and Nicad battery for “eternal” power. As shown in the photos, my version was built into a cheap solar garden light, currently being sold just about everywhere in almost any form you can imagine. Using garden lights was a real boon, especially as they can be found for around $10. The innards consist of a solar cell charging a Nicad battery through a diode, followed by a simple circuit that switches the LED on at sunset. Conversion consists of evicting the original circuit board and wiring the weather station in its place. General arrangement The weather station is in the form of three separate logical building blocks. First, let’s look at the power supply siliconchip.com.au which is a DC-DC switchmode converter operating in boost mode – see Fig.2. Current from the low voltage supply flows through inductor L1 and into pin 8 of the MAX757. The boost effect relies on the fact that the current in a coil is slow to start flowing and equally slow to stop once it does – think of it as an electronic version of a flywheel. The current is allowed to get up to a good clip and then pin 8 suddenly switches it off. The energy in the coil then has nowhere to go except through diode D1 and into the 100µF storage capacitor – think of the capacitor as a water tank. The clever part is that the final voltage on the 100µF capacitor can be controlled by varying the ratio of the on to off times of pin 8. Pin 2 monitors the voltage output and it is compared to a reference voltage generated by a potential divider made up of the 33kΩ and 11kΩ resistors. The MAX757 also has provision for a low voltage warning at pin 4. I haven’t used it in the weather station but have provided for the possibility. Thus, R2 is deleted and a wire link is put in place of R6. The PICAXE is the heart of the circuit. The temperature sensor outputs a digital signal which is read by a PICAXE command specifically designed for this particular sensor, called “READTEMP”. The humidity is handled by an A/D conversion fol- lowed by a mathematical equation that is evaluated by the PICAXE to arrive at a humidity reading from 0-100%. It is important to note that both of the readings are heavily processed to turn them into something that can be read by people. For example, the temperature sensor is itself controlled by its internal microprocessor and the PICAXE must establish communications, download the temperature reading (in a special format), convert it to a more human readable form and then finally convert it to the ASCII for- The circuit is built into a low-cost solar-powered garden light and is powered from the existing solar cell and Nicad battery. December 2004  57 Fig.2: there are basically three parts to the circuit: (1) the weather station circuitry consisting of PICAXE IC1 and the humidity and temperature sensors; (2) a radio modem transmitter; and (3) a simple switchmode power supply. mat which computers use for display purposes. The radio modem then has the simple job of taking the resulting data and making sure it arrives at the computer’s COM port in good order, exactly as a good post office does. There is an unusual feature in that the modem is actually powered by the PICAXE! Construction Start the construction by building the power supply section only, taking extra care to correctly orient the ca58  Silicon Chip pacitors and diode. Once it’s finished, connect a 1.5V battery and measure the output at pin 6; it should be very close to 5V. Next up is the modem, which is assembled as per the included instructions. You will be making life hard for yourself if you don’t test the modem before use – this is most easily done by soldering a temporary link between the SEND pin and ground. Plug the receiver into your computer, fire up Hyperterminal at 1200 bits/sec and apply 5V to the transmitter. If you get a neat series of ‘012345678:;<=>’ then you can be fairly sure the modem works. If it does, remove the temporary link and put the modem aside. Next, solder in the two sensors and put the programmed PICAXE into its socket, then solder in the modem. Apply power – if all is well, you will get a line on your screen with the temperature and humidity, followed a short time later by a line feed. If not, the first thing to check is the power and control lines to the modem transmitter. The power is supplied by pin 8 of the PICAXE and SEND is controlled by pin 7. Power should of course sit at 5V and the SEND pin should blip down to 0V twice every 15 seconds or so. Data is supplied