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PICAXE-18X 4-channel datalogger Pt.3: adding a humidity sensor, more memory & a liquid crystal display In the first two parts of this series, we described how to build and program the PICAXE-18X Datalogger, and showed how to add a batterybacked real-time clock (RTC). In this final instalment, we look at adding a few more goodies, including a humidity sensor, a liquid crystal display and more memory. By CLIVE SEAGER T here are various humidity sensors on the market but the recommended device for use with the PICAXE-18X Datalogger is the Honeywell HIH-3610-001. This sensor is a direct humidity-to-voltage device with built-in conditioning circuitry. It is supplied in a small 3-pin single-in-line package. Two of the pins connect to a regulated +5V power source, while the third gives a linear output voltage that’s proportional to humidity. This means that it can be connected directly to the Datalogger (via connector CT4) without any additional circuitry (see photo above). As with all humidity sensors, take care not to physically touch the sensing area of the device, as moisture/ oils from the hand could damage the sensitive sensor element. A sample graph of the response of the humidity sensor is shown in Fig.1. 78  Silicon Chip When used with the PICAXE, the voltage output of the sensor is measured by the internal analog-to-digital converter (ADC) and stored in a variable (eg, b1) as a number between 0 and 255. Each ADC step is 5V/256 = 0.0195V (using a regulated 5V supply). The graph in Fig.1 shows an offset of approximately 0.8V, which equates to an ADC value of 41 (0.8 / 0.0195). The RH (relative humidity) slope is set at about 0.0306V per %RH, or 1.57 ADC steps per %RH. The actual %RH can be calculated using the following formula: %RH = (ADC value - offset)/(slope of graph) = (ADC value - 41)/1.57 However, as the PICAXE programming language cannot handle fractions, the divide by 1.57 is actually performed by first multiplying by 100 then dividing by 157, as follows: %RH = [(ADC value - 41) x 100]/157 Checking these test values against a calibrated probe using the test program in Fig.2 proved the PICAXE system to be very accurate. However, you may need to “tweak” the offset and slope figures depending on sensor calibration, power supply voltage, temperature, etc. An application note showing how to apply temperature compensation is available from www.phanderson.com/ picaxe/hih3610.htm Note: in order to preserve the accuracy of the humidity sensor’s readings, the Datalogger must be powered from a well-regulated 5V supply. Controlling humidity One of the things I enjoy most about my job is observing the interesting and varied electronics projects turned out by high school students for their electronics course work. Within the UK curriculum, 60% of the final grade is allocated to a practical project and many students produce some wonderful pieces of work. For example, a school I visited recently had produced a number of PICAXE projects linked to a local wildlife water park. One of their most interesting projects was an incubator for working with eggs from rare breed birds. Movement, humidity and temperature are the crucial factors when incubating eggs. Movement and temperature are relatively easy to control but commercial humidity controllers are very expensive, and generally involve www.siliconchip.com.au The fan control switch was built onto a small piece of Veroboard (in turn controlled by the Picaxe Datalogger) and controls a small fan mounted on the lid of the container. Fig.1: this diagram shows the pin out of the humidity sensor and its response characteristics. some form of pump which releases water onto a sponge. The surface area of this sponge is crucial and may often need physical adjustment. The disadvantage of this system is that the response time is extremely slow and so the humidity can fluctuate wildly. It is also not very hygienic as the sponge rapidly attracts bacteria. Therefore, the students had devised a very novel and low-cost alternative. Their humidity unit is based around a plastic food container filled with water. Two holes are cut in the lid of the box. One hole provides an opening for a small 5V fan, which is attached to the box with hot-melt glue. The second hole (measuring about 20mm x 50mm) is covered with a piece of plastic, which is “hinged” along one edge with adhesive tape to form a “flap”. When the fan is switched on, air pressure lifts the plastic flap, allowing moisture-laden air from the fan’s draft to escape from the box. When the fan is switched off, the plastic flap falls and effectively seals the box (in practice, the small surface area exposed through the fan blades makes little difference to the operation of the system). This system provides a very large effective surface area when the fan is running, which means that the humidity can be much more accurately controlled. In fact, it works so well that www.siliconchip.com.au Fig.2: Test Program main: readadc 1,b1 let b1 = b1 - 41 * 100 / 157 debug b1 pause 500 goto main ‘read humidity value ‘change to %RH ‘display on screen ‘wait 0.5 second ‘loop the wildlife park managed to hatch exotic eggs in the students’ incubator that had never before hatched in the commercial units! The Datalogger controls the fan with the aid of a simple interface circuit (Fig.3). The circuit can be constructed on a small strip of Veroboard (or similar) and wired to the Datalogger’s piezo sounder output (Output 0). Serial LCD add-on The students had also used a serial LCD module within the incubator to display the temperature and humidity readings. The LCD module consists of a conventional 16-character, 2-line LCD “piggy-backed” onto a microcon- Fig.3: the fan control circuit is connected to the unused piezo output terminals on the Datalogger. troller-based PC board. The microcontroller’s task is to receive serial data from the Datalogger and generate the signals necessary to display the data on the LCD. Only three wires are required to connect a serial LCD to the Datalogger. The V+, 0V and Data pins on the Datalogger’s LCD connector (CT9) go to the corresponding pins on the serial LCD. Note that the LCD module includes a polarity protection diode in series with the V+ input. The 0.7V drop across this diode causes a reduction in LCD contrast (“brightness”) when operating from a 4.5V battery supply. If you experience this problem, replace the diode with a wire link – but make sure that you have the supply leads around the right way first! Note: the serial LCD module described here is an optional add-on that can be purchased as a kit of parts. Complete assembly instructions are included with the kit. The program to regulate and display the humidity is shown in Fig.4 (target value 60% RH). This program also displays the temperature to three decimal places. The temperature and light values are read every five seconds and displayed on the serial LCD via March 2004  79 Fig.4: Humidity Program SYMBOL temperature = w0 SYMBOL temperatureLSB = b0 SYMBOL temperatureMSB = b1 SYMBOL humidity = w1 SYMBOL scratchpad = w2 SYMBOL address = w3 SYMBOL counter = w4 main: for counter = 1 to 12 '12 * 5 seconds = 1 minute ' 'Read and correct RH value ' do_RH: readadc 1,humidity let humidity = humidity - 41 * 100 / 157 ' 'Switch fan on or off as necessary ' control_fan: if humidity < 60 then fan_on fan_off: fan_on: low 0 goto do_temp 'switch fan on high 0 'switch fan off ' 'Read raw 12 bit data from DS18B20 temperature sensor ' do_temp: readtemp12 7,temperature ' 'Format data into two bytes: 'temperatureMSB = value before decimal place 'temperatureLSB = value after decimal place ' temperatureMSB = temperature / 16 scratchpad = temperatureLSB & $0F * 625 / 10 temperature = scratchpad ' 'Display humidity and temperature on serial LCD ' do_display: serout 6,N2400,(254,128,”RH% = ", #humidity) serout 6,N2400,(254,192,”Temp = ", #temperatureMSB, ".") if temperatureLSB > 100 then skip0 serout 6,N2400,("0") 'display leading 0 if required skip0: serout 6,N2400,(#temperatureLSB) pause 5000 'wait 5 seconds next counter 'next loop ' '1 minute is up so log temperature and humidity ' log_data: high 3 'LED green low 5 'write enable i2cslave %10100010, i2cfast, i2cword writei2c address,(humidity) pause 10 'select EEPROM 1 'write the value 'wait write time i2cslave %10100100, i2cfast, i2cword writei2c address,(temperatureLSB) pause 10 'select EEPROM 2 'write the value 'wait write time i2cslave %10100110, i2cfast, i2cword writei2c address,(temperatureMSB) pause 10 'select EEPROM 3 'write the value 'wait write time high 5 low 3 'write protect 'LED off let address = address + 1 if address < 32767 then main end 'increment address 'if memory not full 80  Silicon Chip Where To Get The Parts The complete Datalogger kit (Part No. AXE110), memory expansion kit (Part No. AXE111) and serial LCD kit (Part No. AXE033) are available from Microzed and their distributors. Contact Microzed on (02) 6772 2777 or check out their web site at www.microzed. com.au The Honeywell HIH-3610-001 humidity sensor is available from Farnell, Cat. 393-7446. Check out their on-line catalog at www. farnellinone.com.au or phone 1300 361 005. the serout command. The values are also logged in EEPROM once every minute (see last months article for more details). The serout command codes “254,128” and “254,192” in the program listing are “cursor” commands that move the cursor to the top and bottom lines of the LCD, respectively. Temperature measurements The DS18B20 temperature sensor (supplied with the Datalogger kit) is a 12-bit digital device with a maximum resolution of 0.0625°C. Much of this accuracy is lost with the PICAXE readtemp command, which automatically rounds and corrects the value to the nearest whole degree. However, the PICAXE-18X part also has a readtemp12 command, allowing all 12 bits of the temperature reading to be retained for maximum accuracy. The program in Fig.4 shows how to separate the raw 12-bit data into two bytes – the “whole degree” and the “fraction” after the decimal place. These values are then displayed to three decimal places on the LCD. Expanding the memory. The Datalogger kit is supplied with a single 24LC16B EEPROM. This provides enough space for 512 bytesized readings for four sensors, or 1024 readings for two sensors. For some experiments, you may want to add more memory and with the PICAXE-18X Datalogger, this is very easy to do! The simplest upgrade involves replacing the 24LC16B EEPROM with the larger, pin-compatible 24LC256 www.siliconchip.com.au The memory expansion board plugs into the I2C connector at one end of the Datalogger. Want really bright LEDs? We have the best value, brightest LEDs available in Australia! Check these out: Luxeon 1 and 5 watt LEDs All colours available, with or without attached optics, as low as $10 each Lumileds Superflux LEDs These are 7.6mm square and can be driven at up to 50mA continuously. •Red and amber: $2 each •Blue, green and cyan: $3 each device. This increases the available space to 8192 byte-sized readings for four sensors. In programming terms, the only real difference between the two chips is the i2cslave command used. For the 24LC256, use the i2cslave, %10100000, i2cword command as it has a word address rather than a byte address. If desired, up to seven more 24LC256 EEPROMs can be added to the datalogger with the addition of a memory expansion board. This multiplies the available memory by eight times! The memory expansion board is a small PC board with seven sockets to accept the Connecting the LCD is as easy as running three wires back to the Datalogger board, either direct or via a socket (not included with the kit) as shown here. additional EEPROMs. It simply plugs into the Datalogger via the 5-pin I2C expansion connector (CT8). The program shown in Fig.4 stores data in the 24LC256 EEPROMs in positions 1, 2 and 3 on the expansion board. Up to 32,768 readings can therefore be made, giving over 22 days of logging with the 1-minute sampling SC interval shown. About the Author Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. Asian Superflux LEDs Same size and current as the Lumileds units, almost the same light output, but a fraction of the price. •Red and amber: Just 50 cents each! •Blue, green, aqua and white: $1 each. Go to www.ata.org.au and check out our webshop or call us on (03)9388 9311. Silicon Chip Binders REAL VALUE AT $12.95 PLUS P&P H Each binder holds up to 12 issues H SILICON CHIP logo printed on spine & cover Price: $A12.95 plus $A5 p&p each. Buy five and get them postage free (available only in Australia). Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. www.siliconchip.com.au March 2004  81