to the modem via pin 9 and you should see siliconchip.com.au Fig.3: follow this wiring diagram to assemble the Weather Station PC board. The Radio Modem Transmitter board can be mounted at right angles on this board and connected using tinned copper wire links – see photo below right. Above: this photo shows how the board assembly is mounted on the bottom of the plastic solar-cell housing. two brief “blurts” of data at the same 15-second repeat rate. If you get lines of data on your screen but the temperature or humidity value is wrong or non-existent, check the orientation and electrical integrity of the sensors. For the experimenter I have included several features to more easily adapt the weather station to various tasks. Probably the most siliconchip.com.au important is that the PICAXE powers the modem, specifically intended for low-power operation. The PICAXE December 2004  59 Radio modem transmitter & receiver circuits Fig.4: reproduced from the November 2003 issue, this diagram shows the Radio Modem Transmitter (Tx) circuit. IC1 (an 8-pin PIC microcontroller) receives serial data – in this case from the PICAXE on the Weather Station PC board – and then sends it to the receiver via a UHF transmitter module. Fig.6: parts layout for the Radio Modem Transmitter PC board. Fig.7: parts layout for the Radio Modem receiver board. Fig.5: again described in the November 2003 issue, the front end of the receiver circuit is almost a mirror image of the transmitter. PIC microcontroller IC1 receives data from the UHF receiver module and – after decoding and error checking – passes it on to the PC’s serial port via a MAX232 receiver/driver chip (IC2). The optional external DATA output is not used for the Weather Station. 60  Silicon Chip siliconchip.com.au Parts List 1 Toko 22µH inductor (494LYF0084M) 1 PC board, code 07112041, 61 x 39mm 1 18-pin DIL IC socket Semiconductors 1 1N5819 Schottky diode (D1) 1 PICAXE-18A microcontroller (IC1) 1 MAX757CPA DC-DC converter (IC2) 1 Dallas DS18B20 smart temperature sensor 1 Honeywell HIH-3610-001 humidity sensor 1 Radio modem Tx/Rx pair Capacitors 1 220µF 10V electrolytic 1 100µF 16V low ESR electrolytic* 1 100n (0.1µF) monolithic (code 100n or 104) Resistors 1 33kΩ 1 10kΩ 1 11kΩ 1 4.7kΩ R2 – optional, see Fig.3 R6 – wire link, see Fig 3 * A low ESR capacitor will give fractionally higher conversion efficiency; they can be found at Altronics and Jaycar. I personally don’t bother and just fit an ordinary capacitor. demo code transmits readings every 15 seconds or so and metaphorically drives around in circles in between times. Low power operation Low power operation usually requires that the transmitter power is turned off (by pulling pin 8 low) and the PICAXE is put to SLEEP between transmissions. You may also find that 15 seconds is too short a time between transmissions for the battery to maintain and so the PICAXE may need to sleep for, say, five minutes at a time. There are several extra inputs and outputs on the board, perfect for extra tasks such as reading rainfall or solar radiation, turning on fans, etc. If a common theme emerges, I may develop some extra modules to plug into the basic unit. As mentioned, the MAX757 DC-DC siliconchip.com.au Demonstration PICAXE Program ; DEMO.BAS by Nenad Stojadinovic ; This software is freeware and may be freely distributed ; This software is written for the PICAXE 18A or 18X and reads temperature from ; a Dallas Semiconductor DS18S20 and relative humidity from a Honeywell ; HIH-3610 sensor. Either the temperature or humidity section of the program can be ; used on its own, just cut out the bit you don’t need. Likewise the screen control ; section can be modified or cut out for use on LCDs. ; The temp and RH data is converted to ASCII and sent out at 1200 baud (N,8,1). ; Provision is made to control a radio modem transmitter, as detailed in Silicon Chip, ; Nov 2003. Output pin OUT 3 remains high until all sensor data is accumulated by the ; PICAXE and then it is pulled low for 100ms to trigger a radio transmission. Email to ; vladimir<at>u030.aone.net.au for more information. ; This software is based on articles originally published in Silicon Chip, Nov 03 and ; March 04. ; OUT 3 connected to the transmitter’s DATA line (high when idle) ; OUT 2 connected to the transmitters POWER input ; OUT 1 connected to the transmitters SEND line (high when idle) ; DS18B20 temperature sensor connected to IN 6 ; Honeywell HIH-3610-001 relative humidity (RH%) sensor connected ; to IN 2 which makes an A/D conversion of the sensor voltage ; NOTE 1: putting the “#” before the register transmits the data in ASCII format ; that is suitable for computer comms progams (eg, Hyperterminal) or LCD displays ; such as the AXE 033. ; NOTE 2: This program sends a comma (44) after the temperature and a line feed (10) ; and carriage return (13) after the humidity. You can change this to anything you like, ; and you can also delete the "T = " and "RH = ". For example, you might like to log the ; temp and RH over a period of time and this is easily done using MS Excel by reading ; in the received data as comma separated variables (CSV). ; NOTE 3: The humidity sensor relies on a very accurate 5V power supply. Test the ; sensor by breathing on it – if it goes over 100% RH,check the power supply voltage ; or adjust the equation (see the SC article in Nov. 04). Note though that humidity ; sensors are not all that accurate – a few percent variation is considered very good. high 3 ;Initialise DATA line high 2 ;Initialise POWER line high 1 ;Initialise SEND line wait 1 ;Wait for it all to settle main: readtemp if serout goto neg: let serout serout humid: readadc let serout 6,b1 b1 > 127 then neg 3,t1200,(“T = “,#b1,44) humid ;Read the sensor, store in b1 ;Is temperature negative? ;Send the temperature stored in b1 ;Finished sending positive temp b1 = b1 - 128 3,t1200,(“T = -”) 3,t1200,(#b1,44) ;Adjust negative value ;Send a minus sign ;Send the temperature stored in b1 2,b2 ;Read the humidity, store in b2 b2 = b2-41*100/157 3,t1200,(“RH% = “,#b2) ;Send the humidity stored in b2 low pause high 1 100 1 wait serout low pause high 2 3,t1200,(10,13) 1 100 1 ;Wait for trans to finish ;Screen control characters ;Lower SEND line to transmit ;Wait a bit ;Put the SEND line back to idle wait 15 ;Wait 15 seconds goto main ;Do it all again ;Lower SEND line to transmit ;Wait a bit ;Put the SEND line back to idle December 2004  61 The solar cell housing is easily dismantled, by undoing a few self-tapping screws. The transparent top cover is shown immediately above, while at right is the solar cell section. The solar cell charges a Nicad battery via a diode and on/off slide switch, as shown at top right. converter has a low-voltage indicator built in and this is implemented using optional resistors R2 and R6. If you want to use it, it’s simply a matter of downloading the data sheet from Maxim and then following the instructions. Antennas I have found that the best antenna Where To Get The Parts Kits plus individual parts for this project are available from the author. Prices are as follows: (1). Weather Station PC board plus switchmode supply components....... $28.50 (2). Add PICAXE-18A ............................................................................... $10.50 (3). Add DS18B20 temperature sensor....................................................... $6.00 (4). Add HIH-3610-001 humidity sensor.................................................... $35.00 (5). Package deal of Tx/Rx modem plus Weather Station PC board     plus switchmode power supply components....................................... $93.00 All prices include postage within Australia and GST. To order, write or email the author as follows: Nenad Stojadinovic, PO Box 320, Woden, ACT 2606. email: vladimir<at>u030.aone.net.au Note: please contact the author before ordering as prices are very volatile (eg, the humidity sensor has gone up by 400% in the last year)! 62  Silicon Chip for the buck is a simple length of insulated wire. Just cut it to 165mm and solder it on – no adjustment is necessary. In my case, it was possible to feed the antenna down into the plastic garden light post and I found that the range was excellent. For those who want to build the transmitter into a smaller space, a coil antenna may be more appropriate (see photo). It simply consists of 24 turns of 25-gauge enamelled copper wire wound onto a 3mm drill bit, with a total length of about 19mm (see photo). It will need to be adjusted by stretching out or compacting the coils until you get adequate range (remember that radio spectrum users consider it impolite to blast out more power than you need, even at the low levels we are generating). Note, however, that you will never get the range of the simple piece of wire. If you need a professional appearance or real range, you can’t go past the rubber duck antenna featured in SC the previous articles. siliconchip.com.au